Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Introduction
Pediatric anesthesiologists care for a diverse patient population. Anesthetizing a 500-g neonate can be followed immediately by care for a 100-kg adolescent. This broad range of patient ages and sizes requires a diverse array of anesthesia equipment. This chapter discusses the types of pediatric anesthesia equipment and highlights the unique adaptations for their use in children.
Humidifiers
During normal breathing, the nasal mucosa and upper airway serve as a reservoir for condensed water. This water evaporates during normal respiration and reduces the heat expended by the body to warm and humidify inhaled gases. For example, in typical atmospheric conditions, air in the pulmonary periphery contains 44 mg of water per liter at 37° C (100% relative humidity); breathing ambient air at a temperature of 22° C, water content of 10 mg/dL would require the addition of 34 mg/L of evaporated water. This evaporation results in energy expenditure, with resultant cooling of the respiratory mucosa. The caloric expenditure of humidification consumes approximately five times the energy required to heat the inspired gases; this may amount to 20% of the basal metabolic rate of an infant ( ). Nasopharyngeal humidification is bypassed routinely with the placement of airway devices such as tracheal tubes, tracheostomy tubes, and laryngeal masks. Benefits of heating and humidifying anesthetic gases include mitigation of intraoperative hypothermia, decreased atelectasis, and improved mucociliary clearance. Consequences of overhumidification include impaired mucociliary clearance related to reduced mucus viscosity, atelectasis, surfactant inactivation, infection, thermal injury, and accumulation of secretions ( ). Partial humidification of gas in the anesthesia breathing circuit takes place within the carbon dioxide absorber, which uses an exothermic reaction that may raise the water vapor content to as much as 29 mg/L. Further humidification is accomplished by reducing the amount of fresh-gas flow, thereby increasing rebreathing of humidified gases and using a heat and moisture exchanger (HME, or “artificial nose”). HMEs use a fine mesh to cause condensation of exhaled water vapor. HMEs add dead space and may increase the resistance to breathing for some infants and children, although these changes are usually tolerable. A low fresh-gas flow rate alone is often inadequate at maintaining humidity and temperature in infants and neonates; therefore active or passive humidification devices should be incorporated when ventilating this population ( ). HMEs increase airway humidification and preserve temperature in anesthetized children at a lower cost than active humidification systems ( ). HMEs require 20 minutes to achieve optimal saturation of the membrane, during which time they are less efficient. HMEs come in a variety of sizes, enabling selection based on size of the patient so dead space or resistance can be minimized. Specially designed HMEs filter out infectious pathogens and minimize the risk of cross-infection between patients ( Fig. 17.1 ) ( ). HMEs can become obstructed in patients with significant airway secretions.
Active humidification is the most efficient means by which to heat and humidify inspired gases ( ). A servo-controlled, shielded, heated wire in the fresh-gas line helps to prevent cooling and condensation of the water as it passes through the inspiratory limb. A probe near the patient connection should monitor the inspiratory gas temperature, because overheating of the inspired gases can injure the airway ( ). Active humidifiers may also increase the compression volume of the breathing circuit; thus compensatory increases in the tidal volume during controlled ventilation may be necessary except in ventilators with compliance compensation ( ). Nuisance alarms from overly sensitive temperature alarms and circuit disconnections from the additional connections into the circuit unfortunately can deter the use of active humidifiers.
Anesthesia breathing systems
landmark article began the modern era of breathing systems for pediatric anesthesia that began as a topic of controversy. When Ayre used Magill’s technique of tracheal intubation for the repair of cleft lip and palate in infants, he noted adverse results. Breathing through a “closed” high-resistance system, these infants often developed “rapid, ‘sighing’ respirations” and “ashy pallor and sweating.” They exhibited a “dark congested oozing at the site of operation.” Postoperatively, the infants were “in varying degrees of shock: some . . . for days.” The contribution of hypotension or hypovolemia to this picture remains unknown because Ayre did not measure blood pressure and had difficulty quantifying blood loss.
Ayre noted dramatic clinical improvement when he adopted an open T-piece breathing system. The T-piece, an extremely simple device, consists of an inspiratory limb, a connection to the patient, and an expiratory limb. It has neither unidirectional nor overflow valves, and there is no breathing bag. The expiratory limb serves as a reservoir for fresh gas; a means of monitoring the infant’s respirations; and, if the distal end is intermittently occluded, a means of providing positive pressure ventilation. If the volume of the expiratory limb is one-third of the tidal volume, rebreathing can be virtually eliminated during spontaneous ventilation with a fresh-gas flow that is twice the minute ventilation ( ). Ayre attributed the salutary effect of the T-piece to marked reductions in resistance to gas flow and rebreathing.
Nonrebreathing and partial rebreathing systems
Despite its apparent benefits, the T-piece is far from ideal. The major flaws are its release of anesthetic gases into the operating room and its inability to provide assisted or controlled ventilation. A series of modifications occurred. first proposed the addition of a breathing bag to the expiratory limb. Another system, the Magill attachment, introduced fresh gas distal to a breathing bag and an overflow valve near the patient connection. These and other variations were brought together under a single classification scheme proposed by , in which each system was distinguished on the basis of the location of its fresh-gas inflow and overflow valves relative to the patient connection.
These Mapleson circuits share the benefit of reduced resistance to breathing by virtue of the absence of unidirectional valves and canisters; the elimination of these components results in varying degrees of rebreathing that depend on the fresh-gas flow ( Fig. 17.2 A). Rebreathing can be beneficial because it conserves heat, humidity, and anesthetic gases. Yet in the absence of carbon dioxide monitoring, the consequences of hypercarbia and respiratory acidosis probably outweigh these benefits.
Each circuit has different rebreathing characteristics, depending on the locations of the fresh-gas inflow, the overflow valves, and the fresh-gas flow rate; the respiratory rate (i.e., expiratory time) and tidal volume, carbon dioxide production, and the mode of ventilation (i.e., spontaneous or controlled) also contribute to the degree of rebreathing. The following sections describe the Mapleson A (Magill attachment) and D systems. The B and C systems are rarely used today. The Mapleson E system is the T-piece described previously.
Mapleson A system
The Mapleson A system results in no rebreathing during spontaneous ventilation when the fresh-gas flow is more than 75% of the minute ventilation; it requires a larger fresh-gas flow to eliminate rebreathing during controlled ventilation ( Fig. 17.2 B) ( ; ). This design is impractical in the operating room, because the proximal location of the overflow valve makes it cumbersome for scavenging waste gases, difficult to adjust during head and neck surgery, and potentially dangerous, because the heavy valve can dislodge a small tracheal tube.
Mapleson D system
A proximal fresh-gas inflow and a distal overflow valve characterize the Mapleson D system. It is a modification of the T-piece, in which a breathing bag and an overflow valve have been added to the distal expiratory limb. Although it requires slightly more fresh-gas flow to eliminate rebreathing during spontaneous ventilation than the Mapleson A system, it is the most economical during controlled ventilation ( ). On balance, considering both spontaneous and controlled ventilation, the Mapleson D requires the lowest fresh-gas flow rates among all Mapleson circuits. This system has become the most widely used of the Mapleson circuits for pediatric anesthesia.
The precise flow dynamics in the Mapleson D system have been a subject of discussion that has produced a variety of complex recommendations ( ; ; ; ; ; ; ). To eliminate rebreathing, higher fresh-gas flows are needed during spontaneous ventilation than during controlled ventilation. With spontaneous ventilation, rebreathing is eliminated by provision of fresh-gas flow equal to the mean inspiratory flow rate ( ; ). If an inspiratory/expiratory ratio is 1:1 or 1:2, the mean inspiratory flow rate is two to three times the minute ventilation. Although Spoerel and others ( ) have demonstrated that a normal arterial carbon dioxide tension (Pa co 2 ) can be maintained during spontaneous ventilation at fresh-gas flows as low as 100 mL/kg per minute, increased minute ventilation (and thus more respiratory work) is required to compensate for rebreathed carbon dioxide.
The recommendations for fresh-gas flow during controlled ventilation are complex and varied ( ; ; ). This reflects the importance of several factors that were summarized by ( Fig. 17.3 ). When a high fresh-gas flow (>100 mL/kg per minute) is used, the Pa co 2 is governed by minute ventilation (ventilation limited). At low fresh-gas flow (<90 mL/kg per minute), Pa co 2 is independent of minute ventilation, varying instead as a function of the amount of rebreathing, which is governed by the fresh-gas flow rate (flow limited).
Additional important factors that govern the magnitude of rebreathing include carbon dioxide production, respiratory rate, and respiratory waveform characteristics (e.g., inspiratory flow, inspiratory and expiratory times, and expiratory pause) ( ). Adjustments to the ventilatory pattern that allow the fresh-gas flow to constitute a larger proportion of the inspired gas (e.g., slow inspiratory time or low inspiratory flow) or that enable exhaled gases to be more completely washed out (e.g., long expiratory pause or slow rate) reduce the amount of rebreathing. With a Mapleson D circuit that has controlled ventilation and low fresh-gas flow (flow limited), an attempt to reduce the Pa co 2 by increasing the respiratory rate would reduce the expiratory pause and thus promote rebreathing ( Fig. 17.4 ). In this situation, the increased ventilation is offset by an increased fraction of inspired carbon dioxide (Fi co 2 ), resulting in no net change in Pa co 2 . To wash out the exhaled gas at this higher respiratory rate and take advantage of the increased minute ventilation, the fresh-gas flow must be increased. These fresh-gas flow and ventilatory recommendations are predicated for a normal metabolic rate and thus normal carbon dioxide production ( ; ). Conditions that increase carbon dioxide production (e.g., fever, catabolic state, or malignant hyperthermia) must be met with a proportional increase in fresh-gas flow and ventilation.
Bain modification of mapleson D
The Bain modification of the Mapleson D circuit incorporates the fresh-gas supply within the expiratory limb in a coaxial arrangement ( Fig. 17.5 ) ( ). This circuit is light and streamlined with only a single hose to the patient. This arrangement also provides some countercurrent warming of the inspired gases and effective scavenging of expired gases. Its major disadvantage lies in the inability to directly inspect the integrity of the inspiratory limb. described an indirect test of the Bain inspiratory-limb integrity in which the oxygen flush is passed through the circuit for several seconds. If the inspiratory limb is intact, the rapid flow of gas through it exerts a Venturi effect on the expiratory limb, resulting in a slight negative pressure and collapse of the breathing bag. With a leak from the inspiratory limb into the expiratory limb, the pressure in the latter rises, tending to inflate the reservoir bag. The rebreathing characteristics of the Bain circuit are identical to those of any other Mapleson D. The major advantages of these Bain circuits for pediatric anesthesia are their relatively lower resistance to breathing offered by an open system, the countercurrent warming of gases, and the low profile of a single hose attached to the patient.
The primary sources of resistance in an anesthesia delivery system are the tracheal tube, the valves, and the carbon dioxide absorber. With modern equipment, the tracheal tube represents a major source of resistance in the neonate ( ; ). Lightweight, large-diameter, modern disc valves exert resistance in two ways. There is a minimum, flow-independent resistance necessary to displace the valve, usually much less than 1 cm H 2 O ( ). A much higher resistance may be required when the expiratory valve is wet. At high gas flows (more than 30 L/min), the valves also become a source of turbulent resistance proportional to the flow through them. Carbon dioxide absorption canisters are also a source of turbulence, and their resistance is inversely proportional to the length of the path the gas must take through the resistor. Thus modern absorbers are short and wide to minimize this path of resistance.
Approximately a half-century after Ayre introduced the T-piece, the extent to which his work applies remains unclear. Perhaps the infants Ayre studied were subjected to the significant resistance imposed by the valves of that era. Although the neonate has a lower proportion of fatigue-resistant fibers in the diaphragm, infants as young as 2 weeks old have been shown to compensate for increases in resistance of 200% without changes in blood gases, at least for relatively short lengths of time ( ). The Mapleson systems’ benefits must be weighed against their inherent problems on an individual basis. If a practitioner seldom uses Mapleson circuits and is unfamiliar with their characteristics, or if a practitioner has to make substantial alterations to an anesthesia machine to accommodate them, then the additional risks of the situation must be considered. Even if a circle system is used for most children, it is important to understand the flow requirements of the Mapleson circuits, because they continue to be used for resuscitation and transporting patients.
Circle systems
When functioning properly, a circle system minimizes environmental pollution and enables lower fresh-gas flows than the Mapleson D circuit, while conserving heat, humidity, and anesthetic agent. Compared with the resistance of an endotracheal tube, the additional resistance imposed by the addition of unidirectional valves and a carbon dioxide absorption canister is trivial. Because the ratio of a young child’s tidal volume to the volume of the inspiratory limb is small, changes in the anesthetic concentration can take some time to reach equilibrium unless higher fresh-gas flows are used. It is important to be vigilant for the manifestations of stuck (resistance) or floating (rebreathing) unidirectional valves, because both can have harmful consequences in small children.
Special circle-breathing systems have been designed for infants and children that incorporate the same components as the standard adult circuits but have been modified to minimize dead space and resistance to breathing. Most incorporate short, narrow-caliber hoses and Y connections to minimize the weight and compliance of the circuit. These smaller hoses, along with smaller carbon dioxide absorber canisters, are also used to reduce the time needed to effect a change in the concentration of vapor or gases in the circuit.
Carbon dioxide absorbers
Rebreathing of expired gases to conserve humidity and heat, in addition to minimize environmental pollution, requires carbon dioxide absorbers to scrub carbon dioxide from the exhaled gas. Anesthetic gas interaction with absorbent materials can produce toxic degradation products or flash fires. Compound A, a compound causing nephrotoxicity in animals, can be produced by high concentrations of sevoflurane, low-flow techniques, and dry absorbent, although amounts produced do not appear to cause clinical toxicity, even in human patients with renal insufficiency or prolonged anesthetics ( ; ). Carbon monoxide generation seen with all inhalational anesthetics has been noted especially with dry absorbents and those formulated with strong bases. Interactions with newer absorbents (Drägersorb Free, Draeger Medical, Telford, PA; Amsorb Plus, Armstrong Medical, Coleraine, Northern Ireland) produce less or none of both of these compounds ( ; ). Concomitant with the production of carbon monoxide is the production of heat. As a result of this exothermic reaction, multiple fires, especially with sevoflurane and desiccated Baralyme absorber (a product no longer manufactured), have been reported in the anesthesia circuit. In the laboratory with this combination and an oxygen-enriched environment, temperatures can reach in excess of 200° C, causing flames to appear in the circuit ( ). Again, the use of newer absorbents with less carbon monoxide production and the routine replacement of absorbents to reduce desiccation minimize this problem.
Anesthesia machines
Anesthesia machines serve two basic functions: to allow the anesthesia provider to control the concentrations of gases and anesthetic vapors delivered to the patient and to support both spontaneous and mechanical ventilation. No anesthesia machines are designed specifically for pediatric applications, and for the most part, the available machines do not function any differently for pediatric patients than they do for adults. An anesthesia machine intended to be used for pediatric patients should, however, have certain characteristics.
Gas and anesthetic vapor delivery
Oxygen
All modern anesthesia machines are designed to ensure continuous oxygen delivery from either pipeline or cylinder supplies and to notify the user if the oxygen-supply pressure fails. Because pressurized gas in the oxygen supply does not guarantee that there is oxygen flowing to the patient, an oxygen analyzer located in the inspiratory limb of the anesthesia circuit is an essential safety monitor for every anesthetic ( ). Indeed, an inspired oxygen-concentration monitor is considered to be the standard of care for every anesthetic according to the ( Box 17.1 ). The oxygen analyzer is crucial when caring for the anesthetized pediatric patient because certain clinical conditions mandate precise control of the delivered oxygen concentration, and the oxygen analyzer confirms delivery of the desired oxygen concentration.
BOX 17.1
Standards for Basic Anesthetic Monitoring Committee of Origin: Standards and Practice Parameters
(Approved by the ASA House of Delegates on October 21, 1986, and Last Affirmed on December 13, 2020)
These standards apply to all anesthesia care, although, in emergency circumstances, appropriate life support measures take precedence. These standards may be exceeded at any time based on the judgment of the responsible anesthesiologist. They are intended to encourage quality patient care, but observing them cannot guarantee any specific patient outcome. They are subject to revision from time to time, as warranted by the evolution of technology and practice. They apply to all general anesthetics, regional anesthetics, and monitored anesthesia care. This set of standards addresses only the issue of basic anesthetic monitoring, which is one component of anesthesia care. In certain rare or unusual circumstances, (1) some of these methods of monitoring may be clinically impractical, and (2) appropriate use of the described monitoring methods may fail to detect untoward clinical developments. Brief interruptions of continual † monitoring may be unavoidable. These standards are not intended for application to the care of the obstetric patient in labor or in the conduct of pain management.
Standard I
Qualified anesthesia personnel shall be present in the room throughout the conduct of all general anesthetics, regional anesthetics, and monitored anesthesia care.
Objective
Because of the rapid changes in patient status during anesthesia, qualified anesthesia personnel shall be continuously present to monitor the patient and provide anesthesia care. In the event there is a direct known hazard—for example, radiation—to the anesthesia personnel that might require intermittent remote observation of the patient, some provision for monitoring the patient must be made. In the event that an emergency requires the temporary absence of the person primarily responsible for the anesthetic, the best judgment of the anesthesiologist will be exercised in comparing the emergency with the anesthetized patient’s condition and in the selection of the person left responsible for the anesthetic during the temporary absence.
Standard II
During all anesthetics, the patient’s oxygenation, ventilation, circulation, and temperature shall be continually evaluated.
Oxygenation
Objective
To ensure adequate oxygen concentration in the inspired gas and blood during all anesthetics.
Methods
- 1.
Inspired gas: During every administration of general anesthesia using an anesthesia machine, the concentration of oxygen in the patient breathing system shall be measured by an oxygen analyzer with a low oxygen concentration limit alarm in use. *
- 2.
Blood oxygenation: During all anesthetics, a quantitative method of assessing oxygenation such as pulse oximetry shall be employed. * When the pulse oximeter is utilized, the variable pitch pulse tone and the low-threshold alarm shall be audible to the anesthesiologist or the anesthesia care team personnel. * Adequate illumination and exposure of the patient are necessary to assess color. *
Ventilation
Objective
To ensure adequate ventilation of the patient during all anesthetics.
Methods
- 1.
Every patient receiving general anesthesia shall have the adequacy of ventilation continually evaluated. Qualitative clinical signs such as chest excursion, observation of the reservoir breathing bag, and auscultation of breath sounds are useful. Continual monitoring for the presence of expired carbon dioxide shall be performed unless invalidated by the nature of the patient, procedure, or equipment. Quantitative monitoring of the volume of expired gas is strongly encouraged. *
- 2.
When an endotracheal tube or laryngeal mask is inserted, its correct positioning must be verified by clinical assessment and by identification of carbon dioxide in the expired gas. Continual end-tidal carbon dioxide analysis in use from the time of endotracheal tube/laryngeal mask placement until extubation/removal or initiating transfer to a postoperative care location shall be performed using a quantitative method such as capnography, capnometry, or mass spectroscopy. * When capnography or capnometry is utilized, the end-tidal CO 2 alarm shall be audible to the anesthesiologist or the anesthesia care team personnel. *
- 3.
When ventilation is controlled by a mechanical ventilator, there shall be in continuous use a device that is capable of detecting disconnection of components of the breathing system. The device must give an audible signal when its alarm threshold is exceeded.
- 4.
During regional anesthesia and monitored anesthesia care, the adequacy of ventilation shall be evaluated by continual observation of qualitative clinical signs and/or monitoring for the presence of exhaled carbon dioxide.
Circulation
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here
