Breathing Circuits PDF

4 Breathing Circuits SCOTT G. WALKER | SENTHIL PACKIASABAPATHY | GEORGE SHEPLOCK | MICHAEL A. ACQUAVIVA CHAPTER OUTLINE...

4 Breathing Circuits SCOTT G. WALKER | SENTHIL PACKIASABAPATHY | GEORGE SHEPLOCK | MICHAEL A. ACQUAVIVA CHAPTER OUTLINE and humidity. It converts continuous gas flow from the anesthesia machine to the intermittent flow of breathing, Introduction facilitates controlled or assisted respiration, and provides other History of Device Development functions such as gas sampling and pressure and spirometric Classifications of Breathing Circuits measurements. Chemical Absorption of Carbon Dioxide The desirable characteristics of a breathing circuit include Dilution with Fresh Gas (1) low resistance to gas flow, (2) minimal rebreathing of the Use of Valves to Separate Exhaled Gases from Inhaled preceding exhaled gases, (3) removal of carbon dioxide at the Gases rate at which it is produced, (4) rapid changes in delivered Use of Open-­Drop Ether or a T-­Piece without a Reservoir to gas composition when required, (5) warmed humidification Release Exhaled Carbon Dioxide Into the Atmosphere of the inspired gases, and (6) safe disposal of waste gases. The Components of a Breathing Circuit components of a breathing circuit include (1) the breathing Connection of the Patient to the Breathing Circuit tubing, (2) respiratory valves, (3) reservoir bags, (4) carbon Breathing Tubing dioxide absorption canisters, (5) a fresh gas inflow site, (6) a Unidirectional Valves pop-­off valve leading to a scavenger for excess gas, (7) a Y-­piece Breathing Bags with a mask or tube mount, and (8) a face mask, laryngeal Volume Reflector mask, or tracheal tube. Other devices that may be included are Gas Inflow and Pop-­Off Valves (1) filters, (2) humidifiers, (3) valves for positive end-­expiratory pressure (PEEP), and (4) detecting mechanisms for airway Carbon Dioxide Absorption pressure, spirometry, and gas analysis. Although these circuit Indicator Dyes components can be assembled in many ways, contemporary Mesh Size and Channeling systems are usually configured by the manufacturer and permit Other Reactions with Absorbents little intervention by the user in regard to their configuration. Mixing Devices Understanding the advantages and limitations of the different Bacterial Filters configurations allows the user to select the most appropriate type for varying clinical settings. Analysis of Specific Circuits Circle Breathing Systems Placement of the Carbon Dioxide Absorber, Fresh Gas History of Device Development Inflow, and Pop-­Off Valve Flow and Concentration Breathing circuits have been an important concern from the start. Semiclosed Systems: Mapleson Classification Because of a delay in the production of his inhaler, Morton was late Proprietary Semiclosed Systems to his first public exhibition of the “Somniferon” (ether) inhaler in Semiopen Systems 1846. The earliest circuits were mechanically simple; differences Pediatric Breathing Systems among them were related to the characteristics of the primary Positive End-­Expiratory Pressure anesthetic agent. Because nitrous oxide and ether anesthetic mixtures were weak (less potent) or slow to produce anesthesia, Circuit Malfunction and Safety it was necessary to exclude air and helpful to include oxygen enrichment. The rapid onset of action and potency of chloroform, on the other hand, demanded precise control. It became apparent Introduction that the unique features of each agent were important. The ability to assist respiration was advantageous, as was conservation of The anesthesia machine serves to create a desired mixture of costly agents and avoidance of large leaks of flammable ones. anesthetic gases, vapors, oxygen, and air (as well as other gases In the twentieth century, a large number of relatively small such as helium and carbon dioxide, albeit less frequently). but more highly engineered improvements were made as other The patient is the recipient of these prepared gas mixtures of demands on the breathing circuit were recognized. In 1915, known composition, and the breathing circuit is the interface Dennis Jackson described the first carbon dioxide absorber to between the anesthesia machine and the patient. This circuit save on the cost of nitrous oxide for animal studies.1 Ralph Waters delivers the gas mixture from the machine to the patient as brought the idea into the OR, designing a to-­and-­fro absorption it removes carbon dioxide, excludes operating room (OR) air, canister that used soda lime.2,3 Bryan Sword introduced the and conditions the gas mixture by adjusting its temperature first circle breathing circuit in 1930.4 Thus, low-­flow absorption 100 Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  4 Breathing Circuits 101 systems were already in use when cyclopropane made them dioxide is absorbed, and all other exhaled gases are rebreathed. essential. A return to high flows in the United States was brought The quantities of fresh oxygen and anesthetics equal those lost about by the poor performance of vaporizers for halothane in the as a result of uptake, metabolism, and circuit leaks.3,17,18 1950s, with the demonstration that such flows could eliminate carbon dioxide without the use of soda lime.5,6 DILUTION WITH FRESH GAS Stimulated by Magill’s use of a number of pieces of apparatus put together in differing configurations for differing purposes, Because of the intermittent nature of carbon dioxide excretion Mapleson described a variety of Magill circuits.7 The original (during exhalation only) and the continuous inflow of fresh Ayre’s T-­piece was modified by numerous practitioners; the gas, the choice of inflow rate—as well as the locations of the Jackson-­Rees circuit represents one such example.8 A variety inflow site, reservoir bag, and pop-­ off valves—contributes of proprietary nonrebreathing valves were introduced, and the to the efficiency of carbon dioxide removal. When fresh gas circuits named for them included the Stephen-­Slater,9 the Fink,6 flows are 1 to 1.5 times the minute volume (approximately the Ruben,10 and the Frumin.11 10 L/min in an adult), dilution alone is sufficient to remove Partial rebreathing and functionally nonrebreathing carbon dioxide.17,19–23 Such systems then behave the same as a circuits—such as the Bain,12 Humphrey ADE,13 and Lack14 nonrebreathing system. systems—found various proponents. Ingenious switching valves permitted transformation from one circuit to another,13 USE OF VALVES TO SEPARATE EXHALED GASES which then often led to difficulty in remembering which circuit FROM INHALED GASES was optimal for what purpose. Today, in addition to factors of convenience and economy, Systems that use nonrebreathing valves are examples of this circuits are used: (1) to control heat and humidity; (2) to measure method of carbon dioxide removal.6,9,11,24,25 A circuit that by patient variables such as tidal volume, respiratory frequency, virtue of high flows behaves as if it were nonrebreathing is not airway pressure, and inspired and expired gas concentrations; considered a nonrebreathing circuit in this analysis. and to (3) control contamination of the OR environment by the agents themselves. The 150-­year history of the development of USE OF OPEN-­DROP ETHER OR A T-­PIECE the breathing circuit offers the practitioner a number of choices. WITHOUT A RESERVOIR TO RELEASE EXHALED All commonly used circuits accomplish their goals more or CARBON DIOXIDE INTO THE ATMOSPHERE less equivalently, but the simple act of increasing fresh gas flow, for example, may markedly increase the work of breathing.15 Although similar to the second method above, systems that used Therefore, it is vital that the anesthesiologist understand the open-­drop ether or a T-­piece without a reservoir were not truly functional characteristics of each circuit. breathing circuits. The T-­pieces with an expiratory reservoir rely on dilution of carbon dioxide by both fresh gas and room air for its removal; these have been included in semiclosed Classifications of Breathing Circuits circuits later.26 A widely used nomenclature was developed that classified circuits as open, semiopen, semiclosed, or closed, according to whether a reservoir is used and whether rebreathing occurs. Components of a Breathing Circuit An open system has no reservoir and no rebreathing; a semiopen The circuits described previously have many features in system has a reservoir but no rebreathing; a semiclosed system common; they connect to the patient’s airway through a face has a reservoir and partial rebreathing; and a closed system mask, laryngeal mask, or tracheal tube adapted to the breathing has a reservoir and complete rebreathing. Variations on this circuit through a Y-­piece or elbow. The system may include classification included the type of carbon dioxide absorber and valves to permit directional gas flow, and a reservoir bag is unidirectional valves used. almost always present, which can be used to manually force Because of confusion with this traditional nomenclature, gas into the lungs. Fresh gas must be supplied to the circuit, Hamilton recommended its abandonment in favor of both a and excessive gas must be allowed to escape. In some, carbon description of the hardware (e.g., circle filter system, coaxial dioxide is absorbed in a chemical filter. A variety of ancillary circuit, T-­piece) and the gas flow rates being used.16 Identifying the devices may also be present, such as humidifiers, spirometers, circuits by eponym—such as Adelaide, Bain, Hafnia, Humphrey, pressure gauges, filters, gas analyzers, PEEP devices, waste gas Jackson-­ Rees, Lack, Magill, and Waters—did not help in scavengers, and mixing and circulating devices. understanding the function or application of the circuit. Almost all anesthesia machines are equipped with some form of a circle CONNECTION OF THE PATIENT breathing circuit with the ability for carbon dioxide absorption TO THE BREATHING CIRCUIT during low-­flow anesthesia and elimination through the pop-­off valve during high-­flow anesthesia. Because an understanding of Either an anesthesia mask, supraglottic device, or a tracheal tube how circuits work is essential, breathing circuits in this chapter connects the circuit to the patient. Masks are made from rubber are organized by method of carbon dioxide elimination. Methods or (now) clear plastic to make secretions or vomitus visible (Fig. for removal of carbon dioxide are discussed. 4.1). Most have an inflatable or inflated cuff, a pneumatic cushion that seals to the face. Masks are available in a variety of sizes and styles to accommodate the wide variety of facial contours. CHEMICAL ABSORPTION OF CARBON DIOXIDE For example, a prominent nasal bridge may prevent a tight fit if Semiclosed and closed systems (i.e., circle and to-­and-­fro) rely the mask’s cuff is flat at that point. A prominent chin (mentum) on chemical absorption of carbon dioxide. Exhaled carbon with sunken alveolar ridge causes a leak at the corner of the Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 102 PART 1 Gases and Ventilation mouth, and the volume of the mask contributes to apparatus selection of mask sizes and styles is more rational than a “one dead space. The mask should fit between the interpupillary line size fits all” approach because a poorly fitting mask can result over the nose and in the groove between the mental process and in trauma to the patient. This is especially true when the mask the alveolar ridge (Fig. 4.2). The average length of this area is must be positioned above the eyebrows because it can cause 85 to 90 mm in adults. The newest disposable plastic masks are pressure on, and possibly damage to, the optic and supraorbital available in a wide range of sizes, intended to fit the faces of nerves. Masks often have a set of prongs for attachment to a small children and large adults equally well. Choosing from a rubber mask holder or head strap; however, if pulled too tight, this mask holder may obstruct the airway. Masks connect to the Y-­piece or elbow via a 22-­mm (⅞-­inch) female connection. BREATHING TUBING The tubing used in breathing circuits typically is approximately 1 meter in length, has a large bore (22 mm) to minimize resistance to gas flow, and has corrugations or spiral reinforcement to permit flexibility without kinking. The internal volume is 400 to 500 mL/m of length. Although these tubes were formerly made of conductive rubber, disposable plastic tubing has almost completely replaced rubber. Electrical conductivity is no longer necessary when breathing tubing is used with nonflammable agents. The advantage of plastic is that it is lightweight; however, it is not biodegradable and thus is disposable by design although not by use. Plastic tubing for a breathing circuit is supplied sterile despite the lack of convincing epidemiologic data to support Fig. 4.1 The modern, clear plastic anesthesia mask. (Courtesy K. Premmer, MD.) the necessity of sterile tubing.27,28 On occasion, it is necessary A B C Fig. 4.2 (A) to (C) The mask’s cushion fits over the nose at the interpupillary line and above the mental process. (Courtesy K. Premmer, MD.) Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  4 Breathing Circuits 103 to pass a breathing circuit on to the sterile surgical field (e.g., UNIDIRECTIONAL VALVES during a laryngectomy or an ex utero intrapartum treatment procedure). By convention, the ends of the tubing are 22 mm Unidirectional valves are incorporated into a breathing circuit in internal diameter (ID) and are identical in design. Tubing to direct respiratory gas flow. They are commonly disks on knife should be inspected before use because manufacturing errors edges or rubber flaps or sleeves. The essential characteristics of can result in obstruction of the lumen.29,30 Compliance of the respiratory valves in breathing circuits are low resistance and tubing varies from nearly 0 to more than 5 mL/m/mm Hg of high competence.35,36 The valves must open widely with little applied pressure, and modern plastic tubing has lower values pressure and must close rapidly and completely with essentially than formerly-­used rubber tubing. Apparent distensibility is no backflow. even greater because compression of gas under pressure, to the Circle and nonrebreathing systems use two nearly identical order of 3% of the volume, occurs at typical inflation pressures. valves: the inspiratory valve opens on inspiration and closes Inflation of a patient’s lungs to 20 cm H2O peak inspiratory on exhalation, preventing backflow of exhaled gas in the pressure compresses 30 to 150 mL of gas in the tubing.31 inspiratory limb. The expiratory valve works in a reciprocal This volume is not delivered to the patient’s lungs, but some fashion to prevent rebreathing. These valves can be mounted fraction of it may be measured by a spirometer within the anywhere within the inspiratory and expiratory limbs of the circuit, adding a form of apparatus dead space to the system. The exact fraction depends on where the spirometer is placed in the circuit with respect to the unidirectional valves. Resistance to gas flow in standard, corrugated breathing tubes is exceedingly small—less than 1 cm H2O/L/min of flow.32 When it is desirable to have the anesthesia machine at some distance from the patient’s head, several tubes may be connected in series with connectors 22 mm (⅞ inch) in outside diameter (OD). Alternatively, extra-­long tubing is available, including tubing that can be compressed to 200 mL of volume in approximately 50 cm of length or that can be stretched to nearly 2 m with an 800-­mL volume. These “concertina” extensions do not increase the resistance of the system by any appreciable amount and affect the apparatus dead space only by A their compliant volume (Fig. 4.3). The pattern of gas flow through the circuit is almost always turbulent because of the corrugations in the tubing, which promote both radial mixing and longitudinal mixing. In documenting performance of one circuit, Spoerel33 demonstrated complete mixing of dead space and alveolar gas after gas had passed through 1 m of such tubing. A change in gas composition at one end, such as when the delivered gas is altered at the anesthesia machine, completes a change in the inspired concentration at the patient connection within two to three breaths. The change in inspired concentration is nearly exactly the change in delivered concentration when high fresh gas flows are used (≥10 L/min). The change decreases to nearly imperceptible as inflow is decreased toward that of closed B systems. Lengths of breathing tubing are sometimes used to connect ventilators to the bag mount and to connect to scavenging devices. Per international standard, either a 19-­or 30-­mm diameter ends on the scavenger fittings prevent inappropriate connections. Tubing of smaller diameter is made for use in circle systems designed specifically for infants and children, and their resistance to gas flow is insignificantly increased. With less compression volume, measured ventilation is more accurate. Reusable rubber tubing is connected to the mask or tube by a separate Y-­piece. Disposable sets often incorporate a Y that may or may not be detachable. Such a Y may be rigid, and it may incorporate an angle elbow or a pair of swivel joints. Although the swivel joints are convenient, they offer a greater chance of leaking; most connectors have negligible leakage, C but those with swivels are twice as likely to leak.34 Any circuit Fig. 4.3 “Concertina” style breathing circuit tubes can be either should be tested before use by determining the oxygen inflow compressed (A and B) or stretched (C) to change in length and volume required to maintain 30 cm H2O of pressure in the circuit (see without significantly affecting apparatus dead space. (Courtesy King also Chapter 25). Systems, Noblesville, IN.) Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 104 PART 1 Gases and Ventilation circuit. The only critical feature of their location is that one must be placed between the patient and the reservoir bag in each limb. Properly positioned and functioning, they prevent any part of the circle system from contributing to apparatus dead space.37 Thus, the only apparatus dead space in such a circuit is the distal limb of the Y-­connector and any tube or mask between it and the patient’s airway. The respiratory valves on most modern anesthesia machines are located near, or incorporated into, the carbon dioxide absorber canister casing along with a fresh gas inflow site and excess gas (pop-­off) valve. In the past, unidirectional valves have been incorporated into the housing of the Y-­piece to decrease the apparatus dead space A effect of compliance volume, but they have fallen into disfavor because of the weight they add to the mask. More importantly, they cause an obstruction to respiration if they are accidentally incorporated backward to the conventional valves in the circle.38 When valved Y-­pieces were used, it was recommended that circle system valves be removed. Failure to reinsert the circle system valves when a normal (nonvalved) Y-­piece was used has caused needless complications. The common valves in anesthetic circuits are dome valves consisting of a circular knife edge occluded by a very light rigid disk of slightly larger diameter (Fig. 4.4A,B). The disk lifts off the knife edge when flow is initiated by the patient’s inspiratory effort, when positive pressure is applied to the reservoir bag, or when the ventilator bellows empties. The disk B is contained either by a small cage or by the dome itself. It must be hydrophobic so that water condensation does not cause it to stick to the knife edge and thereby increase the resistance to opening. Most modern disks are made of hydrophobic plastic and are light and thin. When properly functioning, the disk in a unidirectional valve can be lifted with a circuit pressure of 0.31 cm H2O or less. Most unidirectional valves are mounted vertically, with the disk oriented horizontally, so that it will fall properly into the closed position and seal the circuit from backflow. The valve disks also can be oriented vertically, as on the absorber block of the GE-Datex-Ohmeda S5/ADU workstation (GE Healthcare, Waukesha, WI; see Fig. C 4.4D). Failure to seal converts a large volume of the circuit into apparatus dead space, resulting in rebreathing. The top of the valve is covered by a removable clear plastic dome so that the disk can be easily seen and periodically cleaned or replaced. A nonrebreathing system requires two appropriately placed one-­way respiratory valves (Fig. 4.5). Nonrebreathing valves permit the patient to inspire fresh gas from a reservoir and exhale alveolar gas into the room or into a scavenger. Such valves usually consist of a pair of leaflets in the same housing: one opens during inspiration, the other opens during exhalation. The early nonrebreathing valve designs, such as the Digby-­ D Leigh or Steven-­Slater, required the anesthesiologist to occlude Fig. 4.4 (A) Typical dome valve incorporated into a circle absorber the expiratory valve with a finger if assisted or controlled housing. The valve is in the open position with gas flowing. (B) Be- ventilation was needed (see Fig. 4.5A).9,24 Modern designs that cause of backpressure, the plastic disk seats on the knife edge and the use springs, magnets, or flaps automatically close the expiratory valve is closed. (C) One-­way valves on the Datex-­Ohmeda machine valve when respiration is controlled.39–44 Other designs use (GE Healthcare, Waukesha, WI). (D) Datex-­Ohmeda ADU absorber the pressure difference across the inspiratory valve to inflate block showing unidirectional valves mounted vertically. (Courtesy K. Premmer, MD.) a mushroom-­shaped balloon (Frumin) valve (see Fig. 4.5B),11 or to depress a dome-­shaped cover on the expiratory (Fink) valve.6 Resistance is negligible in both designs, but the Frumin vertically oriented to function properly.45 Those that use flexible valve has the marked advantage of collapsing if the inspiratory rubber leaflets or collapsible rubber tubing to provide the supply is inadequate, permitting inspiration of room air. The sealing function are not positional. Most nonrebreathing valves Frumin valve also is lighter and more compact than the others. connect to masks and/or tracheal tubes, but a valve can be built Some nonrebreathing valves are position sensitive and must be into a mask.44 Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  4 Breathing Circuits 105 Expiratory leaflet Fig. 4.6 The Ruben nonrebreathing valve (left) and Ambu E-­2 valve (right). (Courtesy the Sheffield Department of Anaesthesia Museum, Sheffield, United Kingdom.) jam.47 Such resuscitator valves should not be used in anesthesia, nor should they be used for transporting patients who are still exhaling anesthetic agents. BREATHING BAGS Inspiratory leaflet Breathing bags, also known as reservoir bags or counterlungs, have three principal functions: (1) they serve as a reservoir for anesthetic gases or oxygen, from which the patient can inspire; A To patient (2) they provide the means for a visual assessment of the existence and rough estimate of the volume of ventilation; and (3) they serve as a means for manual ventilation. A reservoir Expiratory function is necessary because anesthesia machines cannot mushroom valve provide the peak inspiratory gas flow needed during normal spontaneous inspiration solely from fresh gas flow. Although the respiratory minute volume of an anesthetized adult is rarely PB more than 12 L/min, the peak inspiratory flow rate may reach 50 L/min, with 20 L/min not uncommon. For example, assume P2 a patient is breathing at a rate of 20 breaths/min with a tidal volume of 500 mL and a minute volume of 10 L/min. If the inspiratory-­to-­expiratory ratio (I:E) is 1:2, each breath takes 1 second for inspiration and 2 seconds for exhalation. The tidal P1 volume of 500 mL inspired in 1 second is an average inspiratory flow (volume per unit time) of 500 mL/sec or 30 L/min. This is many times greater than the commonly used fresh gas flows. The peak flow in mid-­inspiration may be 30% to 40% higher. Assessment of the presence and volume of spontaneous ventilation is affected by the fresh gas flow. In low-­ flow techniques, virtually all the gas inhaled by the patient comes Inspiratory from the reservoir bag, and its excursion thus reflects tidal leaflet volume. If the fresh gas inflow rate from the machine exceeds 10 L/min, most of the gas inhaled by the patient comes from the B To patient fresh gas supply, and the reservoir bag shows little excursion. Fig. 4.5 These nonrebreathing valves incorporate two leaflets that open In a spontaneously breathing patient with a circuit gas inflow alternately on inspiration or exhalation. (A) In the simplest form, the valve rate of 6 L/min, nearly half the tidal volume comes from the functions well during spontaneous ventilation (solid arrows), but an attempt fresh gas inflow, halving the apparent tidal volume as indicated to inflate the patient’s lungs manually blows open both inspiratory and ex- piratory leaflets (dotted arrow) unless the anesthesiologist simultaneously by movement of the bag. occludes the expiratory valve with a finger. Several nonrebreathing valves Reservoir bags for anesthesia machines usually are ellipsoid have been designed to overcome the necessity for manual assistance of so they can be easily grasped with one hand. They are made of valve function. (B) Whenever gas flow opens the inspiratory leaflet, the pres- nonslippery plastic or latex in sizes from 0.5 to 6 L. To improve sure at point P1 is greater than at point P2 or PB. This pressure difference inflates the mushroom-­shaped expiratory balloon, sealing the expiratory grip, some have an hourglass shape or a textured surface; limb. If no gas is supplied to the inspiratory limb, spontaneous effort on the nonlatex bags are available for use with patients who have a part of the patient lowers both P1 and P2 well below atmospheric pressure latex sensitivity. The optimally sized bag can hold a volume that (PB) so that the mushroom valve collapses and the patient inspires room air. exceeds the patient’s inspiratory capacity; that is, a spontaneous deep breath should not empty the bag. For most adults, a 3-­L Self-­inflating resuscitators for air or air-­oxygen mixtures use bag meets these requirements and is easy to grasp. Bags with similar pairs of valves to control gas flow.10,46,47 The Ruben valve a nipple at the bottom for use as an alternate pop-­off site are has an expiratory bobbin-­shaped structure that, when open, available but are rarely used. occludes the inspiratory limb (Fig. 4.6). Anesthetic vapors and In circle systems, the reservoir bag usually is mounted at secretions tend to expand this bobbin slightly, causing it to or near the carbon dioxide absorbent canister via a T-­shaped Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 106 PART 1 Gases and Ventilation ventilation. It can accurately deliver tidal volumes ranging from 20–2000 mL. 30 GAS INFLOW AND POP-­OFF VALVES Pressure (mm Hg) 20 Gases are delivered from the anesthesia machine common gas outlet to the circuit via thick-­walled tubing connected to a nipple incorporated into the circuit. In circle systems this gas inflow nipple is incorporated with the inspiratory unidirectional valve 10 or the carbon dioxide-­absorbent canister housing. The preferred fresh gas inflow site is between the carbon dioxide absorber and the inspiratory valve. The location for other circuits depends 0 on whether breathing is spontaneous, assisted, or controlled 0 3 6 9 12 15 because the type of breathing influences the efficiency of carbon Volume (L) dioxide elimination. Fig. 4.7 As an anesthesia reservoir bag is filled from its evacuated vol- Pop-­off valves—also known as overflow, outflow, relief, spill, ume to its nominal volume, the pressure increases little; as the rubber is or adjustable pressure-­limiting (APL) valves—permit gas to leave slightly stretched, however, a small increase in volume rapidly raises the the circuit, matching the excess to the inflow of fresh gas. The pressure to some maximum, depending on the shape and wall thickness of the bag. Further increase in the bag’s volume causes a decrease in efficiency of an APL valve is related in part to the placement of pressure. The falling pressure with rising volume follows Laplace’s law: the fresh gas inflow. There are many different designs, but most P = 2T/r, where P is pressure, the constant T is a function of the bag’s are constructed like a dome valve loaded by a spring and screw thickness and material, and r is the radius. cap (Fig. 4.9). The valve should open at a pressure of less than 1 cm H2O. As the screw cap is tightened down, more and more gas pressure in the circuit is required to open it, permitting PEEP fitting, usually near the pop-­off valve. The bag also may be during spontaneous ventilation or pressure-­limited controlled placed at the end of a length of corrugated tubing leading respiration. The number of clockwise turns from fully open to from the T-­connector to provide some freedom of movement fully closed should be one or two: fewer turns make it difficult for the anesthesiologist. The pressure-­volume characteristics to set a desired circuit pressure accurately, whereas more make of overinflated bags become important if the pop-­off valve it tedious to use. The exhaust from any of the commonly used is accidentally left in the closed position and gas inflow pop-­off valves can be collected by scavenging system transfer continues (Fig. 4.7). Rubber bags become pressure limiting with tubing connected at this point.51 maximum pressures of 40 to 50 cm H2O, although prestretching The GE-Datex-Ohmeda GMS absorber uses an APL valve may favorably lower the maximum distending pressure.48–50 similar in design to that shown in Fig. 4.9A, which basically is a Disposable bags may reach twice the pressure of rubber bags spring-­loaded disk. When the spring is fully extended, it exerts and then rupture abruptly. a pressure of approximately 1 cm H2O on the disk to hold the valve closed. This is necessary because the waste gas scavenging VOLUME REFLECTOR interface is connected downstream of the APL valve and transfer tubing. If an active scavenging system is used—that is, if The Maquet FLOW-i anesthesia workstation (Getinge United suction is applied to the interface—the negative pressure could States, Wayne, NJ) has a volume reflector integrated into the potentially be applied to the patient circuit (see Chapter 5). To circle system. The volume reflector is a rebreathing device prevent this, the GE-Datex-Ohmeda scavenger interface uses a that consists of a rigid reservoir and is based on the servo ICU negative-­pressure relief (“pop-­in”) valve that opens at a pressure ventilator platform. It replaces the ‘bag-­in-­bottle’ bellows and of −0.25 cm H2O to allow room air to enter the interface. Thus piston used in traditional circle systems, hence it has no moving the greatest negative pressure needed to open the APL valve parts. It has a volume of 1.2 L and is open at both ends. The total (−0.25 cm) is less than the least spring tension needed to keep volume of the FLOW-i breathing system is 2.9 L. The exhaled gas the valve closed (∼1 cm H2O). This arrangement, with the use mixture from the patient is introduced at one end of the volume of an active scavenging system, protects against application of reflector. During automatic ventilation, the stored gas mixture excess negative pressure to the breathing circuit. In the fully in the volume reflector is returned to the patient by applying a closed position, the maximum spring pressure applied to the flow of oxygen from the reflector gas module. Thus, the driving GE-Datex-Ohmeda APL valve disk is 75 cm H2O. Thus, in the gas pillar virtually moves back and forth in the reflector (Fig. manual/bag mode, the circuit pressure in a GE-Datex-Ohmeda 4.8). The amount of exhaled gas returning to the patient via the breathing system is limited to 75 cm H2O. Note that in the CO2 absorber is determined by the ratio between the minute ventilator mode, the circuit pressure is limited by high pressure-­ volume and the set fresh gas flow. There is no physical barrier limit settings on the GE-Datex-Ohmeda ventilator (up to 100 between the driving gas and the exhaled gas. The design ensures cm H2O with the GE-Datex-Ohmeda 7800 and 7900 ventilators; minimal mixing between the exhaled gas and the oxygen in the see Chapter 6). volume reflector. Oxygen-­driven by design, the volume reflector In Dräger Medical (Telford, PA) anesthesia delivery systems, helps to minimize the risk for delivering a potentially dangerous the design of the APL valve differs from those described earlier hypoxic mixture in the presence of leaks during low-­ flow (see Fig. 4.9B,C). This design uses a needle valve instead of a anesthesia. An automatic compensatory increase of oxygen flow spring-­loaded disk, and adjusting the knob varies the size of the from the reflector gas module will occur if the system detects a opening between the needle valve and its seat, which in turn low oxygen concentration at the Y-­piece. The volume reflector adjusts the amount of gas permitted to flow to the scavenger cannot be emptied, thus guaranteeing continuous uninterrupted system. A check valve prevents gas from the scavenging system Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  4 Breathing Circuits 107 Fig. 4.8 The Maquet FLOW-i Volume Reflector. FA, Alveolar concentration; FD, delivered concentration; FI, inspired concentration. See text for explanation. (From “Hypoxic guard systems in anesthesia systems,” Critical Care News, http://www.criticalcarenews.com/clinical-­articles/hypoxic-­ guard-­systems/. Accessed December 7, 2019). from entering the breathing system. With this design, the needle CO2 + H2 O → H2 CO3 (4.1) valve can be totally closed; it therefore does not function as a true pressure limiter. H2 CO3 + 2NaOH → Na2 CO3 + 2H2 O(4.2) Special types of pop-­ off valves permit spontaneous or H2 CO3 + 2KOH → K2 CO3 + 2H2 O  assisted respiration without tedious adjustment.40,52–54 The (4.3) simplest is the Steen valve (Fig. 4.10), which essentially is two Na2 CO3 + Ca(OH)2 → 2NaOH + CaCO3 knife-­edge valves of the dome type, one inverted over the other, (or K2 CO3 ) (or 2KOH)(4.4) that share a common disk.24 A relatively slow flow of gas during the latter part of exhalation, up to 10 L/min, lifts the valve disk In barium hydroxide lime—or Baralyme (Allied Healthcare, at one side only so that the exhaled gas escapes around the disk. St. Louis, MO), which is no longer being produced (see Chapter An abrupt increase in pressure lifts the valve vertically, seals it 23)—Ba(OH)2 replaces the NaOH and KOH in equations 4.2, against the upper knife edge, and closes the circuit so that no gas 4.3, and 4.4, with BaCO3 the product. is lost. The Georgia valve adds a light spring loading to the same Wet soda lime is composed of calcium hydroxide (∼80%), design, which increases the range of gas flows it can exhaust; sodium hydroxide and potassium hydroxide (∼5%), water this is necessary for use with mechanical ventilators.55 Most (∼15%), and small amounts of inert substances such as silica current anesthesia ventilators have such an automatic pop-­off and clay for hardness. The potassium hydroxide and sodium valve built in so that gas is exhausted only at end exhalation (see hydroxide function somewhat like a catalyst to speed the initial also Chapter 6). reaction, forming sodium and potassium carbonates. The sodium and potassium carbonates react over the course of minutes with the calcium hydroxide to form calcium carbonate and water, Carbon Dioxide Absorption regenerating sodium and potassium hydroxides. Soda lime is In partial rebreathing and nonrebreathing systems, carbon exhausted when all the hydroxides have become carbonates. Soda dioxide is vented to room air. When a closed system is used, lime can absorb 19% of its weight in carbon dioxide;5 thus 100 g however, the exhaled carbon dioxide must be otherwise of soda lime can absorb approximately 26 L of carbon dioxide. removed. Carbon dioxide in the presence of water is hydrated A novel carbon dioxide absorbent was created in 1999. Calcium to form carbonic acid. When carbonic acid reacts with a metal hydroxide lime (Amsorb, Armstrong Medical Ltd., Coleraine, hydroxide, the reaction is one of neutralization that results in Northern Ireland, U.K.) is composed of calcium hydroxide (70%); the formation of water and a metal bicarbonate or carbonate a compatible humectant, calcium chloride (0.7%); and two setting and the generation of heat. This reaction is used in anesthesia for agents, calcium sulfate (0.7%) and polyvinylpyrrolidone (0.7%) carbon dioxide absorption.56 In the reactions shown below, only (to improve hardness and porosity); and water (14.5%).57 By the molecular forms of the reactants are written. The reactions adding calcium chloride as a humectant, the calcium hydroxide actually proceed by initial ionization in the thin film of water at remains damp and eliminates the need for sodium or potassium the surfaces of the absorbent. In soda lime: hydroxide. With removal of the strong alkali, calcium hydroxide Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 108 PART 1 Gases and Ventilation Adjusting knob 19-mm taper Leaflet Spring Needle valve Screw cap seat Check valve Needle valve Knife edges A B Lock ring C Fig. 4.9 Adjustable pressure-­limiting (APL) or “pop-­off” valves. (A) Spring-­loaded design. When the cap is fully tightened down, the spring is com- pressed enough to prevent the valve leaflet from lifting at any airway pressure. When the top is loosened and the spring is not compressed, the valve opens at a pressure equal to the weight of the leaflet divided by its area, usually isoflurane) > Ca(OH)2 B T H 2O AGT Baralyme Fig. 4.12 The degradation of volatile anesthetics by carbon dioxide absorbents. (A) Production of compound A from sevoflurane is promoted by warmer, drier absorbent and by higher concentrations of agent (AGT) and lower fresh gas flows (FGF). In the presence of water, sevoflurane produces methanol, which promotes the breakdown of compound A into compound B and other low-­toxicity products C, D, and E. (B) The phenomenon of carbon monoxide (CO) production from the difluoromethyl ethers (desflurane, enflurane, and isoflurane) is often the result of prolonged high gas flows, which dry out the absorbent. Higher temperatures (T) and agent concentrations increase CO production in this setting, and the use of barium hydroxide lime (Baralyme) results in more CO production than the use of soda lime. TABLE 4.1 Composition of CO2 Absorbents Commonly Used in Contemporary Anesthesia Systems Amsorb Plus Sodasorb (Grace (Armstrong Medical Litholyme (Allied Discovery Medisorb (GE Drägersorb 800 Ltd., Coleraine, Healthcare Spiralith Sciences, Healthcare, Plus (Draeger, Northern Ireland, Products, St. (Micropore, Elkton, Composition Deerfield IL) Chicago, IL) Telford, PA) U.K.) Louis, MO) MD) Ca(OH)2 76.5% 81% 82% >75% >75% 0% NaOH 2.25% 1%–2% 2% 0% 0% 0% KOH 2.25% 0.003% 0.003% 0% 0% 0% LiOH 0% 0% 0% 0% 0% 95% LiCl 0% 0% 0% 0% Mixed Expired > Inspired and excretion of carbon dioxide. Oxygen concentration in mixed exhaled gas usually is 4% or 5% lower than that in inspired gas. PA CO2 > PE CO2 > PI CO2 Although oxygen uptake remains relatively constant during The most efficient configurations place the pop-­off valve anesthesia (approximately 250 mL/min standard temperature where the highest concentration of carbon dioxide is found and pressure dry [STPD]) in the average adult, provided that during the phase of breathing in which the circuit pressure is Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  4 Breathing Circuits 115 FGF TABLE 4.2 Spectrum of Carbon Dioxide Elimination A Maximum Carbon Maximum Carbon Dioxide Elimination To Dioxide Retention No mixing of fresh Complete mixing of No mixing of fresh and alveolar fresh and alveolar and alveolar B gas occurs. gas occurs. gas occurs. FGF Fresh gas goes to The mixture is Alveolar gas is the patient. inhaled. rebreathed. Alveolar gas goes Fresh gas and Fresh gas goes FGF to the pop-­off mixed gas are to the pop-­off valve. “popped off” valve. FGF C simultaneously. DESIRABLE ACCEPTABLE UNDESIRABLE C FGF 2. Model studies embody nonphysiologic states, particularly D lack of responsiveness to carbon dioxide and simplified flow patterns. 3. Studies were done in awake volunteers, whose metabolic FGF rates and physiologic responses differ from those of anes- E thetized patients. 4. Imprecise endpoints were used, such as rebreathing of car- bon dioxide identified by capnography rather than by an in- FGF F crease in alveolar or arterial carbon dioxide concentration. However, consensus has been reached in one regard: systems classified as Mapleson A (Magill attachment, Lack, and Humphrey A) are most efficient for spontaneous, unassisted ventilation, and those classified as Mapleson D, E, or F (Jackson-­ Fig. 4.15 Mapleson classification of breathing systems. Note that the semiclosed systems (top four) contain most of the components of a Rees, Bain, Humphrey DE) are most efficient for assisted or circle system: tubing, connectors, bag, fresh gas inflow (FGF), and pop-­ controlled ventilation. off site. They lack carbon dioxide absorbers because carbon dioxide is A general criticism of the published analyses of breathing lowered by the addition of fresh gas and elimination of carbon dioxide– circuits is a failure to distinguish between the quantity of re-­ rich gas preferentially through the pop-­off valve. They also lack separate inspiratory and expiratory limbs; one tubing serves both purposes. The inspired carbon dioxide and minimum inspired carbon dioxide Mapleson A system (Magill attachment) is optimal for spontaneous res- tension. The most common tool, a capnograph, displays a signal of piration. The Mapleson C system is a simple bag and mask. Moving the airway concentration as a function of time. Thus, if airway carbon pop-­off to the bag tail is a major improvement (Cʹ), because this permits dioxide falls slowly with inspiration, just reaching zero near more mixing of fresh and exhaled gas than the Mapleson C (this modi- end inspiration, it is interpreted as no rebreathing, even though fication, not one of Mapleson’s, has been added by the author). The (B) circuit is wasteful of fresh gas in both spontaneous and controlled res- a significant amount of carbon dioxide has been re-­inspired. piration. The (D) circuit is similar to the (A) except that it exchanges the Conversely, if airway carbon dioxide falls to a low but nonzero inflow and pop-­off sites. (A) is optimal for spontaneous breathing, and concentration for all of inspiration, carbon dioxide excretion may (D) is best for controlled breathing. The Mapleson E system (E) is essen- be adequate despite perceived rebreathing if total ventilation is tially an Ayre’s T-­piece with an added reservoir. If the reservoir is short, it is an open, not a semiclosed, system. This system is simple but lacks increased. Inspired carbon dioxide is properly calculated as the the convenience of a bag for ventilatory assistance or control. A bag can integral of instantaneous flow multiplied by instantaneous carbon be added to it ([F], or Jackson-­Rees modification), which may or may not dioxide concentration, which is difficult or impossible to measure possess an adjustable pop-­off valve to help assist or control ventilation. with simple instrumentation. (See also sections on volumetric All these circuits share a common advantage. Vigorous hyperventilation capnography in Chapters 8 and 10.) A better analysis is based on cannot reduce the patient’s carbon dioxide tension much below normal if FGF is kept between one and two times the patient’s normal respira- the equation of defining alveolar ventilation (V̇A ): tory minute volume. V̇CO2 V̇A = (4.5) FA CO2 above atmospheric pressure. This occurs at end-­ exhalation V̇A = V̇E − (VDS × f)(4.6) during spontaneous breathing and during inspiration with manually assisted or controlled breathing. Where f = frequency in breaths per minute A great deal of attention has been devoted to determining Equation 4.5 states that alveolar ventilation is the quotient of the lowest gas flow that can be safely used in clinical anesthesia. carbon dioxide production and alveolar fractional concentration of A variety of claims have been made for the various circuits and carbon dioxide. Because the fractional volume of alveolar carbon for proprietary modifications of the circuits, such as Bain, Lack, dioxide (FAco2) is proportional to the partial pressure of alveolar Humphrey ADE, and Mera F circuits. Unfortunately, much of carbon dioxide (PAco2), a specific Pco2 defines one and only one the published work has one or more of the following flaws: V̇A in a given patient. Equation 4.6 demonstrates that any increase 1.  Theoretical analyses embody unrealistic assumptions in dead space ventilation ( VDS × f ) can be accommodated by about mixing and breathing patterns, especially the I:E an equivalent increase in minute volume (V̇E), keeping alveolar ratio and expiratory flow. ventilation, and hence carbon dioxide elimination, constant. Any Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 116 PART 1 Gases and Ventilation FGF End inspiration 2700 mL 2400 mL FGF ½ Second of 3000 mL Fig. 4.16 Mapleson A circuit, spontaneous breath- exhalation ing. Stippled areas indicate carbon dioxide-­containing gas. Top, The end of a normal spontaneous inspiration for a normal adult patient. As the patient begins to ex- 2150 mL hale, carbon dioxide-­free gas flows from the upper dead FGF space, then carbon dioxide-­rich gas flows into the corru- gated tube and, together with continuing fresh gas flow End exhalation 3000 mL (FGF), fills the bag a half second later (middle). The rest of the expirate goes out through the pop-­off valve, along with carbon dioxide-­containing gas in the tubing, which is pushed toward the pop-­off valve by the fresh gas flow. 2000 mL Optimally, at end exhalation (bottom), the circuit has largely been flushed of carbon dioxide. amount of carbon dioxide may be rebreathed, at a concentration with a rate of 20 breaths/min, tidal volume (VT) of 400 mL, I:E equal to or below alveolar carbon dioxide, if an increase in minute ratio of 1:2, a sinusoidal inspiratory flow averaging 24 L/min, a volume maintains alveolar ventilation. near exponential expiratory flow with a half-­time of less than Even if instantaneous Pco2 reaches zero near end-­ 0.5 second, a functional residual capacity (FRC) of 2400 mL, inspiration, a significant volume of carbon dioxide may be a fresh gas flow of 6 L/min, a bag of 3 L nominal volume at rebreathed, requiring an appropriate increase in minute volume. the pop-­off valve opening pressure, and a corrugated tube of A numerical example may help clarify this. Consider a patient 500 mL volume. The top diagram shows the condition at end in need of 4 L/min of V̇A with FAco2 of 0.05 (5%) and a current inspiration, after the lung has inspired 400 mL over 1 second, V̇E of 6 L/min with a respiratory rate (f) of 20 breaths/min. The consisting of 100 mL of fresh gas flow and 300 mL from the patient could rebreathe 2.5% carbon dioxide and keep FAco2 at circuit. All of the circuit has been flushed with fresh gas, and 5% if the apparent alveolar ventilation doubled to 8 L/min. If carbon dioxide is found only in alveolar gas (stippled area). In the dead space ventilation did not change—it probably would, the first 0.5 second of exhalation, 250 mL of gas are exhaled and, but not much, depending on whether tidal volume or f were together with 50 mL of fresh gas, have distended the reservoir increased—a total minute volume of 10 L would suffice. Clearly, bag to 3 L and have just opened the pop-­off valve (see Fig. 4.16). an inspired carbon dioxide load can be compensated for. Note Carbon dioxide–containing alveolar gas has penetrated partway that the ventilatory response to carbon dioxide of an awake down the breathing tube, but the exact distance depends on (1) person (slope of 2 L/min/mm Hg) would require a rise of less the dead space; (2) the shape and volume of zones I and II of the than 2 mm Hg PAco2. However, at a sensitivity to carbon dioxide capnogram (see Chapter 10 for capnogram zones); and (3) the frequently seen in an anesthetized person (e.g., 0.5 L/min/mm longitudinal mixing, or conical flow pattern, in the tube. In the Hg), the carbon dioxide tension would have to rise nearly 8 next 1.5 seconds of exhalation, the rest of the expired alveolar mm Hg. Thus, a significant difference is apparent between gas (150 mL) has exited the pop-­off valve, and 150 mL of fresh spontaneous breathing, for which ventilation is set by the gas has flushed the carbon dioxide–containing expirate in the patient’s PAco2 and responsiveness, and controlled ventilation, breathing tube back and out through the pop-­off valve. If the for which minute volume is set by the anesthesiologist. sum of the fresh gas flow (150 mL) and the carbon dioxide-­free Any gas mixture that contains carbon dioxide can be dead space gas from zone I exceeds the penetration of zone II considered to consist of a fraction of carbon dioxide-­free gas and alveolar gas, the situation at the end of exhalation, as shown and a fraction of alveolar gas. Any rebreathing of alveolar gas in the bottom diagram of Fig. 4.16, is the result. This generally is simply added dead space and can be compensated for by is true when fresh gas flow exceeds 55% of the respiratory increasing overall ventilation. For example, given 4 L/min of minute volume.106,107 In this particular model, about 100 mL alveolar ventilation and 2 L/min of dead space ventilation, what of fresh gas exits the pop-­off valve with each breath along with will happen if a patient suddenly inspires 1% carbon dioxide? the carbon dioxide–containing alveolar expirate. In studies Each unit of alveolar ventilation now holds only four-­fifths of anesthetized patients, the fresh gas flow that maintains of the previous level of newly produced carbon dioxide, so carbon dioxide homeostasis in Mapleson A circuits used with increasing alveolar ventilation by 25% will result in the same spontaneous breathing has been found to be 70% to 100% of the degree of carbon dioxide elimination. minute volume, depending on the many variables.107–109 During assisted or controlled ventilation, two different Mapleson A Configurations and things happen to decrease efficiency. First, the bag must be Carbon Dioxide Removal squeezed during inspiration, both to deliver the entire tidal Consider first the Mapleson A circuit shown in Fig. 4.16. The volume (400 mL) and to vent the fresh gas flow that comes in assumptions for this model include spontaneous breathing over an entire respiratory cycle (in this case, 1/20 of 6 L/min, Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  4 Breathing Circuits 117 FGF End inspiration 1600 mL 3% 3000 mL—5% Fig. 4.17 Mapleson D circuit, controlled breathing. At end inspiration, the anesthesiologist has squeezed the FGF bag from its nominal volume of 3 L down to 1600 mL. Of 2700 mL 3% this amount, 800 mL went into the patient’s lungs along Mid exhalation with 200 mL of fresh gas; 600 mL of the bag’s contents, with about 3% carbon dioxide, left the circuit through 2150 mL—5.5% the pop-­off valve (top). During the next 2 seconds, the patient exhales nearly all of the tidal volume (900 mL); this, with the continuing fresh gas flow (FGF), fills the bag FGF (middle). Because of the fresh gas flow, the bag’s carbon 3000 mL 3% dioxide concentration content is diluted below alveolar End exhalation gas. Furthermore, the patient’s expiratory flow diminish- es toward the end-­expiratory pause, and the fresh gas 2000 mL—6% flows into the patient end of the circuit. This is the gas that will enter the patient’s lungs first on the next inspira- tion (bottom). or 300 mL). Now all the exhaled tidal volume flows into show normal carbon dioxide homeostasis with a fresh gas the breathing tubing, followed by the continued fresh gas flow of 70% of total minute ventilation in Mapleson D circuits flow during the end-­ expiratory pause. During the next during controlled breathing, if minute volume is 150 mL/kg or compression, some alveolar gas may reenter the airway until greater.12,110 the circuit pressure rises to the threshold of the pop-­off valve. Thereafter, some carbon dioxide and some fresh gas go both to PROPRIETARY SEMICLOSED SYSTEMS the lung and to the pop-­off valve. Understandably, the effect would depend on the rate of compression of the bag, that is, Although a variety of pieces of anesthesia hardware can be used the inspiratory flow, lung and chest wall compliance, airway to assemble Mapleson circuits A through F, several specific resistance, volume of dead space, I:E ratio, and fresh gas flow. circuits with eponymous identities have been introduced that Thus, during assisted ventilation, the Mapleson A circuit is far offer specific advantages. These include the Jackson-­Rees, Bain, less efficient than during spontaneous ventilation in terms of Lack, Mera F, and Humphrey ADE. The last four are conveniently preventing rebreathing. coaxial; they have a tube-­ within-­ a-­tube arrangement that moves the physical location of the fresh gas inflow and/or the Mapleson D Configurations and expiratory valve away from the patient connection elbow while Carbon Dioxide Removal preserving the advantages of the A, D, or F circuits. The Bain A typical circuit for controlled ventilation is shown in Fig. 4.17. and Lack circuits are shown in Fig. 4.18. The assumptions for this model are (1) a fresh gas flow of 6 L/ The Bain circuit is basically a coaxial Mapleson D design. min, (2) a minute volume of 10 L, using a VT of 1000 mL and Instead of a separate small-­bore tube for delivery of fresh gas respiratory rate of 10, (3) an FRC of 2000 mL, (4) an expiratory to the patient elbow, the delivery tube enters the corrugated flow nearly exponential with a 0.5-­second half-time, (5) an I:E expiratory tube near the bag mount and pop-­off valve and runs ratio of 1:2, and (6) a peak inspiratory gas flow rate of 30 L/ coaxially to the patient end, where the end is secured by a plastic min. At inspiration the bag is squeezed to deliver 1000 mL to the “spider” in the center of the tube. Thus fresh gas is delivered at patient in 2 seconds and to blow 600 mL out the pop-­off valve. the patient end, and the pop-­off valve exhausts gas at the bag A total of 200 mL of fresh gas entered in these 2 seconds, which end of the corrugated tube, a Mapleson D arrangement. Various results in the state shown in the top panel of Fig. 4.17. recommendations for fresh gas flow have been published. One Two seconds after exhalation begins, the patient has exhaled commercial brand has a package insert that recommends a 900 mL, which is diluted with 200 mL of fresh gas as it enters the fresh gas flow of 100 mL/kg/min. Such recommendations circuit. This has refilled the bag to 2700 mL. In the next 2 seconds often were based on an instantaneous inspired carbon of exhalation, the rest of the tidal volume, 100 mL, and 200 mL dioxide concentration of zero for some portion of the cycle. more of fresh gas have filled the breathing tubing, and the bag However, with suitably augmented minute ventilation—150 has regained its initial volume of 3 L. The lungs now contain 6% mL/kg or more, instead of the 90 mL/kg for a normal person carbon dioxide because this gas has been slowly increasing as a at rest—adequate carbon dioxide elimination results from a result of continued carbon dioxide delivery to a progressively fresh gas flow of 70 mL/kg/min during assisted or controlled smaller alveolar volume. The bag contains 3% carbon dioxide, ventilation.12,33,110 Although not recommended for prolonged but in the breathing tubing the concentration falls toward zero, periods, spontaneous ventilation requires a greater fresh gas the fresh gas fraction of inspired carbon dioxide (FiCO2). In fact, flow, up to 150 mL/kg/min.25,111,112 if two-­thirds of the fresh gas flow is washed into the lungs, it will The Bain circuit may malfunction if the central tube (fresh provide 4 L/min of carbon dioxide–free alveolar ventilation, gas delivery) becomes disconnected, either where it enters the and the PAco2 will be normal despite obvious rebreathing of corrugated outer tube or from its retaining spider at the patient some carbon dioxide. In fact, studies of anesthetized patients end. Either disconnection effectively increases the apparatus Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 118 PART 1 Gases and Ventilation Mapleson A Pop-off Mapleson A Inflow during the subsequent inspiration because the expiratory valve Mapleson D Inflow Mapleson D Pop-off closes, and little gas flows backward in the inner (expiratory) tubing. Reports that the Lack system is more efficient, equally Magill efficient, or less efficient than a Magill attachment may be found in the literature, but the differences are always slight.108,116,117 Fresh gas flow at 70% of minute volume results in negligible carbon dioxide rebreathing. It is important to note that with spontaneous breathing, increasing fresh gas flow is of little value Bain in lowering arterial Pco2, which is set by the patient’s intrinsic FGF respiratory control centers. The Mera F system, introduced in Japan in 1978, uses a modern circle canister-­valve assembly as the mounting for a Lack coaxial Bain-­type circuit. Byrick and colleagues112 found that at 100 mL/kg/min fresh gas flow, the Mera F functioned as well FGF as the standard Bain circuit. During controlled respiration, FGF this flow kept PAco2 at 45 ± 9 mm Hg during light anesthesia, despite a lower minute volume, because of a slightly improved Semiopen capture of dead space gas for re-­inspiration. The Universal F system (King Systems, Noblesville, IN) is essentially equivalent in design and function to the Japanese Mera F and is based on the same patents, but it is manufactured Fig. 4.18 Comparison of the Mapleson A and D circuits with the Bain, and available in the United States (Fig. 4.19A,B). It also uses Lack, and semiopen circuits. From the top, A schematic of the Maple- a coaxial arrangement and mounts on standard circle system son A and D circuits with an indication for placement of the inflow and hardware. Differences from the Mera F include larger diameter pop-­off valves that distinguish A from D. The Bain circuit, shown at end expiration, uses a small-­bore fresh gas delivery tube to deliver fresh gas inspiratory and expiratory tubing to decrease resistance, to the patient end of the circuit. The Lack circuit looks similar externally, corrugation of the inspiratory tube to prevent kinking, and the but the inner tube is now an expiratory limb that delivers exhaled gas ability to be used as a transport system. When used with circle to the pop-­off valve at the bag end. In a semiopen circuit, the breathing system hardware, the Universal F system is functionally no tube is inspiratory and all exhaled gas exits at the valve. Only fresh gas is found in the tubing. FGF, Fresh gas flow. different than a standard circle system circuit, with the exception that the inspiratory limb is enclosed by the expiratory limb for the purpose of uncluttering the tubing apparatus and providing dead space, and for any given minute volume, it reduces alveolar improved humidification of inspired gas. For patient transport, ventilation accordingly.113 Disconnection at the bag end is by the Universal F system can be converted to a Bain circuit by the far the more serious problem, and visual inspection alone may attachment of an oxygen source to the inspiratory limb and a not identify the problem. Because of this, two tests have been reservoir bag with a pop-­off valve to the expiratory limb (Fig. proposed: one uses a very low oxygen flow (50 mL/min) and 4.19C,D). As with a standard Bain circuit, it is important that occlusion of the inner tube with a finger or plunger from a small a fresh gas flow appropriate for the mode of ventilation being disposable syringe;114 the flowmeter bobbin should fall with used be supplied when using this circuit for transport (70 to 100 occlusion of the inner tube. Alternatively, filling the reservoir mL/kg/min for controlled ventilation and ≤150 mL/kg/min for bag with gas and operating the oxygen flush will normally create spontaneous ventilation). Following transport, this system can a Venturi effect that partially empties the bag.115 function as part of a nonrebreathing system on an intensive care The Lack circuit (coaxial Mapleson A) appears similar to the unit ventilator. Bain circuit externally. Near the bag are both a pop-­off valve and The Enclosed Afferent Reservoir (EAR) breathing system a fresh gas inflow nipple. However, the central tube is larger in (Fig. 4.20) reflects an attempt to create a semiclosed system diameter and serves as an expiratory limb, leading from a spider that retains efficiency during both spontaneous and controlled that centers it coaxially to the pop-­off valve.14 The circuit is long ventilation without the use of switching valves. Use of the term enough that this central tube has a volume of 500 mL. Fresh gas afferent denotes a system in which the reservoir is located on flows between the external corrugated tube and the central tube the portion of the system closely associated with the fresh gas to the patient connection end. This is essentially a Mapleson A supply. Mapleson A systems, including the Lack system, are circuit and is optimal for spontaneous breathing. Fig. 4.18 shows afferent reservoir systems; they preferentially exhaust alveolar the Lack system at end expiration. The first part of exhalation gas during the expiratory phase of spontaneous respiration, has passed retrograde between the corrugated hose and inner but with controlled ventilation, the pop-­ off opens during tube toward the bag, which is simultaneously filling with fresh inspiration as well, which limits efficiency. Efferent systems gas. Because this first part contains little or no carbon dioxide, have the reservoir closely associated with the pop-­off valve little carbon dioxide is found in the outer channel. When the bag and include Mapleson systems D through F. As previously reaches its nominal volume, the pressure rises enough to open noted, they are most efficient during controlled ventilation. the pop-­off valve; for the rest of exhalation, carbon dioxide-­ An enclosed afferent system prevents venting of gas from the rich gas passes into the inner channel. If expiratory flow falls to expiratory valve during controlled inspiration by enclosing the nearly zero at the end-­expiratory pause, fresh gas flows toward reservoir in a chamber; as pressure in the chamber is increased the patient through the outer channel and even into the inner to deliver a breath, the expiratory valve is forced to close. It has tube, pushing alveolar gas out through the pop-­off valve. Any been demonstrated that efficiency during controlled ventilation carbon dioxide remaining in the inner tube is not rebreathed using the EAR system is similar to that of the Bain.118 During Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  4 Breathing Circuits 119 Inspiratory limb Y-piece Coaxial tubing Expiratory limb A Oxygen supply