Breathing Circuits - Clinical Tree

Introduction

The anesthesia machine serves to create a desired mixture of anesthetic gases, vapors, oxygen, and air (as well as other gases such as helium and carbon dioxide, albeit less frequently). The patient is the recipient of these prepared gas mixtures of known composition, and the breathing circuit is the interface between the anesthesia machine and the patient. This circuit delivers the gas mixture from the machine to the patient as it removes carbon dioxide, excludes operating room (OR) air, and conditions the gas mixture by adjusting its temperature and humidity. It converts continuous gas flow from the anesthesia machine to the intermittent flow of breathing, facilitates controlled or assisted respiration, and provides other functions such as gas sampling and pressure and spirometric measurements.

The desirable characteristics of a breathing circuit include (1) low resistance to gas flow, (2) minimal rebreathing of the preceding exhaled gases, (3) removal of carbon dioxide at the rate at which it is produced, (4) rapid changes in delivered gas composition when required, (5) warmed humidification of the inspired gases, and (6) safe disposal of waste gases. The components of a breathing circuit include (1) the breathing tubing, (2) respiratory valves, (3) reservoir bags, (4) carbon dioxide absorption canisters, (5) a fresh gas inflow site, (6) a pop-off valve leading to a scavenger for excess gas, (7) a Y-piece with a mask or tube mount, and (8) a face mask, laryngeal mask, or tracheal tube. Other devices that may be included are (1) filters, (2) humidifiers, (3) valves for positive end-expiratory pressure (PEEP), and (4) detecting mechanisms for airway pressure, spirometry, and gas analysis. Although these circuit components can be assembled in many ways, contemporary systems are usually configured by the manufacturer and permit little intervention by the user in regard to their configuration. Understanding the advantages and limitations of the different configurations allows the user to select the most appropriate type for varying clinical settings.

History of Device Development

Breathing circuits have been an important concern from the start. Because of a delay in the production of his inhaler, Morton was late to his first public exhibition of the “Somniferon” (ether) inhaler in 1846. The earliest circuits were mechanically simple; differences among them were related to the characteristics of the primary anesthetic agent. Because nitrous oxide and ether anesthetic mixtures were weak (less potent) or slow to produce anesthesia, 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 that the unique features of each agent were important. The ability to assist respiration was advantageous, as was conservation of costly agents and avoidance of large leaks of flammable ones.

In the twentieth century, a large number of relatively small but more highly engineered improvements were made as other demands on the breathing circuit were recognized. In 1915, Dennis Jackson described the first carbon dioxide absorber to save on the cost of nitrous oxide for animal studies. Ralph Waters brought the idea into the OR, designing a to-and-fro absorption canister that used soda lime. ^, Bryan Sword introduced the first circle breathing circuit in 1930. Thus, low-flow absorption systems were already in use when cyclopropane made them essential. A return to high flows in the United States was brought about by the poor performance of vaporizers for halothane in the 1950s, with the demonstration that such flows could eliminate carbon dioxide without the use of soda lime. ^,

Stimulated by Magill’s use of a number of pieces of apparatus put together in differing configurations for differing purposes, Mapleson described a variety of Magill circuits. The original Ayre’s T-piece was modified by numerous practitioners; the Jackson-Rees circuit represents one such example. A variety of proprietary nonrebreathing valves were introduced, and the circuits named for them included the Stephen-Slater, the Fink, the Ruben, and the Frumin.

Partial rebreathing and functionally nonrebreathing circuits—such as the Bain, Humphrey ADE, and Lack systems—found various proponents. Ingenious switching valves permitted transformation from one circuit to another, which then often led to difficulty in remembering which circuit was optimal for what purpose.

Today, in addition to factors of convenience and economy, circuits are used: (1) to control heat and humidity; (2) to measure patient variables such as tidal volume, respiratory frequency, airway pressure, and inspired and expired gas concentrations; and to (3) control contamination of the OR environment by the agents themselves. The 150-year history of the development of the breathing circuit offers the practitioner a number of choices. All commonly used circuits accomplish their goals more or less equivalently, but the simple act of increasing fresh gas flow, for example, may markedly increase the work of breathing. Therefore, it is vital that the anesthesiologist understand the functional characteristics of each circuit.

Classifications of Breathing Circuits

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. An open system has no reservoir and no rebreathing; a semiopen system has a reservoir but no rebreathing; a semiclosed system has a reservoir and partial rebreathing; and a closed system has a reservoir and complete rebreathing. Variations on this classification included the type of carbon dioxide absorber and unidirectional valves used.

Because of confusion with this traditional nomenclature, Hamilton recommended its abandonment in favor of both a description of the hardware (e.g., circle filter system, coaxial circuit, T-piece) and the gas flow rates being used. Identifying the circuits by eponym—such as Adelaide, Bain, Hafnia, Humphrey, Jackson-Rees, Lack, Magill, and Waters—did not help in understanding the function or application of the circuit. Almost all anesthesia machines are equipped with some form of a circle breathing circuit with the ability for carbon dioxide absorption during low-flow anesthesia and elimination through the pop-off valve during high-flow anesthesia. Because an understanding of how circuits work is essential, breathing circuits in this chapter are organized by method of carbon dioxide elimination. Methods for removal of carbon dioxide are discussed.

Chemical Absorption of Carbon Dioxide

Semiclosed and closed systems (i.e., circle and to-and-fro) rely on chemical absorption of carbon dioxide. Exhaled carbon dioxide is absorbed, and all other exhaled gases are rebreathed. The quantities of fresh oxygen and anesthetics equal those lost as a result of uptake, metabolism, and circuit leaks. ^, ^,

Dilution with Fresh Gas

Because of the intermittent nature of carbon dioxide excretion (during exhalation only) and the continuous inflow of fresh gas, the choice of inflow rate—as well as the locations of the inflow site, reservoir bag, and pop-off valves—contributes to the efficiency of carbon dioxide removal. When fresh gas flows are 1 to 1.5 times the minute volume (approximately 10 L/min in an adult), dilution alone is sufficient to remove carbon dioxide. ^, Such systems then behave the same as a nonrebreathing system.

Use of Valves to Separate Exhaled Gases from Inhaled Gases

Systems that use nonrebreathing valves are examples of this method of carbon dioxide removal. ^, ^, ^, ^, A circuit that by virtue of high flows behaves as if it were nonrebreathing is not considered a nonrebreathing circuit in this analysis.

Use of Open-Drop Ether or a T-Piece without a Reservoir to Release Exhaled Carbon Dioxide Into the Atmosphere

Although similar to the second method above, systems that used open-drop ether or a T-piece without a reservoir were not truly 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 circuits later.

Components of a Breathing Circuit

The circuits described previously have many features in common; they connect to the patient’s airway through a face mask, laryngeal mask, or tracheal tube adapted to the breathing circuit through a Y-piece or elbow. The system may include valves to permit directional gas flow, and a reservoir bag is almost always present, which can be used to manually force gas into the lungs. Fresh gas must be supplied to the circuit, and excessive gas must be allowed to escape. In some, carbon dioxide is absorbed in a chemical filter. A variety of ancillary devices may also be present, such as humidifiers, spirometers, pressure gauges, filters, gas analyzers, PEEP devices, waste gas scavengers, and mixing and circulating devices.

Connection of the Patient to the Breathing Circuit

Either an anesthesia mask, supraglottic device, or a tracheal tube connects the circuit to the patient. Masks are made from rubber or (now) clear plastic to make secretions or vomitus visible ( Fig. 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. For example, a prominent nasal bridge may prevent a tight fit if the mask’s cuff is flat at that point. A prominent chin (mentum) with sunken alveolar ridge causes a leak at the corner of the mouth, and the volume of the mask contributes to apparatus dead space. The mask should fit between the interpupillary line over the nose and in the groove between the mental process and the alveolar ridge ( Fig. 4.2 ). The average length of this area is 85 to 90 mm in adults. The newest disposable plastic masks are available in a wide range of sizes, intended to fit the faces of small children and large adults equally well. Choosing from a selection of mask sizes and styles is more rational than a “one size fits all” approach because a poorly fitting mask can result in trauma to the patient. This is especially true when the mask must be positioned above the eyebrows because it can cause pressure on, and possibly damage to, the optic and supraorbital nerves. Masks often have a set of prongs for attachment to 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.

Fig. 4.1, The modern, clear plastic anesthesia mask.

Fig. 4.2, (A) to (C) The mask’s cushion fits over the nose at the interpupillary line and above the mental process.

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 the necessity of sterile tubing. ^, On occasion, it is necessary to pass a breathing circuit on to the sterile surgical field (e.g., during a laryngectomy or an ex utero intrapartum treatment procedure). By convention, the ends of the tubing are 22 mm in internal diameter (ID) and are identical in design. Tubing should be inspected before use because manufacturing errors can result in obstruction of the lumen. ^, Compliance of the tubing varies from nearly 0 to more than 5 mL/m/mm Hg of applied pressure, and modern plastic tubing has lower values than formerly-used rubber tubing. Apparent distensibility is even greater because compression of gas under pressure, to the order of 3% of the volume, occurs at typical inflation pressures. Inflation of a patient’s lungs to 20 cm H ₂ O peak inspiratory pressure compresses 30 to 150 mL of gas in the tubing. This volume is not delivered to the patient’s lungs, but some fraction of it may be measured by a spirometer within 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 H ₂ O/L/min of flow. 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 their compliant volume ( Fig. 4.3 ).

Fig. 4.3, “Concertina” style breathing circuit tubes can be either compressed (A and B) or stretched (C) to change in length and volume without significantly affecting apparatus dead space.

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, Spoerel 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 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, but those with swivels are twice as likely to leak. Any circuit should be tested before use by determining the oxygen inflow required to maintain 30 cm H ₂ O of pressure in the circuit (see also Chapter 25 ).

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