Hazards of the Anesthesia Delivery System

Critical Incidents

The critical incident (CI) technique was first described by Flanagan in 1954 and was developed to reduce loss of military pilots and aircraft during training. It was modified and introduced into anesthesia by Cooper et al., who interviewed staff and resident anesthesiologists in a large metropolitan teaching hospital. They collected and analyzed 1089 descriptions of CIs during anesthesia. A mishap was labeled a CI when it was clearly an occurrence that could have led, if not discovered or corrected in time, or did lead to an undesirable outcome, ranging from increased length of hospital stay to death or permanent disability. Other CI study inclusion criteria were as follows: that each incident involves an error by a member of the anesthesia team or a failure(s) of the anesthesiologist’s equipment to function properly; it occurred during patient care; it could be clearly described; and the incident was clearly preventable. Of the 1089 CIs, 70 represented errors or failures that had contributed in some way to a “substantive negative outcome” (SNO), defined as mortality, cardiac arrest, canceled operative procedure, or extended stay in the postanesthesia care unit, intensive care unit, or in the hospital. While 30% of all CIs were related to equipment failures, including breathing circuit disconnections, misconnections, ventilator malfunctions, and gas flow control errors, only three (4.3%) of SNO incidents involved equipment failure, suggesting that human error was the dominant problem. Although equipment failures rarely cause death, CIs related to equipment are common and have prompted improvements in equipment design, in construction, and in monitoring.

Many studies have demonstrated that anesthesia caregivers perform poorly when it comes to identifying problems with their anesthesia delivery systems. Buffington et al. intentionally created five faults in a standard anesthesia machine and then invited 190 attendees at a Postgraduate Assembly of the New York State Society of Anesthesiologists to identify them within 10 minutes. The average number of discovered faults was 2.2; 7.3% of participants found no faults and only 3.4% found all five. The authors concluded that greater emphasis was needed in educational programs on the fundamentals of anesthesia machine design and detection of hazards.

In an effort to improve patient safety and proper use of the anesthesia machine, the American Society of Anesthesiologists (ASA), the Anesthesia Patient Safety Foundation, and others supported the use of checklists to enable the anesthesia practitioner to ensure that the anesthesia delivery system was functioning normally prior to the start of an anesthetic. In 1986, the US Food and Drug Administration (FDA), in cooperation with the ASA, machine experts, and manufacturers, published a generic apparatus checklist to enable the practitioner to check out anesthesia equipment thoroughly prior to its use (see Chapter 25 ). March and Crowley evaluated the 1986 FDA recommended checklist and individual practitioner checklists in order to determine whether the existence of the FDA checklist would improve detection of anesthesia machine faults. Participants in this study were given machines with four faults that were detectable if the FDA checklist was used properly. The results of the study revealed that 25.8% of faults were found with the practitioner’s checklist and 29.9% were found with the FDA checklist. In either case, the results were poor and indicated that the mere introduction of the FDA checklist in 1986 did not improve the ability of these anesthesiologists to detect machine faults. However, it should be noted that in this study no attempt was made to ensure that the FDA checklist was used properly during the checkout procedure.

Kumar et al. conducted a random survey of 169 anesthesia machines and ancillary monitors in 45 hospitals in Iowa. The machines ranged in age between 1 and 28 years (the oldest being 1958 vintage). Five machines had no backup source of oxygen, 60 had no functioning oxygen analyzer, and 15 had gas leaks of greater than 500 mL/min (2 proximal to the common gas outlet and 13 in the patient circuit). Fourteen of the 383 vaporizers tested did not meet the manufacturer’s calibration standards, and 20 had been added downstream of the machine common gas outlet. Of the 123 machines with ventilators, 16 had no alarm for low airway pressure and only 31 had a high-pressure alarm. Of the ventilators surveyed, 59% were of the hanging bellows design and 41% of the standing design; 95.5% had a scavenging system, but in 24.3%, the scavenging circuit connectors were indistinguishable from the breathing circuit connectors, a potentially hazardous situation. The use of these old machines increases the risk for development of problems related to the delivery system; in addition, equipment users may not be as educated as they should be in their ability to detect such problems.

In 1993, the Australian Anaesthesia Patient Safety Foundation published results of the Australian Incident Monitoring Study (AIMS) that had collected 2000 CIs. Of these, 177 (9%) were due to equipment failure, in general, and 107 of the 177 (60%) involved the anesthesia delivery system. Failures included problems due to unidirectional valves, ventilator malfunctions, gas or electrical supply, circuit integrity, vaporizers, absorbers, and pressure regulators. Concerning problems with ventilation, it was recommended that critical areas be doubly or triply monitored and that monitoring equipment be self-activating.

Adverse Outcomes

In the absence of mandatory reporting systems and concerns for litigation, it is difficult to accurately estimate the frequency of adverse outcomes associated with use of anesthesia delivery systems. Case vignettes have provided some insight into how adverse outcomes have arisen and led to development of standards for monitoring. The role of equipment failures leading to malpractice litigation in the United States has been studied by the ASA Closed Claims Project (CCP). The ASA CCP is a structured evaluation of adverse anesthetic outcomes obtained from the closed claims files of 35 US professional liability insurance companies. A 1997 analysis of 3791 claims, of which 76% occurred during the period 1980–1990, found that gas delivery equipment (GDE) problems accounted for 72/3791 (2%). Of these 72, 39% were related to the breathing circuit, 21% to vaporizers, 17% to ventilators, 11% to gas tanks or lines, and 7% to the anesthesia machine. Death or brain damage occurred in 76% of the 72 cases. Initiating events were circuit misconnects, disconnects and gas delivery system errors. The primary mechanism of injury was inadequate oxygenation in 50% of the claims, excessive airway pressure in 18%, and anesthetic overdose in 13%. Misuse was judged to have occurred in 75% and equipment failure in only 24%. Anesthesia caregivers were considered responsible in 70% of use error cases and ancillary staff (e.g., technicians) to have contributed in 30%. Predominant mechanisms of injury were hypoxemia, excessive airway pressure, and anesthetic agent overdose. Overall, the reviewers deemed 78% of claims to have been preventable by the use or better use of monitoring. Payment or settlements were made in 76% of the claims (median payment, $306,000; range, $542 to $6,337,000) so that the claims were notable for a high severity of injury, high cost, and a prominent role for equipment misuse.

In 2013, Mehta et al. published an update of the 1997 CCP study. At that time, of a total of 9806 claims (of which 6022 were from surgical or obstetrical cases), 115 claims were related to GDE. The claims were categorized by type of equipment; provider error only, equipment failure only, or both; and whether the adverse outcome was preventable by a pre-use checkout of the anesthesia delivery system ( Table 23.1 ). In contrast to the 1997 study, the number of vaporizer-related claims exceeded breathing circuit claims and there was an increase in supplemental oxygen supply–related claims. The authors considered that 35% of the 115 claims were preventable by a preanesthesia equipment check.

As of December 2018, the CCP database included 11,034 claims, of which 125 were related to anesthesia GDE. The most recent GDE claim was for an event in 2014. Thus far, however, it appears that GDE problems are decreasing as a proportion of surgical anesthesia claims. Anesthesia gas delivery claims represented 4% of surgical or obstetric general anesthesia claims from the 1970s, 3% from the 1980s, 1% from the 1990s, and 1% from the period 2000–2014. There were only 49 anesthesia gas delivery system claims from 1990–2014. These included 17 vaporizer problems, 11 breathing circuit problems, 11 oxygen tank and supplemental oxygen line events, 6 ventilator problems, and 4 anesthesia machine problems. The outcomes in anesthesia GDE claims from 1990–2014 seem to be less severe than earlier claims.

In the period 1990–2014, 43% of anesthesia gas delivery system claims resulted in severe injury or death compared to 80% in 1970–1989. Among the 49 claims from 1990–2014 were 15 deaths, 11 awareness claims, 11 pneumothorax claims, and 5 permanent brain damage claims. Payments reflect the lower severity of injury, with a median payment (in 2017 dollars) of $322,000 in the 1990–2012 claims compared with $923,000 (adjusted to 2017 dollars) for earlier GDE claims. Thirty-nine (80%) of the 49 post-1990 claims resulted in payment.

With patient safety as the primary concern, over the past several years, the basic gas machine has evolved into the present, more sophisticated anesthesia delivery system/workstation. The most current voluntary consensus standard describing the features of a modern anesthesia workstation is that published by the International Organization for Standardization in 2011. This standard superseded the American Society for Testing and Materials (ASTM F 1850-00) (republished in 2005), which described the requirements for anesthesia workstations and their components. ASTM F-1850 superseded the ASTM F1161-88 published in 1988 and reapproved in 1994 (see also Chapter 27 , Standards and Regulatory Considerations). It is anticipated that the use of a state-of-the-art anesthesia gas delivery system that meets the latest standard, together with adoption of the Standards for Basic Anesthetic Monitoring, first published by the ASA in 1986 and periodically updated, will enhance patient safety. As in the case of monitoring standards, however, absolute confirmation may be difficult.

Complications caused by the anesthesia delivery system may be operator-induced (misuse or “use error”) or attributable to failure of a component. The categories of oxygen delivery, carbon dioxide elimination, circuit pressure and volume problems, inhaled anesthetic agent doses, problems of humidification of inhaled gases, and electrical failure are discussed in detail.

Hypoxemia

Hypoxemia, which for the purposes of this chapter is defined as a PaO ₂ less than 60 mm Hg, may be caused by problems with the anesthesia delivery system or by problems within the patient. If the patient is adequately ventilated and the alveolar oxygen concentration is as expected, then a problem with the patient is the cause of hypoxemia. Pulmonary conditions that cause shunting, venous admixture, ventilation-perfusion mismatch, or, less likely, diffusion defects can cause hypoxemia. Examples of these conditions are pneumonia, atelectasis, pulmonary edema, pneumothorax, hemothorax, pyothorax, pulmonary embolism, alveolar proteinosis, and bronchospasm. In addition, conditions that decrease mixed venous oxygen, such as anemia and shock, may also cause or contribute to hypoxemia ( Box 23.1 ).

The anesthesia delivery system may cause hypoxemia by delivering insufficient oxygen to the lungs and thereby reducing the alveolar oxygen concentration ( Box 23.2 ). Inadequate ventilation, caused by either apnea or low minute ventilation, is a well-described cause of alveolar hypoxia. These problems can arise due to failure to initiate manual or mechanical ventilation or to resume mechanical ventilation after deliberately suspending it such as for median sternotomy or taking of X-rays. Some workstations offer a setting whereby ventilation and gas flow can be suspended for up to 1 minute (see Fig. 5.19 ). This “pause fresh gas flow (FGF)” feature allows time for airway management with near-zero pollution of the operating room by unscavenged anesthetic. It also prevents the potential liability of forgetting to turn the vaporizer back on (if turned off during intubation, as some prefer).

Hypoxemia may also be due to failure to recognize a major leak or disconnection in the breathing circuit even though ventilation is attempted. It is important that the caregiver be fully familiar with the components of the breathing system as the source of the leak may not always be obvious, such as from a circuit condensate drain valve. The anesthesia delivery system may also cause hypoxemia by delivering insufficient oxygen from the machine to the breathing circuit.

Insufficient or low inspired oxygen concentrations can be definitively detected via the use of an oxygen analyzer. The oxygen analyzer is a critical monitor because although it may appear that pure oxygen is being delivered from the oxygen flowmeter, if the gas in the flowmeter is not in reality oxygen, then the patient will receive a hypoxic gas mixture. Without the oxygen analyzer, this condition would not be recognized. The pulse oximeter, while a valuable patient monitor, does not replace the oxygen analyzer. A low SpO ₂ reading on a properly functioning pulse oximeter merely indicates that the patient’s hemoglobin is poorly saturated with oxygen. However, only the oxygen analyzer in the breathing circuit would be able to determine that the cause was inadequate delivery of oxygen to the patient. Therefore, these two monitors are complementary and both must be employed to ensure patient safety.

The oxygen analyzer is not without limitations. In order to act as a valuable safety device, it must have an adequate power source and be properly calibrated. In addition, it must be positioned such that it is sampling the gases that the patient will breathe. An analyzer placed by the inspiratory unidirectional valve may indicate a normal oxygen concentration, but if there is a disconnection between that point and the patient, the patient will not receive that gas. For this reason, it is critical that the anesthesiologist understand the equipment design and the limitations of this device. The analyzer must function normally, the circuit valves must be present, the circuit must be intact, and the patient’s lungs must be ventilated if the reading on the oxygen analyzer is to reflect the oxygen that is being delivered to the alveoli. Vigilant observation of the patient’s ventilation, the integrity of the breathing system, and the oxygen analyzer ensure proper delivery of oxygen to the patient.

The gas entering the anesthesia workstation from the hospital pipeline gas supply system or the oxygen cylinders may contain a gas other than oxygen. The central liquid oxygen reservoir may be filled with a gas other than oxygen (e.g., liquid nitrogen). The pipelines throughout the hospital may be crossed so that nitrous oxide or some other gas may be flowing through the oxygen pipeline (see Chapter 1 ).

Placement of a nitrous oxide wall adapter on one end of an oxygen hose would allow that hose to be connected to the wall’s nitrous oxide source and the anesthesia machine’s oxygen inlet, in which case nitrous oxide would flow through the oxygen flowmeter on the machine. ^, If, during use of an anesthesia workstation, one suspects that a gas other than pure oxygen is being delivered via the pipeline, the pipeline hose must be disconnected and the backup oxygen tank opened. If the tank is opened but the pipeline remains connected, oxygen will not flow from the tank because the pressure in the hospital central pipeline is usually 55 psig, whereas the pressure in the reserve oxygen cylinder is reduced to 45 psig.

Most contemporary anesthesia workstations are equipped with an auxiliary oxygen flowmeter and oxygen outlet, for connecting a nasal cannula, for example. One must recognize that there is no analysis of the gas flowing from this outlet and so it is presumed to be oxygen. In the event of a pipeline crossover, the hypoxic gas would also be delivered from the auxiliary oxygen flowmeter. Mudumbai et al. used medical simulation to investigate the response of 20 third-year anesthesia residents to an anesthesia machine pipeline crossover scenario. A significant number used the auxiliary oxygen flowmeter as a presumed external source of oxygen in their response to this crisis, contributing to delays in definitive treatment.

There is a report that modification of the gas-specific oxygen quick-connect on a wall outlet oxygen flowmeter allowed it to be connected into a nitrous oxide wall outlet. When a self-inflating resuscitation bag was connected to the oxygen flowmeter in the course of resuscitating two patients, the anesthesia caregivers unknowingly delivered nitrous oxide, with catastrophic outcomes. When a gas presumed to be oxygen is being delivered in the absence of an oxygen analyzer, if the patient does not appear to be responding appropriately, one must always consider the possibility of a “wrong gas” and ventilate instead with room air.

Because of potential problems with the wall oxygen source, it has been suggested that only oxygen cylinders be used. However, this approach overlooks the possibility that an oxygen cylinder may be empty, the valve faulty, a tank key not available, or it may contain a gas other than oxygen. ^, A nitrous oxide or another gas cylinder may be attached to the oxygen hanger yoke if the pin index system is defeated by removal of a pin or by placement of more than one washer (Bodek seal) between the yoke and the cylinder. Lorraway et al. studied a total of 20 second-year and fourth-year anesthesia residents in a simulation of an oxygen pipeline supply failure. They found that the majority of the participants either did not have the knowledge to change the oxygen cylinder or did not attempt to change the oxygen, even after prompting, demonstrating an apparent basic deficiency in their training. Finally, crossed pipes within the anesthesia machine would allow a gas other than oxygen into the oxygen flowmeter. Delivery of a hypoxic gas via the oxygen pipeline or from a cylinder has been the basis for some now classic movies (e.g., “Coma,” 1978; “Green for Danger,” 1946).

Turning on the oxygen flow control valve may result in no oxygen gas flow. The hospital’s central oxygen system may be empty, shut down, or otherwise unavailable to deliver oxygen. ^, In one report, separation of a brazed joint between the stainless steel liquid oxygen storage vessel and the brass pipe fitting connecting to the hospital oxygen pipeline resulted in spillage of 8000 gallons of liquid oxygen into the atmosphere. Fortunately in this case, there was a bulk liquid oxygen storage tank in another location that maintained the oxygen supply to the facility.

The oxygen hose to the anesthesia machine may become disconnected from the wall, or the back-up cylinders may be empty, absent, or turned off. There may be a leak in the intermediate pressure oxygen system in the machine. The oxygen flow control valve or oxygen piping in the machine may be obstructed, thereby preventing the flow of oxygen to the flowmeter. ^, The flowmeter bobbin or rotameter may become stuck, and it may appear that gas is flowing from that flowmeter even when it is not. Leaks in the oxygen flowmeter tube or the low-pressure portion of the anesthesia machine can permit loss of oxygen before it reaches the common gas outlet of the machine and breathing circuit. Oxygen delivery at the flowmeters could potentially be compromised if the flow tubes become filled with water ( Fig. 23.1 ). The source of the water is the air pipeline. The process whereby room air is compressed to supply the pipeline involves ensuring that all moisture is removed. If the dehumidifying process fails, the air pipeline may deliver air and water or, in an extreme case, only water. Once water has entered the machine and flowmeters, the repair can be quite expensive. One author stated that it would cost about $8000 to fix the machine and commented that “…there is something to be said about the safety of being able to see your flowmeters—as opposed to the digital interface in the newer anesthesia machines.”

Contemporary anesthesia workstations are equipped with proportioning systems that prevent the delivery of a <25% oxygen mixture when nitrous oxide is being administered. Older anesthesia machines may not have these safety design features and should be considered obsolete. Because the oxygen “fail-safe” system is sensitive to the pressure of oxygen rather than the flow of oxygen, on older anesthesia machines, the nitrous oxide flowmeter could be turned on without the oxygen flowmeter also being turned on ( Fig. 23.2 ). This could lead to the delivery of a hypoxic gas mixture in the breathing circuit; the oxygen analyzer would detect this problem and the anesthesiologist would have to recognize it and respond by adding oxygen to the mixture. Such a machine would, however, be considered unsafe per the ASA guidelines for determining anesthesia machine obsolescence.

Although nitrous oxide/oxygen proportioning systems help to prevent the delivery of hypoxic mixtures, they are not foolproof and cannot be relied on entirely as the only method to prevent a hypoxic mixture. The (now GE-Datex-Ohmeda) Link-25 proportion limiting system causes the oxygen flow control valve to open further and increase flow if a hypoxic mixture would otherwise result when only oxygen and nitrous oxide are being used. This system can fail if the needle valve is broken in the closed position or if the linkage between the flowmeter controls fails. ^, Limitations of all oxygen/nitrous oxide proportioning systems are that they do not analyze the gas that is flowing through the oxygen flowmeter, nor do they prevent administration of a hypoxic gas mixture if the machine has flowmeters for a third or fourth gas that is hypoxic ( Fig. 23.3 ) (e.g., helium, carbon dioxide).

Abnormalities in the anesthesia breathing circuit can lead to a hypoxic mixture. Absent or incompetent unidirectional valves in the circle system will permit rebreathing of exhaled gas, which, if insufficiently mixed with fresh gas, would present the patient with a hypoxic mixture. In the Mapleson circuits (see Chapter 4 , Breathing Circuits) loss of the fresh gas supply from the machine leads to severe rebreathing of a hypoxic mixture as the patient uses up the oxygen and replaces it with carbon dioxide. Failure of the fresh gas supply in the circle system results in a breathing mixture that becomes progressively hypoxic as the oxygen is consumed and only nitrous oxide remains. Leaks in the breathing circuit lead to loss of gases, which become more pronounced during positive pressure ventilation. If a hanging (descending) bellows ventilator is used, the lost gas may be replaced with entrained room air as the bellows descends by gravity during exhalation ( Fig. 23.4 ). A standing bellows collapses if a significant leak develops. In addition, system leaks can lead to severe hypoventilation. The sources of leaks may be valve housings, circuit hoses, pressure monitoring and gas sampling lines, connection sites, pressure relief valves, and carbon dioxide absorbers. Leaks can also be caused by subatmospheric pressure being applied to the system from unrelieved scavenger suction or by a catheter that has unintentionally been passed into the trachea alongside the tracheal tube. A leak in the ventilator bellows might allow the drive gas in the ventilator bellows housing to enter the patient circuit. It should be noted that some bellows ventilators that are normally driven by compressed oxygen can be reconfigured to be powered by compressed air. Depending on the gas used, this could affect the composition of the breathing mixture. ^,

Closed-system or low-flow anesthesia can lead to the delivery of a hypoxic mixture. The total flow of gas is adjusted to compensate for the uptake of nitrous oxide and oxygen. However, if the oxygen content of the circuit gases is not carefully monitored, the uptake of nitrous oxide may be low and that of oxygen may be high. The resultant mixture could become hypoxic.

Hyperoxia

Administration of a mixture that contains more oxygen than required results in hyperoxia. There are a number of possible causes, including insufficient nitrous oxide or air administered from the flowmeters; flowmeter leaks and inaccurate flowmeters may cause gases to be lost, with resultant high oxygen concentrations. A leak in a ventilator bellows that allows injection of pressurizing or drive gas oxygen (e.g., GE ventilators 7000, 7800, 7900 series) into the bellows may cause the inspired oxygen concentration to increase. ^, Administration of helium and oxygen via separate flowmeters during laser surgery of the airway could be hazardous if the helium supply becomes depleted, or if the oxygen flush were operated, in which case a hyperoxic gas mixture would result and may lead to a fire (see Fig. 23.3 ).

Hyperoxia may be undesirable or even dangerous in certain situations. It has been suggested that bleomycin induces sensitivity to oxygen toxicity and that the minimum FiO ₂ that can maintain an SpO ₂ >90% be used in these patients. Similarly, when there is a risk of fire, such as in airway or head and neck procedures, the FiO ₂ should be kept to the minimum compatible with an acceptable SpO ₂ . ^, In such situations, use of an oxygen analyzer with an appropriately set high oxygen concentration alarm is essential.

Hypercarbia

When carbon dioxide production exceeds elimination, the arterial carbon dioxide tension increases until equilibrium is achieved. During anesthesia, patient factors or delivery system conditions may cause hypercarbia.

Patients who are breathing spontaneously are prone to hypercarbia because of the depressant effects of anesthetics on the central respiratory center, weakness from muscle relaxants, and motor blockade during spinal or epidural anesthesia. Complete or partial airway obstruction can cause hypercarbia. Pulmonary conditions that cause a large shunt and increased metabolic production of carbon dioxide (e.g., from malignant hyperthermia [MH] or administration of sodium bicarbonate) without a concomitant increase in ventilation also cause hypercarbia. Absorption of carbon dioxide insufflated during laparoscopic surgery also often causes hypercarbia.

The anesthesia delivery system can be a source of hypercarbia. Apnea caused by failure to ventilate either manually or mechanically raises the carbon dioxide concentration. Ventilating with an inadequate tidal volume or respiratory rate reduces alveolar ventilation and leads to hypercarbia. Leaks in the machine, circuit, and ventilator and failure to fill the bellows may lead to hypoventilation ( Box 23.3 ).

The anesthesia breathing circuit may contain insufficient gas if the pipeline gas source fails. However, this type of problem can be mitigated by use of the reserve gas cylinders on the anesthesia machine. Inside the machine, leaks may develop either at the oxygen yoke or from a faulty check valve that permits gas to escape into the room. It is also possible for gas to leak from the pipes, flowmeters, vaporizers, vaporizer selector switches, and vaporizer mounts on the machine.

The interface hose from the anesthesia machine (common gas outlet) to the breathing circuit and the breathing circuit itself may be the sources of a leak. Disconnections and leaks of sufficient magnitude lead to hypercarbia. Flow of gas from the machine to the circuit may be obstructed either in the machine or in the interface hose. Leaks in valve housings, tracheal tubes, ventilator hoses, reservoir bag, ventilator bellows, and system relief valves can reduce the volume in the breathing system and cause hypercarbia.

Subatmospheric pressure applied to the breathing system can reduce the system volume and cause hypercarbia. Sources of subatmospheric pressure include vacuum hoses on scavenger interfaces, nasogastric tubes that have been placed in the trachea and suctioned, and sampling catheters from side-stream gas analysis systems (see Chapter 8 , Gas Monitoring).

The anesthesia ventilator can cause hypercarbia if the settings are such that inadequate alveolar ventilation is provided. Either the rate may be too low or the tidal volume may be too small. On ventilators that allow the user to set the tidal volume, ventilatory rate, inspiratory-to-expiratory ratio, and flow of drive gas independently of each other, smaller tidal volumes than desired may be delivered. The reason for this is that the ventilator may cycle to exhalation before the bellows is emptied completely and therefore fails to deliver the preset volume to the patient. Factors that can cause this problem are high respiratory rate, short inspiratory time, (low inspiratory/expiratory [I:E} ratio), low rate of inflow of the ventilator driving gas, and decreased pulmonary compliance. This problem can be discovered by careful observation of the ventilator bellows and monitoring of the exhaled tidal volume.

Poor pulmonary compliance also causes volume to be lost because some of the tidal volume expands the compliant breathing circuit tubing. This volume is included in the tidal volume measured by a breathing system expiratory limb spirometer during exhalation, but it does not contribute to the alveolar ventilation. Pressure-preset ventilators may cycle to exhalation before an adequate tidal volume has been delivered. This also causes hypercarbia if it is unrecognized; therefore, it is important to monitor the tidal volume and minute ventilation at the patient’s airway and not to assume that the volume set on the ventilator will in fact be delivered to the patient.

Fresh gas flows continuously from the anesthesia machine into the breathing system, but gas can only leave the circuit during exhalation. With older models of anesthesia ventilators (e.g., Dräger AV-E, Ohmeda 7000), the fresh gas entering the circuit during the inspiratory phase of ventilation is added to the tidal volume delivered by the ventilator, so that the patient receives a larger tidal volume than was set on the ventilator. These ventilators are described as tidal volume and I:E ratio change-uncompensated (in contrast to the modern ventilator designs that use fresh gas decoupling [FGD] or computerized compensation to ensure that changes in FGF, respiratory rate, and/or I:E ratio do not change the tidal volume that the patient receives). During use of an uncompensated ventilator, if the patient is normocarbic and then the FGF is decreased, or if the I:E ratio is changed such that exhalation is prolonged, the result is a decreased tidal volume (less augmentation by the fresh gas inflow) and an increase in PaCO ₂ ^, ( Fig. 23.5A,B ).

The carbon dioxide absorber may cause hypercarbia by acting as a source of leaks from the circuit or by failing to absorb the carbon dioxide produced by the patient. The absorber may not be closed and sealed properly. Improperly applied gaskets and absorbent granules on the gaskets can prevent the absorber canister from being sealed. Failure to close the handle on the absorber canister can also cause a huge leak. Exhausted granules and channeling of gas through the absorber prevent the absorption of carbon dioxide. This causes rebreathing of exhaled CO ₂ and hypercarbia. Machines equipped with absorber bypass switches (now obsolete) can allow the exhaled carbon dioxide to be deliberately rebreathed. If this is not monitored carefully, the patient may become hypercarbic. Some contemporary workstations use detachable and disposable cartridges of CO ₂ absorbent. Removal of such a cartridge to facilitate an increased level of CO ₂ at the end of a case is effectively the same as activating the absorber bypass switch that was present on some older machines ( Fig. 23.6 ). One must remember to replace the absorber cartridge in the circuit once the CO ₂ is as desired and before starting the next case.

The color of the dye (e.g., ethyl violet) in the carbon dioxide absorbent indicates whether or not the absorbent has been exhausted. However, it has been noted that the ethyl violet indicator can be photodeactivated by fluorescent lights and can thereby give the false impression that the absorbent is fresh when it is in fact exhausted. Some anesthesia workstations are capable of controlled delivery of carbon dioxide ( Fig. 23.7 ). Unintentional or improper use of the carbon dioxide flow meter may cause hypercarbia. Hypercarbia has also been reported when an anesthesia machine N ₂ O hose with an N ₂ O quick-connect fitting was connected to a CO ₂ wall outlet in the OR. In this case, the manufacturer-specific (Ohmeda) quick-connects for CO ₂ and N ₂ O happened to be mirror-images of one another so that rotating the N ₂ O hose connection through 180 degrees allowed it to be inserted into the CO ₂ wall outlet ( Fig. 23.8 ).

An increase in apparatus dead space, can result in hypercarbia. Specifically, the unidirectional inspiratory and expiratory valves may be absent or broken, or they may malfunction in the open position ( Fig. 23.9A,B ). Large-volume tubes (e.g., “goosenecks”) placed between the Y-piece of the breathing circle and the airway increase apparatus (mechanical) dead space and may cause hypercarbia if compensatory ventilatory maneuvers (i.e., larger tidal volumes) are not employed ( Fig. 23.10 ).

The components of the circle breathing circuit must be arranged in such a way as to prevent rebreathing. Three arrangements of the circle system must be avoided :

• 1.

  The fresh-gas inlet must not be placed between the patient and the expiratory unidirectional valve.

• 2.

  The adjustable pressure limit (pop-off) valve must not be placed between the patient and the inspiratory unidirectional valve.

• 3.

  The reservoir bag must not be between the patient and the inspiratory or expiratory unidirectional valves.

The Mapleson systems will permit the rebreathing of carbon dioxide and allow hypercarbia to develop if appropriate precautions are not taken. For example, the Mapleson A circuit (the Magill attachment) should be used only with spontaneously breathing patients, and the FGF must be at least 0.7 times the minute ventilation. In addition, the hose between the patient and the reservoir bag must be long enough so that exhaled carbon dioxide does not reach the reservoir bag ( Fig. 23.11 ).

The Mapleson B and C systems always permit the rebreathing of exhaled carbon dioxide because exhaled gas is directed into a blind pouch. In order to prevent hypercarbia with these systems, FGFs of 1.5 to 2.5 times normal minute ventilation must be used and the patient must be hyperventilated (see Fig. 23.11 ). If the patient is breathing spontaneously, the metabolic work performed increases. However, controlled ventilation with these circuits does not create this problem because the work of breathing is not being performed by the patient.

The T-piece systems, Mapleson D, E, and F, function similarly ( Fig. 23.12 ). FGFs of 2.5 to 3 times minute ventilation prevent rebreathing at calculated normal minute ventilation. Alternatively, reduced FGFs can be employed, but hypercarbia is prevented by hyperventilation. If the fresh gas connection is disrupted, hypercarbia will occur. This may be an especially difficult problem with the Bain circuit (coaxial Mapleson D) because a disconnected or kinked inner hose may go unnoticed ( Fig. 23.13 ) (see also Chapter 4 ). ^,

Hypocarbia

When carbon dioxide elimination exceeds production, PaCO ₂ decreases. When equilibrium between the two processes is achieved, a new steady state develops and PaCO ₂ stabilizes. General anesthesia, neuromuscular blocking agents, and hypothermia reduce metabolic rate. If minute ventilation is not decreased, the patient becomes hypocarbic. In addition, hyperventilation in general causes hypocarbia.

Hyperventilation and the resulting hypocarbia can be caused by simply having either the tidal volume, ventilatory rate, or both set too high. The FGF, which contributes to the minute ventilation in uncompensated ventilators (see earlier), will cause the patient to become hypocarbic if its contribution is not taken into account when the ventilator settings are made. In patients with very compliant lungs, the contribution of the FGF may be very significant. The driving gas from the ventilator may increase ventilation if there is a hole in the bellows. This is more significant with the Dräger AV-E ventilator because drive gas (mixture of oxygen and entrained room air) flows into the bellows housing throughout the inspiratory time so that the gas volume added to the patient circuit exceeds the desired tidal volume, resulting in hyperventilation and hypocarbia. In contrast, with the GE-Datex ventilators, the drive gas (oxygen) volume is essentially the same as the set tidal volume so there should be no increase in the patient’s tidal volume. Drive gas that enters the breathing circuit via a hole in the bellows affects the composition of the circuit gases. The result is that the patient’s lungs will be ventilated with an unintended gas mixture with dilution of any inhaled anesthetic.

Occlusions

The anesthesia circle breathing system is composed of a number of tubes that may become occluded. In general, such occlusions can be found outside the tube, within the wall, or within the lumen of the tube. Tubing misconnections have become less common since the introduction of standard diameters; however, if adapters are used, misconnections are still possible. By standard, breathing circuit tubing connections are 22 mm in diameter (ISO 5367:2014 Anesthetic and respiratory equipment—Breathing sets and connector), waste gas scavenging tubing is 19 mm or 30 mm in diameter, and the common gas outlet and tracheal tube connectors are 15 mm in diameter. Accessories added to the circuit may cause an obstruction to the gas pathway. Filters placed in the circuit, incorrectly connected humidifiers, manufacturing defects in tubing, and failure to completely remove plastic wrapping from breathing system components before connecting them into the circuit have all been reported as causes of total occlusion of the breathing circuit ( Fig. 23.14 ). A freestanding positive end-expiratory pressure (PEEP) valve may cause obstruction if it is incorrectly placed in the inspiratory limb of a circle system. The PEEP valves that use a weighted ball are designed to be mounted vertically on the expiratory side of a circle system. In one case, the weighted-ball PEEP valve was erroneously placed horizontally and reversed in the expiratory limb between the circuit and the exhalation unidirectional valve. When the oxygen flush was operated, the metal ball was driven downstream, totally obstructing the PEEP valve and circuit, preventing exhalation, and causing increased intrathoracic pressure. In another case, the PEEP valve was placed on the inhalation side in reversed fashion; this caused total obstruction on the inhalation side of the circuit and prevented inspiration ( Fig. 23.15 ). Because of such potentially fatal errors, a freestanding PEEP valve must be used with great caution, if it must be used at all. Freestanding bidirectional PEEP valves are safer because incorrect placement will not cause total breathing circuit obstruction.

Use error following performance of the low pressure system leak check in some GE workstations has resulted in complete obstruction of the breathing circuit. In the original low-pressure system leak check, the user disconnects the breathing circuit from the inspiratory connector on the machine, occludes the inspiratory connector port using a manufacturer-provided red rubber plug and then initiates the automated low-pressure system leak check. Once the leak check has been completed, the user must remember to remove the red rubber plug before reconnecting the breathing circuit. Due to the design of the red rubber plug, it was possible to connect the breathing circuit to the plug, thereby completely obstructing the circuit. Because of this possibility, the manufacturer redesigned the plug, which is now green and cannot be connected to the breathing circuit ( Fig. 23.16A–D ).

Although total occlusion of the breathing circuit should activate a pressure or volume alarm in most cases, depending on the system used, these alarms may be fooled when the tracheal tube is totally occluded. Consider a breathing circuit with a pressure-monitoring system incorporating a fixed setting of +65 cm H ₂ O for the high-pressure alarm limit threshold, such as that used on some older model anesthesia machines. When the tracheal tube becomes totally obstructed (due to a kink or total intraluminal obstruction), the pressure rises in the circuit, which satisfies the low-pressure alarm. However, unless the pressure reaches +65 cm H ₂ O, the high-pressure alarm is not activated. The peak pressure achieved in the circuit during inspiration depends on the inspiratory flow control setting (which determines the driving pressure available to compress the bellows), the preset tidal volume, the inspiratory time, and the fresh-gas inflow rate from the anesthesia machine. At low ventilator inspiratory flow settings, the driving pressure of the ventilator may be 50 cm H ₂ O or less, which, when combined with normal rates of fresh-gas inflow from the machine, may result in failure of the peak inspiratory pressure to reach the high-pressure alarm threshold of +65 cm H ₂ O. During exhalation, excess gas is released normally from the patient circuit. The volume alarm may also be fooled in this situation, depending on its low-limit threshold setting. In the system described, the low-volume alarm threshold was fixed at 80 mL. The situation described earlier involved total failure to ventilate the patient and an adverse outcome. In contemporary practice, this should be immediately detectable by continuous capnometry, by pressure, and by volume alarms whose thresholds can be set close to the normal values for that particular patient. An appropriately set high pressure limit on the ventilator should prevent an adverse outcome in such a situation.

Misconnections and obstructions should be preventable and detected by testing of the breathing circuit before use with all accessories in place and in spontaneous, assisted, and controlled ventilation modes. These procedures are described in the recommended pre-use checkout. Occasionally, however, an obstruction can develop because of failure of a component during the case, so that there is no substitute for monitoring and vigilance. High pressure in the breathing system may also be due to obstruction in the gas pathway from the circuit to the waste gas scavenging system. ^, The automated pre-use checkouts used in contemporary workstations vary in their ability to detect complete obstruction of the breathing circuit.

Inadequate Amount of Gas in the Breathing System

An insufficient volume of gas in the breathing system may be caused by inadequate delivery or excessive loss. Inadequate delivery may be due to failure of gas delivery to the machine or from the common gas outlet. A decrease in oxygen supply pressure to the machine may cause a decrease in gas flows set at the flowmeters. Flow setting errors may occur. A disconnection, misconnection, or obstruction between the machine’s common gas outlet and the patient circuit has a similar effect.

Inadequate volume of gas in the circuit may also be caused by excessive removal. An active waste gas scavenging system utilizes wall suction to remove the waste gases from the scavenging interface. Excess negative pressure may be applied to the circuit if the negative-pressure relief (“pop-in”) valve or valves on the interface should become occluded. A similar situation can arise with an open-reservoir scavenging system if the relief ports become occluded while suction is applied to the interface. A high subatmospheric pressure in the scavenging system may open the circuit adjustable pressure limiting (APL) valve, transmitting the subatmospheric pressure to the patient circuit. If a ventilator were being used, unrelieved excess negative pressure in the scavenging system would in most cases tend to hold the ventilator pressure-relief valve to its seat, preventing its opening on exhalation and causing a high pressure to develop in the circuit ( Fig. 23.17 ).

A side-stream sampling (diverting) gas analyzer connected to the patient circuit has been reported as the cause of excessive negative pressure in a breathing circuit where the FGF of 50 mL/min during cardiopulmonary bypass was less than the analyzer’s gas sampling rate of 250 mL/min. The gas sampling rates of commonly used side-stream sampling gas analyzers vary between about 50 mL/min to 300 mL/min. While there is potential for creating negative pressure in the circuit if low FGF rates are being used, contemporary analyzers are designed to annunciate an alarm and limit the negative pressure that can be generated.

Excess gas removal by a sampling device during spontaneous ventilation creates a subatmospheric pressure in the circuit that in turn causes the APL valve to close. This prevents the scavenging system negative-pressure relief valve or valves from relieving the negative pressure in the circuit. In one study (albeit from 1987), the maximum circuit subatmospheric pressure achieved by side-stream sampling devices during testing ranged from −1 to −148 mm Hg. Such low pressures, if transmitted to the patient’s airway, have the potential to cause negative pressure barotrauma and cardiovascular dysfunction.

Excessive volume loss resulting in negative pressures in the breathing system may arise if hospital suction is applied through the working channel of a fiberoptic bronchoscope that has been inserted into the circuit through an airway diaphragm adapter or by a suction catheter that has been accidentally advanced alongside the tracheal tube into the trachea.

Inadequate circuit volume and negative pressure may occur during spontaneous ventilation in the presence of a low FGF rate and inadequate size of reservoir bag (such as a pediatric size bag used with an adult patient). During inspiration, the reservoir bag will collapse, and a negative pressure will be created in the circuit. Circuit APL valves usually have a minimum opening pressure that is slightly greater than that needed to distend the reservoir bag. If the bag were of correct size but noncompliant, or if the APL valve had a low opening pressure, during exhalation, most of the gas would exit through the APL valve rather than fill the bag. The net result would be an inadequate reservoir volume for the next inspiration. Modern circuit pressure monitors incorporate a sub-atmospheric-pressure alarm such that when pressure is less than -10 cm H ₂ O at any time, audible and visual alarms are triggered.

Failure to Initiate Artificial Ventilation

Failure to initiate artificial ventilation is usually attributable to an operator error. The error may be failure to turn on the ventilator (e.g., after tracheal intubation, separation from cardiopulmonary bypass, or during median sternotomy), unintentionally setting a respiratory rate of zero breaths per minute, failure to select the “automatic” (ventilator) setting at the “manual/automatic” selector switch in the circuit, or failure to connect the ventilator circuit hose (either at the patient circuit connector by the selector switch or at the bag mount). Because some older circuit volume and pressure alarms must be deliberately enabled or are enabled only when the ventilator is on, these monitors will fail to detect that the ventilator has not been turned on. In this respect, continuous capnography provides the most sensitive monitor of ventilation. If the delivery system incorporates a standing-bellows ventilator, failure to connect the ventilator tubing to the circuit will cause the bellows to collapse.

With either standing or hanging bellows design, when a ventilator is turned on but the “manual” (bag) mode is selected at the selector switch, then during inspiration, the bellows will attempt to empty against a total obstruction (the closed selector switch) and its failure to empty will be readily observed. Failure to ventilate in this situation is annunciated by both low-pressure and volume alarms in the breathing system. Some older designs of the circle system lack a “manual/automatic” selector switch, and the APL valve must be closed to effect intermittent positive-pressure ventilation (IPPV) when the ventilator hose is connected to the bag mount. In such a case, failure to close the APL valve is yet another cause of failure to initiate IPPV.

Even if the breathing system incorporates a selector switch, there are occasions when the primary anesthesia ventilator fails and a free-standing ventilator may be brought in to provide IPPV. The foregoing considerations then apply if the new ventilator is connected to the circuit via the bag mount connection; that is, the “manual” mode is selected, and the APL valve is closed.

Some contemporary anesthesia workstations provide a single (pause gas flow) control whereby both ventilation and FGF can be temporarily suspended for up to 1 minute after which ventilation and flows are automatically resumed ( Fig. 5.19 ). With this feature, it is not necessary to separately switch off the ventilator when brief interruptions of ventilation are required, for example, when taking an X-ray. In some cases in which the user planned to stop ventilation for a brief period and then forgot to switch the ventilator back on, a bad outcome was prevented because (fortuitously) 100% oxygen was continuously flowing into an intact breathing circuit. In this situation, apneic oxygenation prevents hypoxemia although profound hypercarbia will develop. ^,

Leaks and Disconnections in the Breathing System

Breathing circuit disconnections and leaks are among the most common causes of anesthesia mishaps. ^, Anesthesia breathing systems contain numerous basic connections, and as more monitors, humidifiers, filters, gas flow, and gas sampling adapters are added, additional connections are needed. Each connection is a potential disconnection. Disconnections cannot be totally prevented; although in the past, some have considered the 15-mm connector between the tracheal tube and the circuit a safety “fuse” to prevent unintentional extubation, most now prefer a secure system that does not disconnect. Circuit disconnections and their detection have been the subject of several reviews. Cooper et al. found that disconnections of the patient from the machine were responsible for 7.5% of CIs involving human error or equipment failure. Of these disconnections, about 70% occur at the Y piece. ^,

The risks of disconnection are reduced by secure locking of connecting components, use of disconnect (pressure, volume, capnography) alarms, and, most importantly, user education. Making secure friction connections, such as those between the tracheal tube and elbow adapter or between the adapter and the Y piece, requires that the user employ a pushing and twisting motion rather than merely pushing the two units together. When a disconnection occurs, the anesthesia caregiver must systematically trace the flow of gases through the breathing system, looking for the disconnection in the same way as would be done in the event of a no-gas-flow or obstruction situation.

Most disconnections are detectable by the basic breathing system monitors of pressure, volume, and flow. Pressure monitors annunciate an alarm if the peak inspiratory pressure in the circuit fails to reach the threshold low setting. The alarm setting on the monitor should be user adjustable; the user should be able to set it to a level just below the usual peak inspiratory pressure. Most monitors now provide a continuous graphic display of the circuit pressure as well as of the alarm threshold or thresholds. A response algorithm for the low-pressure alarm condition has been proposed ( Fig. 23.18 ).

The breathing circuit low-pressure alarm can be “fooled” if it is not set at the correct sensitivity. Thus, a circuit disconnection at the Y piece combined with sufficient resistance at the patient connector end may not trigger the low-pressure alarm if inspiratory gas flow from the ventilator is high enough so that the low-pressure alarm threshold is crossed. Examples include unintended extubation of a patient who has a small-diameter tracheal tube where the tube connector offers a high resistance to gas flow, or occlusion of the open patient connector by the drapes. A circuit low-pressure alarm sensing pressure in the absorber may be fooled if there is a high resistance between the inspiratory tubing connector and the Y piece, such as may be attributable to a cascade humidifier in the inspiratory limb of the circle. Humidifiers may also represent the source of a detectable leak in the anesthesia circuit.

A breathing circuit low-pressure alarm is less likely to be fooled when a standing-bellows ventilator is being used, because failure of the bellows to fill adequately during exhalation will lead to lower peak pressures on the next inspiration ( Fig. 23.19 ). With the hanging-bellows design, the peak inspiratory pressure with a disconnect tends to be higher than with a standing-bellows ventilator disconnect, the hanging bellows having filled completely during exhalation. A pressure alarm set to an inappropriately low threshold is therefore more likely to be fooled by a hanging-bellows ventilator.

The common gas outlet of the anesthesia machine was a site of disconnections before the standard use of retaining devices. The diameter of the tubing connecting the common gas outlet with the circuit is relatively narrow and offers relatively high resistance to gas flow compared with the 22-mm-diameter circuit tubing. If a hanging-bellows ventilator were being used with a large tidal volume setting, the machine-to-circuit connector-tubing resistance may be such that during inspiration, the low-pressure alarm limit would be satisfied despite the leak. During exhalation, room air would be entrained via the fresh-gas inflow tubing to refill the bellows (see Fig. 23.17 ). A disconnection of this tubing may also lead to a hypoxic gas mixture in the circuit as air is entrained and oxygen is consumed. Detection of this type of disconnection, which is associated with air entrainment, is aided by an oxygen analyzer with an appropriately set concentration alarm threshold located in the patient circuit.

If, as is recommended, the circuit low-pressure alarm has been set to just below the peak inspiratory pressure, it should be recognized that more false-positive alarms will be generated. Thus, when a tidal volume uncompensated ventilator is used with a set tidal volume, a decrease in FGF, I:E ratio, or inspiratory flow rate or a change in respiratory rate may decrease the peak inspiratory pressure, thereby triggering the alarm. A decrease in rate may decrease peak pressure in a volume ventilator (increased inspiratory time and a lower inspiratory flow rate needed). An increase in rate may also decrease peak pressure due to less uncompensated FGF. However, a false-positive alarm, with an appropriate response, is preferable to failing to detect a potentially hazardous situation, provided that the user does not permanently silence the alarm.

Leaks from the breathing circuit, other than those attributable to component disconnection, may also result in inadequate exchange of gas between system and patient. Leaks may arise in any component because of cracking, incorrect assembly, or malfunction of a system component, particularly the ventilator pressure-relief valve. Sometimes, the design of a component may make a leak more likely. In the Dräger Fabius and Apollo workstations, lifting the APL valve allows gas to leave the circuit. There are several reports of tubing becoming trapped under the APL valve, resulting in a leak and failure to ventilate. As a result, the APL valve was redesigned, and replaced on machines with the older design by field service technicians ( Fig. 23.20 ).

During inspiration, the ventilator pressure-relief valve is normally held closed by the pressure of the driving gas from the bellows housing. If this valve is not held closed during inspiration, then gas in the patient circuit may be vented to the scavenging system rather than going to the patient. Incompetence of the ventilator pressure-relief valve has been reported in connection with pilot-line disconnection or occlusion and valve damage. In such a situation, the loss of volume from the circuit would be detected by appropriately set pressure and volume alarms, but the source of the leak might be less obvious. If a closed-reservoir scavenging system is in use, the diagnosis is made by observation of the scavenging system reservoir bag. The bag normally fills during exhalation, as gas is released from the patient circuit, and empties during inspiration, when the ventilator pressure-relief valve is closed. If the ventilator pressure-relief valve is incompetent, the scavenging system reservoir bag will be seen to fill during the inspiration, as the ventilator bellows empties its contained gas into the scavenging system.

Leaks and malfunctions in the patient circuit are sometimes first detected by an airway respiratory gas monitor when the composition of the gas mixture in the breathing system differs significantly from that expected. Application of negative pressure to the circuit by a malfunctioning scavenging system, or intermittently by a hanging-bellows or piston ventilator during exhalation, may cause entrainment of air into the breathing system through a small leak otherwise unrecognized by pressure, volume, or even carbon dioxide monitoring. ^, A leak of room air or other gases into the patient circuit may result in dilution of the anesthesia gas mixture and, potentially in an extreme case, awareness under anesthesia. Leaks into the patient circuit may occur if there is a hole in the ventilator bellows. In this case, the high pressure in the driving gas circuit forces driving gas into the patient circuit during inspiration. With a GE-Datex-Ohmeda ventilator, the diluting (driving) gas is normally 100% oxygen, but with a Dräger AV-E, it is a mixture of air and oxygen. ^, Such an event might be detected by a change in FiO ₂ , peak inspiratory pressure, tidal or minute volume, end-tidal carbon dioxide, or a multigas or agent analyzer. Contemporary Dräger Fabius and Apollo workstations use a piston ventilator driven by an electric motor. During exhalation, the piston retracts to refill the chamber. If there is inadequate FGF and the reservoir bag is empty, a negative pressure relief valve opens to allow air to be drawn into the piston chamber. This occurs in order to prevent negative pressure being applied to the patient’s lungs. The entrainment of air into the breathing circuit and unintended composition of the patient’s inspired gas mixture would be detected by a respiratory gas analyzer and/or oxygen analyzer.

A leak in the breathing system may be the result of use error. Once the breathing circuit checkout has been completed, it is inadvisable to make any changes unless they are absolutely necessary. Replacing a snap-in disposable absorber cartridge during a general anesthetic is usually safe, but opening an absorber canister during a case (to replace absorbent cartridges) and not being able to close it creates a huge leak in the circle breathing system. A case report describes such an event in a patient whose airway was not easily accessible for connection to an Ambu bag. ^, In this case, the expiratory limb of the circuit was detached from the anesthesia machine and connected to an Ambu bag supplied with oxygen. Ventilation via the expiratory limb of the circuit was maintained for 6 minutes while the absorber canister leak was corrected. The patient suffered no adverse outcome. The efficacy of alveolar ventilation was not reported but the PaCO ₂ and end-tidal carbon dioxide undoubtedly were high considering that the volume of the expiratory limb of the circuit now constituted additional apparatus dead space. A schematic of the expiratory limb ventilation technique is depicted in Fig. 23.21A,B .

High Pressure in the Breathing System

The anesthesia machine provides a continuous flow of gas to the patient circuit. Whenever circuit gas inflow rate exceeds outflow rate, excessive pressures can develop. If these pressures are transmitted to the patient’s lungs, severe cardiovascular compromise, barotrauma, and even pneumothorax may arise. ^,

During spontaneous ventilation, high pressure may be caused by inadequate opening (or even complete closure) of the APL valve, kinking or occlusion of the tubing between the APL valve and the scavenging interface, or malfunction of the interface positive-pressure relief valve. During spontaneous ventilation, the bag will distend to accommodate the excess gas. Reservoir bags are highly distensible and limit the maximum circuit pressure to approximately 45 cm H ₂ O. Nevertheless, such an airway pressure could produce hypotension by inhibiting venous return. Increases in circuit pressure will be more rapid when the fresh-gas inflow rate is high, for example, during prolonged use of the oxygen flush.

Excessive pressure in the circuit may occur during use of an anesthesia ventilator. During inspiration, the ventilator pressure-relief valve is normally held closed (see Fig. 23.19 ). Thus, a high inspiratory gas-flow rate will be associated with increased peak pressures in the circuit. In order to protect the patient’s lungs from excessive pressure, all contemporary ventilators incorporate a high pressure alarm and high pressure limit.

There are many reports of ventilator malfunctions causing excessive circuit pressures. Failure of the ventilator to cycle from inspiration to exhalation results in driving gas continuing to enter (Dräger) or enter but not leave (GE-Datex-Ohmeda) the bellows housing. This causes the ventilator pressure-relief valve to remain closed and excess pressure to build up within the circuit. The pressure increase is limited by the driving gas pressure prevailing in the bellows housing. In Dräger AV-E ventilators, this pressure depends on the setting of the inspiratory flow-control knob. Other reported causes of the ventilator pressure-relief valve failing to open normally include mechanical obstruction of the driving gas exhaust system (e.g., blocked Dräger AV-E muffler), kinking of a Dräger AV-E ventilator pressure-relief valve pilot line during inspiration, failure of a solenoid valve causing persistent inhalation, and diffusion of nitrous oxide into the space between the two pieces of rubber constituting the relief valve diaphragm, causing insidious PEEP. Even with normal ventilator bellows function, high pressures in the circuit may be caused by occlusion of the tubing between the ventilator pressure-relief valve outlet and the scavenging system or by obstruction of the scavenging interface positive pressure relief (“pop-off”) valve. In such cases, as the pressure in the patient circuit rises, the ventilator bellows empties less completely and may even become distorted.

High pressures arising in the circuit are detected by the circuit pressure monitor, which incorporates two types of alarms, a continuing-pressure alarm and a high-pressure alarm. A continuing-pressure alarm is annunciated usually when the circuit pressure remains in excess of +15 cm H ₂ O for more than 10 seconds. A high-pressure alarm is annunciated immediately when the circuit pressure exceeds the high pressure threshold limit, which, in contemporary monitors, is set by the user but may have a default setting of +40 cm H ₂ O, depending on the pressure monitor. When either of these alarms is annunciated during mechanical ventilation, a problem should be suspected with the ventilator circuit. In the absence of a high-pressure limit feature, circuit pressure can be immediately relieved by disconnection of the patient from the circuit at the Y-piece, inspiratory hose, expiratory hose, or by selecting the manual (bag) mode and relieving pressure by opening the APL valve. The incorporation of safety relief valves into the circuit as a protection against high pressures is now the norm, and the opening threshold is usually set to about 5 cm H ₂ O above peak inspiratory pressure. The pressure limit must be set according to the patient, because too low a setting may preclude the ability to ventilate a patient with poor total thoracic compliance.

Pressure limiting devices differ among some of the older model ventilators. The Ohmeda 7800 series ventilators incorporate an inspiratory high-pressure limit such that when the selected threshold (pressure measured in patient circuit downstream of the inspiratory unidirectional valve) is exceeded, the ventilator cycles to exhalation, driving gas circuit pressure falls to zero, and excess patient circuit gas is discharged to the scavenging system via the ventilator pressure-relief valve. The basic model Dräger AV-E ventilators were not pressure-limited, but a pressure-limit control was available and could be retrofitted to certain standing-bellows design AV-E units. The Dräger AV-E pressure-limit control device senses the pressure in the patient circuit at the bellows, and whenever the threshold high-pressure limit is exceeded, a valve opens in the driving gas circuit (bellows housing) to release excess driving gas to the atmosphere. This limits the driving gas pressure such that patient circuit pressure does not exceed the set limit for the remainder of the inspiration. The time cycling (I:E ratio and set ventilatory rate) of the Dräger AV-E is therefore maintained, in contrast to the GE-Datex-Ohmeda 7000/7800 series ventilators. Both the Dräger AV-E and the GE-Datex-Ohmeda approaches to limiting pressure in the patient circuit require a normally functioning ventilator pressure-relief valve, because it is through this valve (the opening pressure of which is controlled by the pressure in the driving gas circuit) that excess gas and pressure are relieved from the patient circuit. If the pressure-relief valve or its outflow path should become obstructed, neither the Dräger AV-E nor the GE-Datex-Ohmeda pressure-limiting mechanisms would be effective in relieving pressure in the patient circuit.

Contemporary computer-controlled anesthesia workstations overcome many of the limitations of older models through use of pressure sensors and electronically controlled solenoid valves to limit excesses of pressure in the breathing system.

Liquid Agent in the Fresh-Gas Piping

Lethal anesthetic agent overdosage may occur when excessive amounts of saturated vapor or even liquid agent enter the bypass portion of the vaporizer, the machine piping between the vaporizer and the common gas outlet, the interface hose, or the breathing circuit. The overdosage situation was more likely when measured-flow type vaporizers (Copper Kettle or Verni-Trol) were used, because calculation or flow setting errors could easily arise. In addition, some older designs of vaporizers could be overfilled such that excess liquid could enter the fresh gas piping. Measured flow systems are no longer in use and are considered obsolete. Modern vaporizers are concentration-calibrated and designed to prevent overfilling (see also Chapter 3 ).

Tilting or tipping of a vaporizer may cause liquid agent to enter the bypass of the vaporizer or the machine piping. One milliliter of liquid potent volatile agent produces approximately 200 mL of vapor at 20°C (see Chapter 3 ). For example, if 1 mL of liquid isoflurane were to enter the common gas piping, it would require approximately 20 L of fresh gas to dilute the resulting vapor to a concentration of 1%, or a minimum alveolar concentration (MAC) of approximately 0.8. It is easy to appreciate how a relatively small volume of liquid agent in the wrong place could have a profound effect on a patient.

If a vaporizer has been tilted or tipped and there is concern that liquid agent may have leaked into the piping of the machine, then with no patient connected to the system, the vaporizer should be drained and then flushed with a high flow rate of oxygen from the anesthesia machine flowmeter ( not the oxygen flush, which bypasses the vaporizer); the vaporizer dial should be set to a high concentration during this procedure. ^, If any doubt still exists as to the safe function of the vaporizer, it must be withdrawn from clinical service until certified safe for use by an authorized service representative. Additional caution was needed with a halothane vaporizer that had been tipped. Liquid halothane contains thymol, a sticky preservative that does not evaporate. Thymol entering the flow-control and temperature-compensating parts of a variable bypass vaporizer could cause vaporizer malfunction even after the halothane has been flushed out of these parts.

Modern removable vaporizers are mounted on the back bar of the anesthesia machine. Contemporary anesthesia vaporizers (e.g., Dräger Vapor 2000 series; GE-Datex Tec 7 series) have antispill designs. In the case of the Dräger Vapor 2000 and 3000 series vaporizers, the dial must be set to the T (transport) position in order to remove the vaporizer from the anesthesia workstation. In this position, the vaporizer sump is isolated from the other parts. The GE-Aladin cartridges also have antispill mechanisms and can be safely tilted when removed from the workstation. The Aladin cartridges must be withdrawn from the slot in the workstation during filling, to prevent overflow of liquid agent into the workstation, which can occur despite the workstation’s overflow mechanism. In the event liquid agent overwhelms the overflow mechanisms and enters the workstation parts of the vaporizer, the vaporizer shuts down and no agent is delivered.

Overfilling of a vaporizer, which led to a halothane overdose and neurological impairment in a 3-year-old boy, has been reported in connection with incorrect use of an agent-specific key fill device. ^, When used correctly, the keyed bottle adaptor must be screwed tightly onto the bottle to ensure a gas-tight joint. The other end of the adapter is inserted into the vaporizer fill port and tightened. Then with the concentration dial turned to OFF, the filler control is opened, the bottle raised, and liquid agent flows into the vaporizer, displacing air from the vaporizer to flow back into the bottle via the air return tube. Overfilling is prevented because the intake of air into the bottle stops when filling has reached the maximum safe level in the vaporizer sump, and because the vaporizer dial is in the OFF position, the air space at the top of the vaporizer sump is sealed. Slow filling of the vaporizer by the correct method described has resulted in individuals speeding up the process by loosening the seal between the agent bottle and key fill adapter, and turning the vaporizer ON. This erroneous filling practice has led to the overfilling of vaporizers, with adverse outcome. ^,