The Anesthesia Machine and Workstation

The Anesthesia Machine as a Component of an Anesthesia Workstation

The modern anesthesia gas delivery system is composed of the anesthesia machine (see the following section), anesthesia vaporizer(s) (see Chapter 3 ), breathing system (see Chapter 4 ), ventilator (see Chapter 6 ), and waste gas scavenging system (see Chapter 5 ). The basic arrangement of these elements is the same in all contemporary anesthesia gas delivery systems. While it may not be intuitive from looking at an anesthesia gas delivery system, Fig. 2.1 shows that the breathing system is the functional center of the anesthesia gas delivery system, because it connects to all the other components, as well as to the patient.

The patient breathes in from and out to the breathing system. If ventilation is spontaneous or manually assisted, gas passes back and forth through the breathing system to and from the reservoir bag. If the lungs are mechanically ventilated, the ventilator bellows or piston acts as a counterlung, exchanging its volume with the patient’s lungs via the breathing system. The patient consumes oxygen and takes up anesthetic agents, which would be depleted from the breathing system if no fresh gas were added. Adding fresh gas is the function of the anesthesia machine. The anesthesia machine receives gases (oxygen and sometimes air, nitrous oxide, heliox or carbon dioxide) under pressure from their sources of storage (see Chapter 1 ), safely creates a gas mixture of known composition and flow rate, and delivers it to a concentration-calibrated vaporizer, which adds a controlled concentration of potent, inhaled, volatile anesthetic agent. The resulting gas mixture of oxygen with other compressed gases and vaporized anesthetics is delivered to the machine’s common gas outlet (CGO). This fresh gas mixture flows continuously from the CGO into the patient breathing system, most commonly a circle breathing system. Typically, more fresh gas is added than is consumed by the patient, and this excess gas must be able to leave the breathing system to prevent a progressive increase in circuit (and consequently airway) pressure. Excess gas leaves the breathing system via the adjustable pressure-limiting (APL) valve during spontaneous or manual ventilation, or via the ventilator pressure relief valve during mechanical ventilation. The waste gas that exits the breathing system enters the scavenging system, which is the safety interface to a facility system that discharges the gas to the outside. An understanding of the structure and function of the anesthesia gas delivery system is essential to the safe practice of anesthesia.

Anesthesia Machine and Workstation

Gas delivery systems continue to evolve as advances in technology and safety are incorporated into current designs. The recent evolution can be traced through the voluntary consensus standards that have been developed with input from manufacturers, users, and other interested agencies. The current applicable international standard is ISO/IEC 80601-2-13: Particular requirement for basic safety and essential performance of an anaesthetic workstation , which was first published in 2011, and confirmed to be up-to-date in 2017. It superseded the American Society for Testing and Materials (ASTM) standard F1850-00, first published in 2000, which introduced the term “workstation” in distinction to “anesthesia (or gas) machine.” The anesthesia workstation is defined as a system for the administration of anesthesia to patients consisting of the gas delivery system, breathing system, anesthetic gas scavenging system, anesthetic vapor delivery system, anesthesia ventilator, and associated monitoring and protection devices. This standard superseded anesthesia machine standard ASTM F1161-88, first published in 1989. The original anesthesia machine standard was American National Standards Institute (ANSI) Z79.8, first published in 1979. All of these standards were cowritten by anesthesia practitioners and were voluntarily adopted by anesthesia machine manufacturers of the day. While voluntary consensus standards are not mandated, it is highly unlikely that a manufacturer would build, or that the Food and Drug Administration (FDA) would allow to be marketed, a workstation that did not comply with the current or most recent voluntary consensus standards.

The evolution of the anesthesia workstation and advances in technology have led to many changes in design. Although basic operations remain the same, the components are more technologically advanced. For example, in most new models the glass tube flowmeters (rotameters) are replaced by digital flow indicators or virtual flowmeters displayed on an electronic information screen. The gas flow–control needle valves may be replaced by electronic flow controllers. Digital pressure gauges have replaced many mechanical compressed gas gauges. Most modern workstations include hardware and software to automate much of the pre-use check. This chapter describes the basic components and functions of a traditional anesthesia machine (i.e., the gas delivery device described in ASTM standard F1850-00) referring to outdated products at times so the reader can appreciate some of the changes that have been made in the most recent models.

Dräger Medical Inc. (Telford, PA) and GE Healthcare (Waukesha, WI) manufacture most of the anesthesia workstations sold in the United States. This chapter reviews the features of a basic anesthesia delivery system, referring to Dräger and GE outdated products when appropriate. The flow of compressed gases from the point of entry into the machine, through the various components, and to the exit at the CGO is described. The function of each component is discussed so that the effects of failure of that component, as well as the rationale for the various machine checkout procedures, can be appreciated. This approach provides a framework from which to diagnose problems that may arise with the machine. Of note, the individual workstation manufacturer’s operator and service manuals represent the most comprehensive reference for any specific model of machine, and the reader is strongly encouraged to review the relevant manuals. The manufacturers also produce excellent educational materials, and a number of simulations are also available on the Internet.

Overview of Gas Flow Through the Anesthesia Machine

The gas flow arrangements of a basic two-gas anesthesia machine are shown in Fig. 2.2 . The machine receives each of the two compressed gases, oxygen (O ₂ ) and nitrous oxide (N ₂ O), from two supply sources: a cylinder source and a pipeline source. The storage and supply of these gases to the operating room (OR) are described in Chapter 1 .

The basic functions of any anesthesia machine are to receive compressed gases from their supplies and to create a gas mixture of known composition and flow rate at the CGO. The relation between pressure and flow is stated in Ohm’s law:

Controlling the flow of gases from high-pressure sources through the machine to exit the CGO at pressures approximating atmospheric requires changes in pressure and/or resistance. Modern anesthesia machines also incorporate safety features designed to prevent the delivery of a hypoxic mixture to the breathing system. These features include the oxygen supply pressure failure alarm, pressure sensor shut-off (“fail-safe”) system, gas flow proportioning systems, and usability features designed to decrease use errors.

The anesthesia machine gas pathways have been conveniently divided by some authors into three systems :

• 1.

  a high-pressure system that includes parts upstream of the first-stage regulator, where compressed gas pressures are between 45 and 2200 pounds per square inch gauge pressure (psig),

• 2.

  an intermediate pressure system that includes parts downstream of the first-stage pressure regulator and upstream of the gas flow control valves, where pressures are between 16 and 55 psig,

• 3.

  a low-pressure system that includes all parts downstream of the gas flow control valves, where pressures are normally slightly greater than atmospheric pressure.

Other authors consider the high-pressure system to be simply all components upstream of the gas flow control valves and the low-pressure system to be all components downstream of the gas flow control valves. Indeed, this agrees with the system descriptions in the U.S. FDA 1993 pre-use checkout recommendations. Either way, most classification schemes agree on the definition of the low-pressure system.

Gas Inlets

The pipeline inlets into the anesthesia machine are gas specific by a national standard ( Fig. 2.3 ), known as the diameter index safety system (DISS), that ensures that a pipeline gas hose cannot be connected to the wrong anesthesia machine gas inlet. The DISS system specifies the internal bore dimensions of the anesthesia machine inlet into which only a compatible hose connector with matching outer dimensions can insert, and the oxygen connector has a different design than all of the others ( Fig. 2.4 ). There are specific DISS connectors for oxygen, air, nitrous oxide, helium, heliox, carbon dioxide, suction, and waste anesthetic gas suction, as well as nitrogen, nitric oxide, xenon, and many nonmedical gases. All anesthesia machine pipeline inlets incorporate a filter that prevents particles greater than 100 micrometers from entering, and a check valve that prevents leakage from the machine if the pipeline is not connected and compressed gas cylinders are in use ( Fig. 2.5 ). Failure of this valve would cause gas to leak from the machine if the pipeline hose were not connected. Upstream of the pipeline check valve is a juncture to a pressure gauge that measures the pipeline gas supply pressure (see Fig. 2.2 ). The upstream location, on the pipeline side of the check valve, means that the gauge reading falls to zero as soon as the pipeline hose is disconnected.

Oxygen can also be supplied to the pipeline inlet from special freestanding E-size cylinders that incorporate a regulator that delivers oxygen at 50 psig to a DISS connector. Thus, if the oxygen pipeline fails, the machine’s oxygen hose could be connected to this tank outlet using a compatible fitting ( Figs. 2.6 and 2.7 ). In remote locations, large H-cylinders with pressure regulators can supply compressed gas via the pipeline inlets ( Fig. 2.8 ).

Compressed gas can also be supplied to the machine from a back-up E-cylinder attached via a hanger yoke ( Fig. 2.9 ). The medical gas pin index safety system ensures that only an oxygen cylinder fits correctly into an oxygen hanger yoke (see Chapter 1 ).

Several considerations apply before a cylinder is hung in a yoke. First, the plastic wrapper that surrounds the cylinder valve must be removed, taking care to place the included plastic or rubber washer (Bodok seal) on the yoke inlet. Checking that the washer is in place, the cylinder is then hung in the yoke by aligning the gas outlet hole with the strainer nipple, and aligning the two yoke pins with the corresponding holes in the cylinder. Then the T-handle is tightened to press the cylinder stem outlet against the washer and the yoke inlet, creating a gas-tight pressure fitting. The cylinder should never be mounted 180 degrees in the wrong direction because a tightened T-handle screw might damage the tank safety relief device which can be confused with the cylinder gas outlet (see Chapter 1 ). Although changing a compressed gas cylinder on an anesthesia machine may seem straightforward, one study showed that a significant number of senior residents in a simulator could not perform this task satisfactorily, possibly because it is generally performed by technical staff.

The compressed gas enters the machine through a sintered metal strainer incorporated into yoke assembly ( Fig. 2.10 ) designed to prevent dirt or other particles greater than 100 microns from entering the machine. The oxygen then flows past a hanger yoke (“floating”) check valve to enter the anesthesia machine at high pressure.

Checking to see that a backup tank contains sufficient oxygen is an important part of the pre-use checkout and also ensures that a tank wrench is available for opening and closing the cylinder valve. However, different locations of the cylinder gauge or the presence of two oxygen hanger yokes can complicate this process. On some anesthesia machines with one oxygen hanger yoke the oxygen cylinder pressure gauge is located upstream of the check valve, on the cylinder side of the check valve, so that the gauge indicates the pressure in the cylinder. In this situation, the gauge reads zero when the cylinder is closed and immediately indicates the pressure in the cylinder when it is open. On other anesthesia machines, and all those with two oxygen hanger yokes ( Fig. 2.11 ), the oxygen cylinder pressure gauge is located downstream of the check valve, and indicates the pressure in the high-pressure system of the anesthesia machine. In this case, the gauge may read a higher-than-zero pressure when the tank is closed and stay at that pressure when the tank is open. If so, the only way to be sure of the pressure in the tank is to (1) close the oxygen cylinder(s), (2) disconnect the oxygen pipeline, (3) turn on oxygen flow to bleed off the high oxygen pressure line, and (4) open the oxygen cylinder. The gauge indicates the true pressure in the oxygen cylinder only if the gauge reading increases when the oxygen cylinder is opened. To check a second oxygen cylinder on a different yoke, close the first cylinder and begin from step 3. Then reconnect to the oxygen pipeline.

The check valve in each hanger yoke is designed to prevent gas from flowing out of the machine through an unoccupied yoke, but it is best practice to insert a yoke plug ( Fig. 2.12 ) into an unoccupied yoke to keep dust out of the inlet and as a backup measure. In the situation of two oxygen yokes (see Fig. 2.11 ), the check valves also prevent transfilling of one oxygen cylinder to the other when both are open, since without a check valve, oxygen would tend to flow from the full tank to the empty one. Without check valves, the transfilling and sudden compression of oxygen into the empty cylinder could cause a rapid temperature rise in the pipes, gauge, and tank with an associated risk of fire. This is known as an adiabatic change, in which the state of a gas is altered without the gas being permitted to exchange heat energy with its surroundings.

Gauges and Sensors

Pipeline and cylinder supply pressure gauges ( Fig. 2.13 ) in traditional machines are of the Bourdon tube design. In principle, the Bourdon tube is a coiled metal tube sealed at its inner end and open to the gas pressure at its outer end (see Chapter 9 ). As gas pressure increases, the coiled tube tends to straighten. A pointer attached to the inner-sealed end thereby moves across a scale calibrated in units of pressure. If the Bourdon tube were to burst, the inside of the gauge could be exposed to high pressure. The gauge is therefore constructed with a special heavy glass window and a mechanism designed to act as a pressure fuse so that gas is released from the back of the casing if the pressure were to suddenly increase. The cylinder and pipeline pressure gauges for the gases supplied to the machine are generally situated in a panel on the front of the anesthesia machine (see Fig. 2.13 ). They are designed to be easy to read, typically have colored and lettered labels to indicate the gas, and icons to indicate whether cylinder or pipeline pressures are being displayed (see Chapter 18 ). Newer workstations typically use electronic transducers to measure cylinder and pipeline pressures, with the readings displayed on an electronic information screen ( Fig. 2.14 ). Electronic pressure transducers measure the voltage through a Wheatstone bridge that is connected to a strain gauge, the resistance of which changes as it bends when force is applied ( Fig. 2.15 ). An advantage of electronic pressure transducers is that they can be monitored by the workstation’s alarm system. A disadvantage is that the cylinder and pipeline pressures are not always displayed in a constant location, and may even be hidden during some workstation operational modes.

Pressure Regulators

A pressure regulator is a device that converts a variable, high-input gas pressure to a constant, lower output pressure. Pressure regulators are used in a number of places within anesthesia machines. All anesthesia machines have cylinder pressure regulators, sometimes termed the first-stage regulators (see Fig. 2.2 ). These reduce the pressures of compressed gases coming from cylinders at variable pressures of up to 2200 psig, depending on the gas composition and volume within the cylinder, to a constant lower output pressure, typically 45 psig. As seen in Fig. 2.11 , on anesthesia machines fitted for two oxygen cylinders, oxygen from both yokes flows via a common pathway to a single first-stage regulator. The principles of action of a pressure regulator are shown in Fig. 2.16A and B , and described in the legend.

Failure of the pressure reduction function of a regulator can transmit excessively high pressure downstream. To protect against such occurrences, the regulator incorporates a pressure relief valve in the low-pressure chamber through which excess pressures are vented to the atmosphere. If the diaphragm were to rupture or develop a hole, the regulator would fail and gas would escape around the adjustment screw and spring. The high flow of escaping gas would make a loud sound, suggesting the possibility of a regulator failure. Failure of an oxygen first-stage regulator would cause a significant loss of oxygen pressure, whether coming from the cylinder or the pipeline (see Fig. 2.2 ), resulting in sudden failure of oxygen delivery.

Oxygen Flush

The oxygen flush button activates a simple valve that opens flow from the oxygen pipeline or oxygen first-stage regulator to the CGO. Pressing the oxygen flush button results in a flow of 35–75 L/min of pure oxygen, bypassing any ON/OFF switches, flowmeters, and vaporizers. The pressure at the CGO could increase up to the oxygen supply pressure unless some pressure relief mechanism is present. The oxygen flush can be used for a number of reasons. It can be used to fill the breathing system with oxygen in order to manually ventilate a patient’s lungs in an emergency when the anesthesia workstation is off. It can be used to fill the breathing system when mask ventilating a patient, but it dilutes any inhaled anesthetics in the breathing system. To avoid barotrauma in a patient, caution is necessary when the oxygen flush is activated, particularly during mechanical ventilation. Contemporary machines incorporate a pressure-limiting device in the ventilator to prevent potentially harmful pressures, but the inspiratory pressure could reach this limit if the oxygen flush is activated during the inspiratory phase of positive-pressure ventilation (see Chapter 6 ). The oxygen flush can also be used to provide high pressure oxygen for emergency jet ventilation (but see Common Gas Outlets and Outlet Check Valves, later, for further discussion).

The workstation standard requires that the oxygen flush valve be self-closing and designed to minimize unintended operation by equipment or personnel. A modern design for an oxygen flush button is shown in Fig. 2.17 ; note that the button is recessed in a housing to prevent accidental depression and that the valve is self-closing.

Pressure Sensor Shut-Off (Fail-Safe) Valves and Alarm System

Pressure sensor shut-off valves and oxygen failure protection devices (OFPDs), often referred to as fail-safe valves, were an early safety feature of anesthesia machines. They were introduced at a time when oxygen and nitrous oxide were commonly used, often supplied from cylinders, and before the development of pulse oximetry or the common measurement of inspired oxygen concentration. A recognized hazard was that the oxygen cylinder could be empty and the nitrous oxide would continue to flow. This was sometimes not recognized and the patient would suffer a hypoxic injury or death.

Pressure sensor shut-off valves reduce or interrupt the flow of a second gas when the pressure from the oxygen pipeline or oxygen first-stage regulator falls below a set threshold. Pressure sensor shut-off valves stop the flow of nitrous oxide and other hypoxic gases (e.g., carbon dioxide) to their flowmeters if the oxygen supply pressure falls below the threshold setting. In the event of a catastrophic loss of oxygen to the anesthesia machine, pressure sensor shut-off valves ensure that oxygen is the last gas that flows to the patient. On some older anesthesia machines, even the air channel has a pressure sensor shut-off valve.

Pressure sensor shut-off valves can be constructed to either stop or to proportionally decrease the flow of a second gas as the oxygen pressure decreases below a preset threshold. Fig. 2.18 shows an all-or-nothing type valve from an older GE-Datex-Ohmeda machine. When the oxygen pressure is higher than 25 psig, it presses on the diaphragm with enough force to hold the valve open and allow the flow of nitrous oxide. The valve closes and totally interrupts the flow of nitrous oxide when oxygen pressure decreases below 25 psig. ^, Fig. 2.19 shows a proportional type valve, called an OFPD, from an older Dräger machine. As the oxygen supply pressure falls and the flow of oxygen from the machine’s flowmeter decreases, the OFPDs proportionately reduce the supply pressure of other gases to their flowmeters, synchronously decreasing those flows. The supply of nitrous oxide and other gases is completely interrupted when the oxygen supply pressure falls below 12 ± 4 psig.

The fail-safe valves only ensure that nitrous oxide cannot flow when oxygen supply pressure is lost, and not that a safe ratio of oxygen to nitrous oxide flows is coming from the flowmeters. Thus, a normally functioning fail-safe system would permit flow of 100% nitrous oxide, provided the machine has an adequate oxygen supply pressure. The term “fail-safe” therefore represents something of a misnomer because it does not ensure adequate oxygen flow ( Fig. 2.20 ).

Since the fail-safe valve only senses pressure in the oxygen supply line, it would not be able to detect if a gas other than oxygen was in the oxygen supply line. For example, the oxygen and nitrous oxide supply lines could be crossed during construction of a new OR. Similarly, nitrogen could be introduced into the oxygen supply line during the pressure testing of a new pipeline. These potentially lethal mistakes can only be detected by the workstation’s oxygen analyzer.

All anesthesia machines have an oxygen supply pressure failure alarm that notifies the user if the oxygen supply pressure is below a preset threshold, typically 30 psig. On contemporary machines, this is usually a simple electronic sensor on the oxygen piping downstream of the pipeline inlet and oxygen first-stage regulator ( Fig. 2.21 ). Sometimes, the oxygen electronic pipeline and cylinder pressure gauges serve as the sensors for the oxygen supply pressure alarm. If the oxygen pressure falls below the set point, an electrical switch closes which activates both an audible and visual alarm, and alarm message ( Fig. 2.22 ). The workstation standard requires that whenever oxygen supply pressure falls below the manufacturer-specified threshold, a high-priority alarm is activated. The alarm remains activated until the oxygen supply pressure becomes adequate, and for safety reasons the audio cannot be paused for more than 120 seconds.

Oxygen Ratio Proportioning Systems

A major consideration in the design of contemporary anesthesia machines is the prevention of hypoxic gas mixture delivery to the patient. The fail-safe system described previously only interrupts, or proportionately reduces and ultimately interrupts, the supply of nitrous oxide and other gases to their flowmeters if the oxygen supply pressure to the machine decreases. It does not prevent delivery of a hypoxic mixture to the CGO.

In anesthesia machines with mechanical flow controls, oxygen and nitrous oxide flow controls are physically interlinked either mechanically (older GE-Datex-Ohmeda machines) or mechanically and pneumatically (Dräger machines) so that a fresh gas mixture containing at least 25% oxygen is created at the level of the rotameters when nitrous oxide and oxygen are being delivered. ^, In anesthesia machines with electronic flow controls, proportioning is achieved by the computer interposed between the human user and the electronically controlled gas flow valves.

Datex-Ohmeda and GE anesthesia machines with mechanical flow controls use the Link-25 Proportion-Limiting Control System. In this system, a chain linkage between the oxygen and nitrous oxide flow-control knobs engages whenever the oxygen flow is set less than 25% of the nitrous oxide flow ( Figs. 2.23 and 2.24 ). When engaged, turning the oxygen down further causes the nitrous oxide knob to turn down as well, and turning the nitrous oxide up causes the oxygen knob to turn up too. However, the flow through a flowmeter assembly depends on both the control knob position and the pressure differential across the flow control orifice (see Mechanical Flow-Control Valves later). So, for this system to work, second-stage regulators located just upstream of the flowmeter assemblies tightly control the inlet pressures of oxygen (14 psig) and nitrous oxide (26 psig) to their respective flowmeters (see Figs. 2.2 and 2.24 ).

Dräger anesthesia machines use different types of flow proportioning systems that regulate the oxygen flows to be greater than 25% of the nitrous oxide flows, irrespective of where the flow-control knobs are set. With these systems, when the oxygen flow is below 25% of the nitrous oxide flow, decreasing the oxygen flow further causes the nitrous oxide flow to decrease without its flow-control knob moving, and turning the nitrous oxide knob up does not increase the nitrous oxide flow. Dräger anesthesia machines with mechanical flow controls and glass flowmeters (no longer manufactured or supported by Dräger) use the oxygen ratio controller (ORC; Fig. 2.25 ). ^, Dräger anesthesia machines with mechanical flow controls and electronic flowmeters use the sensitive oxygen ratio controller (S-ORC) proportioning system ( Fig. 2.26 ). However, the general principle of the ORC and S-ORC is otherwise the same: Two connected diaphragms control a slave nitrous oxide flow control valve that is in series with the manual nitrous oxide flow control valve. This slave nitrous oxide flow control valve limits the ratio of nitrous oxide flow to oxygen flow so that it does not exceed 3:1 (i.e., an oxygen concentration lower than 25% in the mixture).

The user has different experiences with the Dräger ORC or S-ORC versus the GE-Datex-Ohmeda Link-25 Proportioning Limiting System. In the Link-25 system, once the oxygen flow has been increased via the link chain and gears, the oxygen flow remains at the increased setting even if the nitrous oxide flow is deliberately decreased. With the ORC and S-ORC systems, if the system is acting to decrease the flow of nitrous oxide because the user has decreased the oxygen flow, when the flow of oxygen is increased again, the nitrous oxide flow will increase to its original setting. Although of elegant designs, the ORC, S-ORC, and Link-25 systems are subject to mechanical and/or pneumatic failure (see Chapter 23 ) and should be tested according to the manufacturer’s instructions during the pre-use machine checkout.

All of the aforementioned proportioning systems function only between nitrous oxide and oxygen and there is no interlinking of oxygen with other gases, such as air or helium, that might be delivered by the machine ( Figs. 2.27 and 2.28 ). Thus, when a third or fourth gas is in use, these proportioning systems afford no protection against a hypoxic mixture at the CGO. Because of this, it is inherently safer to supply helium from tanks that contain a mixture of helium and oxygen (heliox) in a 75:25 ratio.

Many manufacturers now offer anesthesia machines with electronically-controlled flow-controllers and electronic flowmeters. In these, hypoxic gas mixtures are prevented at the level of the user interface, where the gas flows are set on a controlling computer. The user is prevented from setting an oxygen flow that is less than 25% of the nitrous oxide flow. The computer continuously adjusts the flow controllers to achieve the set flows, rapidly responding to any changes in supply pressures.

Even if the fresh gas proportioning system is functioning correctly, an oxygen leak downstream of the flowmeter, or the addition of high concentrations of a potent volatile inhaled anesthetic (e.g., 18% desflurane) downstream of the proportioning systems, could result in a hypoxic mixture (i.e., <21% oxygen) being delivered from the machine’s CGO. Also, with low fresh gas flows, patient oxygen consumption can cause the breathing system oxygen concentration to decrease below 21%. Once again, an oxygen analyzer in the inspiratory side of the patient circuit is essential if a potentially hypoxic mixture is to be detected and thereby prevented.

Fresh Gas Controllers and Flowmeters

Two separate components deliver the intended flow of each gas to the CGO: a variable resistor device controls the flow, and another device measures the flow ( Fig. 2.29A and B ).