Chapter 4. The Anesthesia Machine

• Misuse of anesthesia gas delivery systems is three times more likely than failure of the device to cause equipment-related adverse outcomes. An operator’s lack of familiarity with the equipment or a failure to check machine function, or both, are the most frequent causes. These mishaps account for only about 2% of cases in the ASA Closed Claims Project database. The breathing circuit was the most common single source of injury (39%); nearly all damaging events were related to misconnects or disconnects.
• The anesthesia machine receives medical gases from a gas supply, controls the flow and reduces the pressure of desired gases to a safe level, vaporizes volatile anesthetics into the final gas mixture, and delivers the gases to a breathing circuit that is connected to the patient’s airway. A mechanical ventilator attaches to the breathing circuit but can be excluded with a switch during spontaneous or manual (bag) ventilation.
• Whereas the oxygen supply can pass directly to its flow control valve, nitrous oxide, air, and other gases must first pass through safety devices before reaching their respective flow control valves. These devices permit the flow of other gases only if there is sufficient oxygen pressure in the safety device and help prevent accidental delivery of a hypoxic mixture in the event of oxygen supply failure.
• Another safety feature of anesthesia machines is a linkage of the nitrous oxide gas flow to the oxygen gas flow; this arrangement helps ensure a minimum oxygen concentration of 25%.
• All modern vaporizers are agent specific and temperature corrected, capable of delivering a constant concentration of agent regardless of temperature changes or flow through the vaporizer.
• A rise in airway pressure may signal worsening pulmonary compliance, an increase in tidal volume, or an obstruction in the breathing circuit, tracheal tube, or the patient’s airway. A drop in pressure may indicate an improvement in compliance, a decrease in tidal volume, or a leak in the circuit.
• Traditionally ventilators on anesthesia machines have a double-circuit system design and are pneumatically powered and electronically controlled. Newer machines also incorporate microprocessor control that relies on sophisticated pressure and flow sensors. Some anesthesia machines have ventilators that use a single-circuit piston design.
• The major advantage of a piston ventilator is its ability to deliver accurate tidal volumes to patients with very poor lung compliance and to very small patients.
• Whenever a ventilator is used “disconnect alarms” must be passively activated. Anesthesia workstations should have at least three disconnect alarms: low pressure, low exhaled tidal volume, and low exhaled carbon dioxide.
• Because the ventilator’s spill valve is closed during inspiration, fresh gas flow from the machine’s common gas outlet normally contributes to the tidal volume delivered to the patient.
• Use of the oxygen flush valve during the inspiratory cycle of a ventilator must be avoided because the ventilator spill valve will be closed and the adjustable pressure-limiting (APL) valve is excluded; the surge of oxygen (600-1200 mL/s) and circuit pressure will be transferred to the patient’s lungs.
• Large discrepancies between the set and actual tidal volume are often observed in the operating room during volume-controlled ventilation. Causes include breathing circuit compliance, gas compression, ventilator-fresh gas flow coupling, and leaks in the anesthesia machine, the breathing circuit, or the patient’s airway.
• Waste-gas scavengers dispose of gases that have been vented from the breathing circuit by the APL valve and ventilator spill valve. Pollution of the operating room environment with anesthetic gases may pose a health hazard to surgical personnel.
• A routine inspection of anesthesia equipment before each use increases operator familiarity and confirms proper functioning. The U.S. Food and Drug Administration has made available a generic checkout procedure for anesthesia gas machines and breathing systems.

No piece of equipment is more intimately associated with the practice of anesthesiology than the anesthesia machine (Figure 4-1). On the most basic level, the anesthesiologist uses the anesthesia machine to control the patient’s ventilation and oxygen delivery and to administer inhalation anesthetics. Proper functioning of the machine is crucial for patient safety. Modern anesthesia machines have become very sophisticated, incorporating many built-in safety features and devices, monitors, and multiple microprocessors that can integrate and monitor all components. Additional monitors can be added externally and often still be fully integrated. Moreover, modular machine designs allow a wide variety of configurations and features within the same product line. The term anesthesia workstation is therefore often used for modern anesthesia machines. There are two major manufacturers of anesthesia machines in the United States, Datex-Ohmeda (GE Healthcare) and Dräger Medical. Other manufacturers (eg, Mindray) produce anesthesia delivery systems. Anesthesia providers should carefully review the operations manuals of the machines present in their clinical practice.

Much progress has been made in reducing the number of adverse outcomes arising from anesthetic gas delivery equipment, through redesign of equipment and education.

The American National Standards Institute and subsequently the ASTM International (formerly the American Society for Testing and Materials, F1850-00) published standard specifications for anesthesia machines and their components. Table 4-1 lists essential features of a modern anesthesia workstation. Changes in equipment design have been directed at minimizing the probability of breathing circuit misconnects and disconnects and automating machine checks. Because of the durability and functional longevity of anesthesia machines, the ASA has developed guidelines for determining anesthesia machine obsolescence (Table 4-2). This chapter is an introduction to anesthesia machine design, function, and use.

Most machines have gas inlets for oxygen, nitrous oxide, and air. Compact models often lack air inlets, whereas other machines may have a fourth inlet for helium, heliox, carbon dioxide, or nitric oxide. Separate inlets are provided for the primary pipeline gas supply that passes through the walls of health care facilities and the secondary cylinder gas supply. Machines therefore have two gas inlet pressure gauges for each gas: one for pipeline pressure and another for cylinder pressure.

Pipeline Inlets

Oxygen and nitrous oxide (and often air) are delivered from their central supply source to the operating room through a piping network. The tubing is color coded and connects to the anesthesia machine through a noninterchangeable diameter-index safety system (DISS) fitting that prevents incorrect hose attachment. The noninterchangeability is achieved by making the bore diameter of the body and that of the connection nipple specific for each supplied gas. A filter helps trap debris from the wall supply and a one-way check valve prevents retrograde flow of gases into the pipeline supplies. It should be noted that most modern machines have an oxygen (pneumatic) power outlet that may be used to drive the ventilator or provide an auxiliary oxygen flowmeter. The DISS fittings for the oxygen inlet and the oxygen power outlet are identical and should not be mistakenly interchanged. The approximate pipeline pressure of gases delivered to the anesthesia machine is 50 psig.

Cylinder Inlets

Cylinders attach to the machine via hanger-yoke assemblies that utilize a pin index safety system to prevent accidental connection of a wrong gas cylinder. The yoke assembly includes index pins, a washer, a gas filter, and a check valve that prevents retrograde gas flow. The gas cylinders are also color-coded for specific gases to allow for easy identification. In North America the following color-coding scheme is used: oxygen = green, nitrous oxide = blue, carbon dioxide = gray, air = yellow, helium = brown, nitrogen = black. In the United Kingdom, white is used for oxygen and black and white for air. The E-cylinders attached to the anesthesia machine are a high-pressure source of medical gases and are generally used only as a back-up supply in case of pipeline failure. Pressure of gas supplied from the cylinder to the anesthesia machine is 45 psig. Some machines have two oxygen cylinders so that one cylinder can be used while the other is changed. At 20°C, a full cylinder contains 600 L of oxygen at a pressure of 1900 psig, and 1590 L of nitrous oxide at 745 psig. Cylinder pressure is usually measured by a Bourdon pressure gauge (Figure 4-5). A flexible tube within this gauge straightens when exposed to gas pressure, causing a gear mechanism to move a needle pointer.

Pressure Regulators

Unlike the relatively constant pressure of the pipeline gas supply, the high and variable gas pressure in cylinders makes flow control difficult and potentially dangerous. To enhance safety and ensure optimal use of cylinder gases, machines utilize a pressure regulator to reduce the cylinder gas pressure to 45-47 psig1 before it enters the flow valve (Figure 4-6). This pressure, which is slightly lower than the pipeline supply, allows preferential use of the pipeline supply if a cylinder is left open (unless pipeline pressure drops below 45 psig). After passing through Bourdon pressure gauges and check valves, the pipeline gases share a common pathway with the cylinder gases. A high-pressure relief valve provided for each gas is set to open when the supply pressure exceeds the machine’s maximum safety limit (95-110 psig), as might happen with a regulator failure on a cylinder. Some machines also use a second regulator to drop both pipeline and cylinder pressure further (two-stage pressure regulation). A second-stage pressure reduction may also be needed for an auxiliary oxygen flowmeter, the oxygen flush mechanism, or the drive gas to power a pneumatic ventilator.

Oxygen Supply Failure Protection Devices

Most modern (particularly Datex-Ohmeda) machines use a proportioning safety device instead of a threshold shut-off valve. These devices, called either an oxygen failure protection device (Dräger) or a balance regulator (Datex-Ohmeda), proportionately reduce the pressure of nitrous oxide and other gases except for air (Figures 4-7 and 4-8). They completely shut off nitrous oxide and other gas flow only below a set minimum oxygen pressure (eg, 0.5 psig for nitrous oxide and 10 psig for other gases).

All machines also have an oxygen supply low-pressure sensor that activates alarm sounds when inlet gas pressure drops below a threshold value (usually 20-30 psig). It must be emphasized that these safety devices do not protect against other possible causes of hypoxic accidents (eg, gas line misconnections), in which threshold pressure may be maintained by gases containing inadequate or no oxygen.

Flow Valves & Meters

Once the pressure has been reduced to a safe level, each gas must pass through flow control valves and is measured by flowmeters before mixing with other gases, entering the active vaporizer, and exiting the machine’s common gas outlet. Gas lines proximal to flow valves are considered to be in the high-pressure circuit whereas those between the flow valves and the common gas outlet are considered part of the low-pressure circuit of the machine. When the knob of the flow control valve is turned counterclockwise, a needle valve is disengaged from its seat, allowing gas to flow through the valve (Figure 4-9). Stops in the full-off and full-on positions prevent valve damage. Touch- and color-coded control knobs make it more difficult to turn the wrong gas off or on. As a safety feature the oxygen knob is usually fluted, larger, and protrudes farther than the other knobs. The oxygen flowmeter is positioned furthest to the right, downstream to the other gases; this arrangement helps to prevent hypoxia if there is leakage from a flowmeter positioned upstream.

Flow control knobs control gas entry into the flowmeters by adjustment via a needle valve. Flowmeters on anesthesia machines are classified as either constant-pressure variable-orifice (rotameter) or electronic. In constant-pressure variable-orifice flowmeters, an indicator ball, bobbin, or float is supported by the flow of gas through a tube (Thorpe tube) whose bore (orifice) is tapered. Near the bottom of the tube, where the diameter is small, a low flow of gas will create sufficient pressure under the float to raise it in the tube. As the float rises, the (variable) orifice of the tube widens, allowing more gas to pass around the float. The float will stop rising when its weight is just supported by the difference in pressure above and below it. If flow is increased, the pressure under the float increases, raising it higher in the tube until the pressure drop again just supports the float’s weight. This pressure drop is constant regardless of the flow rate or the position in the tube and depends on the float weight and tube cross-sectional area.

Flowmeters are calibrated for specific gases, as the flow rate across a constriction depends on the gas’s viscosity at low laminar flows (Poiseuille’s law) and its density at high turbulent flows. To minimize the effect of friction between them and the tube’s wall, floats are designed to rotate constantly, which keeps them centered in the tube. Coating the tube’s interior with a conductive substance grounds the system and reduces the effect of static electricity. Some flowmeters have two glass tubes, one for low flows and another for high flows (Figure 4-10A); the two tubes are in series and are still controlled by one valve. A dual taper design can allow a single flowmeter to read both high and low flows (Figure 4-10B). Causes of flowmeter malfunction include debris in the flow tube, vertical tube misalignment, and sticking or concealment of a float at the top of a tube.

Should a leak develop within or downstream from an oxygen flowmeter, a hypoxic gas mixture can be delivered to the patient (Figure 4-11). To reduce this risk, oxygen flowmeters are always positioned downstream to all other flowmeters (nearest to the vaporizer).

Some anesthesia machines have electronic flow control and measurement (Figure 4-12). In such instances, a back-up conventional (Thorpe) auxiliary oxygen flowmeter is provided. Other models have conventional flowmeters but electronic measurement of gas flow along with Thorpe tubes and digital or digital/graphic displays (Figure 4-13). The amount of pressure drop caused by a flow restrictor is the basis for measurement of gas flow rate in these systems. In these machines oxygen, nitrous oxide, and air each have a separate electronic flow measurement device in the flow control section before they are mixed together. Electronic flowmeters are essential components in workstations if gas flow rate data will be acquired automatically by computerized anesthesia recording systems.

Minimum Oxygen Flow

The oxygen flow valves are usually designed to deliver a minimum flow of 150 mL/min when the anesthesia machine is turned on. One method involves the use of a minimum flow resistor (Figure 4-14). This safety feature helps ensure that some oxygen enters the breathing circuit even if the operator forgets to turn on the oxygen flow. Some machines are designed to deliver minimum flow or low-flow anesthesia (<1 L/min) and have minimum oxygen flows as low as 50 mL/min.

Oxygen/Nitrous Oxide Ratio Controller

Vaporizers

Volatile anesthetics (eg, halothane, isoflurane, desflurane, sevoflurane) must be vaporized before being delivered to the patient. Vaporizers have concentration-calibrated dials that precisely add volatile anesthetic agents to the combined gas flow from all flowmeters. They must be located between the flowmeters and the common gas outlet. Moreover, unless the machine accepts only one vaporizer at a time, all anesthesia machines should have an interlocking or exclusion device that prevents the concurrent use of more than one vaporizer.

Physics of Vaporization

At temperatures encountered in the operating room, the molecules of a volatile anesthetic in a closed container are distributed between the liquid and gaseous phases. The gas molecules bombard the walls of the container, creating the saturated vapor pressure of that agent. Vapor pressure depends on the characteristics of the volatile agent and the temperature. The greater the temperature, the greater the tendency for the liquid molecules to escape into the gaseous phase and the greater the vapor pressure (Figure 4-15). Vaporization requires energy (the latent heat of vaporization), which results in a loss of heat from the liquid. As vaporization proceeds, temperature of the remaining liquid anesthetic drops and vapor pressure decreases unless heat is readily available to enter the system. Vaporizers contain a chamber in which a carrier gas becomes saturated with the volatile agent.

A liquid’s boiling point is the temperature at which its vapor pressure is equal to the atmospheric pressure. As the atmospheric pressure decreases (as in higher altitudes), the boiling point also decreases. Anesthetic agents with low boiling points are more susceptible to variations in barometric pressure than agents with higher boiling points. Among the commonly used agents, desflurane has the lowest boiling point (22.8°C at 760 mm Hg).

Copper Kettle

The copper kettle vaporizer is no longer used in clinical anesthesia; however, understanding how it works provides invaluable insight into the delivery of volatile anesthetics (Figure 4-16). It is classified as a measured-flow vaporizer (or flowmeter-controlled vaporizer). In a copper kettle, the amount of carrier gas bubbled through the volatile anesthetic is controlled by a dedicated flowmeter. This valve is turned off when the vaporizer circuit is not in use. Copper is used as the construction metal because its relatively high specific heat (the quantity of heat required to raise the temperature of 1 g of substance by 1°C) and high thermal conductivity (the speed of heat conductance through a substance) enhance the vaporizer’s ability to maintain a constant temperature. All the gas entering the vaporizer passes through the anesthetic liquid and becomes saturated with vapor. One milliliter of liquid anesthetic is the equivalent of approximately 200 mL of anesthetic vapor. Because the vapor pressure of volatile anesthetics is greater than the partial pressure required for anesthesia, the saturated gas leaving a copper kettle has to be diluted before it reaches the patient.

For example, the vapor pressure of halothane is 243 mm Hg at 20°C, so the concentration of halothane exiting a copper kettle at 1 atmosphere would be 243/760, or 32%. If 100 mL of oxygen enters the kettle, roughly 150 mL of gas exits (the initial 100 mL of oxygen plus 50 mL of saturated halothane vapor), one-third of which would be saturated halothane vapor. To deliver a 1% concentration of halothane (MAC 0.75%), the 50 mL of halothane vapor and 100 mL of carrier gas that left the copper kettle have to be diluted within a total of 5000 mL of fresh gas flow. Thus, every 100 mL of oxygen passing through a halothane vaporizer translates into a 1% increase in concentration if total gas flow into the breathing circuit is 5 L/min. Therefore when total flow is fixed, flow through the vaporizer determines the ultimate concentration of anesthetic. Isoflurane has an almost identical vapor pressure, so the same relationship between copper kettle flow, total gas flow, and anesthetic concentration exists. However, if total gas flow changes without an adjustment in copper kettle flow (eg, exhaustion of a nitrous oxide cylinder), the delivered volatile anesthetic concentration rises rapidly to potentially dangerous levels.

Modern Conventional Vaporizers

Temperature compensation is achieved by a strip composed of two different metals welded together. The metal strips expand and contract differently in response to temperature changes. When the temperature decreases, differential contraction causes the strip to bend allowing more gas to pass through the vaporizer. Such bimetallic strips are also used in home thermostats. As the temperature rises differential expansion causes the strip to bend the other way restricting gas flow into the vaporizer. Altering total fresh gas flow rates within a wide range does not significantly affect anesthetic concentration because the same proportion of gas is exposed to the liquid. However, the real output of an agent would be lower than the dial setting at extremely high flow (>15 L/min); the converse is true when the flow rate is less than 250 mL/min. Changing the gas composition from 100% oxygen to 70% nitrous oxide may transiently decrease volatile anesthetic concentration due to the greater solubility of nitrous oxide in volatile agents.

Given that these vaporizers are agent specific, filling them with the incorrect anesthetic should be avoided. For example, unintentionally filling a sevoflurane-specific vaporizer with halothane could lead to an anesthetic overdose. First, halothane’s higher vapor pressure (243 mm Hg versus 157 mm Hg) will cause a 40% greater amount of anesthetic vapor to be released. Second, halothane is more than twice as potent as sevoflurane (MAC 0.75 versus. 2.0). Conversely, filling a halothane vaporizer with sevoflurane will cause an anesthetic underdosage. Modern vaporizers offer agent-specific keyed filling ports to prevent filling with an incorrect agent.

Excessive tilting of older vaporizers (Tec 4, Tec 5, and Vapor 19.n) during transport may flood the bypass area and lead to dangerously high anesthetic concentrations. In the event of tilting and spillage, high flow of oxygen with the vaporizer turned off should be used to vaporize and flush the liquid anesthetic from the bypass area. Fluctuations in pressure from positive-pressure ventilation in older anesthesia machines may cause a transient reversal of flow through the vaporizer, unpredictably changing agent delivery. This “pumping effect” is more pronounced with low gas flows. A one-way check valve between the vaporizers and the oxygen flush valve (Datex-Ohmeda) together with some design modifications in newer units limit the occurrence of some of these problems. Variable bypass vaporizers compensate for changes in ambient pressures (ie, altitude changes maintaining relative anesthetic gas partial pressure).

Electronic Vaporizers

Electronically controlled vaporizers must be utilized for desflurane and are used for all volatile anesthetics in some sophisticated anesthesia machines.

Desflurane Vaporizer

Desflurane’s vapor pressure is so high that at sea level it almost boils at room temperature (Figure 4-15). This high volatility, coupled with a potency only one-fifth that of other volatile agents, presents unique delivery problems. First, the vaporization required for general anesthesia produces a cooling effect that would overwhelm the ability of conventional vaporizers to maintain a constant temperature. Second, because it vaporizes so extensively, a tremendously high fresh gas flow would be necessary to dilute the carrier gas to clinically relevant concentrations. These problems have been addressed by the development of special desflurane vaporizers. A reservoir containing desflurane (desflurane sump) is electrically heated to 39°C (significantly higher than its boiling point) creating a vapor pressure of 2 atmospheres. Unlike a variable-bypass vaporizer, no fresh gas flows through the desflurane sump. Rather, pure desflurane vapor joins the fresh gas mixture before exiting the vaporizer. The amount of desflurane vapor released from the sump depends on the concentration selected by turning the control dial and the fresh gas flow rate. Although the Tec 6 Plus maintains a constant desflurane concentration over a wide range of fresh gas flow rates, it cannot automatically compensate for changes in elevation. Decreased ambient pressure (eg, high elevation) does not affect the concentration of agent delivered, but decreases the partial pressure of the agent. Thus, at high elevations, the anesthesiologist must manually increase the concentration control.

Aladin Cassette Vaporizer

This vaporizer is designed for use with the Datex-Ohmeda S/5 ADU and Aisys machines. Gas flow from the flow control is divided into bypass flow and liquid chamber flow (Figure 4-18). The latter is conducted into an agent-specific, color-coded, cassette (Aladin cassette) in which the volatile anesthetic is vaporized. The machine accepts only one cassette at a time and recognizes the cassette through magnetic labeling. The cassette does not contain any bypass flow channels; therefore, unlike traditional vaporizers, liquid anesthetic cannot escape during handling and the cassette can be carried in any position. After leaving the cassette, the now anesthetic-saturated liquid chamber flow reunites with the bypass flow before exiting the fresh gas outlet. A flow restrictor valve near the bypass flow helps to adjust the amount of fresh gas that flows to the cassette. Adjusting the ratio between the bypass flow and liquid chamber flow changes the concentration of volatile anesthetic agent delivered to the patient. In practice, the clinician changes the concentration by turning the agent wheel, which operates a digital potentiometer. Software sets the desired fresh gas agent concentration according to the number of output pulses from the agent wheel. Sensors in the cassette measure pressure and temperature, thus determining agent concentration in the gas leaving the cassette. Correct liquid chamber flow is calculated based on desired fresh gas concentration and determined cassette gas concentration.

Common (Fresh) Gas Outlet

In contrast to the multiple gas inlets, the anesthesia machine has only one common gas outlet that supplies gas to the breathing circuit. The term fresh gas outlet is also often used because of its critical role in adding new gas of fixed and known composition to the circle system. Unlike older models, some newer anesthesia machines measure and report common outlet gas flows (Datex-Ohmeda S/5 ADU and Narkomed 6400). An antidisconnect retaining device is used to prevent accidental detachment of the gas outlet hose that connects the machine to the breathing circuit.

The oxygen flush valve provides a high flow (35-75 L/min) of oxygen directly to the common gas outlet, bypassing the flowmeters and vaporizers. It is used to rapidly refill or flush the breathing circuit, but because the oxygen may be supplied at a line pressure of 45-55 psig, there is a real potential of lung barotrauma. For this reason, the flush valve must be used cautiously whenever a patient is connected to the breathing circuit. Moreover, inappropriate use of the flush valve (or a situation of stuck valve) may result in backflow of gases into the low-pressure circuit, causing dilution of inhaled anesthetic concentration. Some machines use a second-stage regulator to drop the oxygen flush pressure to a lower level. A protective rim around the flush button limits the possibility of unintentional activation. Anesthesia machines (eg, Datex-Ohmeda Aestiva/5) may have an optional auxiliary common gas outlet that is activated with a dedicated switch. It is primarily used for performing the low-pressure circuit leak test (see Anesthesia Machine Checkout List).

1Pressure unit conversions: 1 kiloPascal (kP) = kg/m · s2 = 1000 N/m2 = 0.01 bar = 0.1013 atmospheres = 0.145 psig = 10.2 cm H2O = 7.5 mm Hg.

The breathing system most commonly used with anesthesia machines is the circle system (Figure 4-19); a Bain circuit is occasionally used. The components and use of the circle system were previously discussed. It is important to note that gas composition at the common gas outlet can be controlled precisely and rapidly by adjustments in flowmeters and vaporizers. In contrast, gas composition, especially volatile anesthetic concentration, in the breathing circuit is significantly affected by other factors, including anesthetic uptake in the patient’s lungs, minute ventilation, total fresh gas flow, volume of the breathing circuit, and the presence of gas leaks. Use of high gas flow rates during induction and emergence decreases the effects of such variables and can diminish the magnitude of discrepancies between fresh gas outlet and circle system anesthetic concentrations. Measurement of inspired and expired anesthetic gas concentration also greatly facilitates anesthetic management.

In most machines the common gas outlet is attached to the breathing circuit just past the exhalation valve to prevent artificially high exhaled tidal volume measurements. When spirometry measurements are made at the Y-connector, fresh gas flow can enter the circuit on the patient side of the inspiratory valve. The latter enhances CO2 elimination and may help reduce desiccation of the CO2 absorbent.

Newer anesthesia machines have integrated internalized breathing circuit components (Figure 4-20). The advantages of these designs include reduced probability of breathing circuit misconnects, disconnects, kinks, and leaks. The smaller volume of compact machines can also help conserve gas flow and volatile anesthetics and allow faster changes in breathing circuit gas concentration. Internal heating of manifolds can reduce precipitation of moisture.

Oxygen Analyzers

General anesthesia should not be administered without an oxygen analyzer in the breathing circuit. Three types of oxygen analyzers are available: polarographic (Clark electrode), galvanic (fuel cell), and paramagnetic. The first two techniques utilize electrochemical sensors that contain cathode and anode electrodes embedded in an electrolyte gel separated from the sample gas by an oxygen-permeable membrane (usually Teflon). As oxygen reacts with the electrodes, a current is generated that is proportional to the oxygen partial pressure in the sample gas. The galvanic and polarographic sensors differ in the composition of their electrodes and electrolyte gels. The components of the galvanic cell are capable of providing enough chemical energy so that the reaction does not require an external power source.

Although the initial cost of paramagnetic sensors is greater than that of electrochemical sensors, paramagnetic devices are self-calibrating and have no consumable parts. In addition, their response time is fast enough to differentiate between inspired and expired oxygen concentrations.

All oxygen analyzers should have a low-level alarm that is automatically activated by turning on the anesthesia machine. The sensor should be placed into the inspiratory or expiratory limb of the circle system’s breathing circuit—but not into the fresh gas line. As a result of the patient’s oxygen consumption, the expiratory limb has a slightly lower oxygen partial pressure than the inspiratory limb, particularly at low fresh gas flows. The increased humidity of expired gas does not significantly affect most modern sensors.

Spirometers

Spirometers, also called respirometers, are used to measure exhaled tidal volume in the breathing circuit on all anesthesia machines, typically near the exhalation valve. Some anesthesia machines also measure the inspiratory tidal volume just past the inspiratory valve or the actual delivered and exhaled tidal volumes at the Y-connector that attaches to the patient’s airway.

A common method employs a rotating vane of low mass in the expiratory limb in front of the expiratory valve of the circle system (vane anemometer or Wright respirometer, Figure 4-21A).

The flow of gas across vanes within the respirometer causes their rotation, which is measured electronically, photoelectrically, or mechanically. In another variation using this turbine principle, the volumeter or displacement meter is designed to measure the movement of discrete quantities of gas over time (Figure 4-21B).

Changes in exhaled tidal volumes usually represent changes in ventilator settings, but can also be due to circuit leaks, disconnections, or ventilator malfunction. These spirometers are prone to errors caused by inertia, friction, and water condensation. For example, Wright respirometers under-read at low flow rates and over-read at high flow rates. Furthermore, the measurement of exhaled tidal volumes at this location in the expiratory limb includes gas that had been lost to the circuit (and not delivered to the patient; discussed below). The difference between the volume of gas delivered to the circuit and the volume of gas actually reaching the patient becomes very significant with long compliant breathing tubes, rapid respiratory rates, and high airway pressures. These problems are at least partially overcome by measuring the tidal volume at the Y-connector to the patient’s airway.

A hot-wire anemometer utilizes a fine platinum wire, electrically heated at a constant temperature, inside the gas flow. The cooling effect of increasing gas flow on the wire electrode causes a change in electrical resistance. In a constant-resistance anemometer, gas flow is determined from the current needed to maintain a constant wire temperature (and resistance). Disadvantages include an inability to detect reverse flow, less accuracy at higher flow rates, and the possibility that the heated wire may be a potential ignition source for fire in the breathing manifold.

Ultrasonic flow sensors rely on discontinuities in gas flow generated by turbulent eddies in the flow stream. Upstream and downstream ultrasonic beams, generated from piezoelectric crystals, are transmitted at an angle to the gas stream. The Doppler frequency shift in the beams is proportional to the flow velocities in the breathing circuit. Major advantages include the absence of moving parts and greater accuracy due to the device’s independence from gas density.

Machines with variable-orifice flowmeters usually employ two sensors (Figure 4-21C). One measures flow at the inspiratory port of the breathing system and the other measures flow at the expiratory port. These sensors use a change in internal diameter to generate a pressure drop that is proportional to the flow through the sensor. Clear tubes connect the sensors to differential pressure transducers inside the anesthesia machine (Datex-Ohmeda 7900 SmartVent). The changes in gas flows during the inspiratory and expiratory phases help the ventilator to adjust and provide a constant tidal volume. However, due to excessive condensation sensors can fail when used with heated humidified circuits.

A pneumotachograph is a fixed-orifice flowmeter that can function as a spirometer. A parallel bundle of small-diameter tubes in chamber (Fleisch pneumotachograph) or mesh screen provides a slight resistance to airflow. The pressure drop across this resistance is sensed by a differential pressure transducer and is proportional to the flow rate. Integration of flow rate over time yields tidal volume. Moreover, analysis of pressure, volume, and time relationships can yield potentially valuable information about airway and lung mechanics. Modifications have been required to overcome inaccuracies due to water condensation and temperature changes. One modification employs two pressure-sensing lines in a Pitot tube at the Y-connection (Figure 4-21D). Gas flowing through the Pitot tube (flow sensor tube) creates a pressure difference between the flow sensor lines. This pressure differential is used to measure flow, flow direction, and airway pressure. Respiratory gases are continuously sampled to correct the flow reading for changes in density and viscosity.

Circuit Pressure

A pressure gauge or electronic sensor is always used to measure breathing-circuit pressure somewhere between the expiratory and inspiratory unidirectional valves; the exact location depends on the model of anesthesia machine. Breathing-circuit pressure usually reflects airway pressure if it is measured as close to the patient’s airway as possible. The most accurate measurements of both inspiratory and expiratory pressures can be obtained from the Y-connection (eg, D-lite and Pedi-lite sensors).

Some machines have incorporated auditory feedback for pressure changes during ventilator use.

Adjustable Pressure-Limiting Valve

The adjustable pressure-limiting (APL) valve, sometimes referred to as the pressure relief or pop-off valve, is usually fully open during spontaneous ventilation but must be partially closed during manual or assisted bag ventilation. The APL valve often requires fine adjustments. If it is not closed sufficiently excessive loss of circuit volume due to leaks prevents manual ventilation. At the same time if it is closed too much or is fully closed a progressive rise in pressure could result in pulmonary barotrauma (eg, pneumothorax) or hemodynamic compromise, or both. As an added safety feature, the APL valves on modern machines act as true pressure-limiting devices that can never be completely closed; the upper limit is usually 70-80 cm H2O.

Humidifiers

Absolute humidity is defined as the weight of water vapor in 1 L of gas (ie, mg/L). Relative humidity is the ratio of the actual mass of water present in a volume of gas to the maximum amount of water possible at a particular temperature. At 37°C and 100% relative humidity, absolute humidity is 44 mg/L, whereas at room temperature (21°C and 100% humidity) it is 18 mg/L. Inhaled gases in the operating room are normally administered at room temperature with little or no humidification. Gases must therefore be warmed to body temperature and saturated with water by the upper respiratory tract. Tracheal intubation and high fresh gas flows bypass this normal humidification system and expose the lower airways to dry (<10 mg H2O/L), room temperature gases.

Prolonged humidification of gases by the lower respiratory tract leads to dehydration of mucosa, altered ciliary function, and, if excessively prolonged, could potentially lead to inspissation of secretions, atelectasis, and even ventilation/perfusion mismatching, particularly in patients with underlying lung disease. Body heat is also lost as gases are warmed and even more importantly as water is vaporized to humidify the dry gases. The heat of vaporization for water is 560 cal/g of water vaporized. Fortunately, this heat loss accounts for about only 5-10% of total intraoperative heat loss, is not significant for a short procedure (<1 h), and usually can easily be compensated for with a forced-air warming blanket. Humidification and heating of inspiratory gases may be most important for small pediatric patients and older patients with severe underlying lung pathology, eg, cystic fibrosis.

Passive Humidifiers

Humidifiers added to the breathing circuit minimize water and heat loss. The simplest designs are condenser humidifiers or heat and moisture exchanger (HME) units (Figure 4-22). These passive devices do not add heat or vapor but rather contain a hygroscopic material that traps exhaled humidification and heat, which is released upon subsequent inhalation. Depending on the design, they may substantially increase apparatus dead space (more than 60 mL3), which can cause significant rebreathing in pediatric patients. They can also increase breathing-circuit resistance and the work of breathing during spontaneous respirations. Excessive saturation of an HME with water or secretions can obstruct the breathing circuit. Some condenser humidifiers also act as effective filters that may protect the breathing circuit and anesthesia machine from bacterial or viral cross-contamination. This may be particularly important when ventilating patients with respiratory infections or compromised immune systems.

Active Humidifiers

Active humidifiers are more effective than passive ones in preserving moisture and heat. Active humidifiers add water to gas by passing the gas over a water chamber (passover humidifier) or through a saturated wick (wick humidifier), bubbling it through water (bubble-through humidifier), or mixing it with vaporized water (vapor-phase humidifier). Because increasing temperature increases the capacity of a gas to hold water vapor, heated humidifiers with thermostatically controlled elements are most effective.

The hazards of heated humidifiers include thermal lung injury (inhaled gas temperature should be monitored and should not exceed 41°C), nosocomial infection, increased airway resistance from excess water condensation in the breathing circuit, interference with flowmeter function, and an increased likelihood of circuit disconnection. These humidifiers are particularly valuable with children as they help prevent both hypothermia and the plugging of small tracheal tubes by dried secretions. Of course, any design that increases airway dead space should be avoided in pediatric patients. Unlike passive humidifiers, active humidifiers do not filter respiratory gases.

Ventilators are used extensively in the operating room (OR) and the intensive care unit (ICU). All modern anesthesia machines are equipped with a ventilator. Historically OR ventilators were simpler and more compact than their ICU counterparts. This distinction has become blurred due to advances in technology together with an increasing need for “ICU-type” ventilators as more critically ill patients come to the OR. The ventilators on some modern machines are just as sophisticated as those in the ICU and have almost the same capabilities. After a general discussion of basic ventilator principles, this section reviews the use of ventilators in conjunction with anesthesia machines.

Overview

Ventilators generate gas flow by creating a pressure gradient between the proximal airway and the alveoli. Older units relied on the generation of negative pressure around (and inside) the chest (eg, iron lungs), whereas modern ventilators generate positive pressure and gas flow in the upper airway.

Ventilator function is best described in relation to the four phases of the ventilatory cycle: inspiration, the transition from inspiration to expiration, expiration, and the transition from expiration to inspiration. Although several classification schemes exist, the most common is based on inspiratory phase characteristics and the method of cycling from inspiration to expiration. Other classification categories may include power source (eg, pneumatic-high pressure, pneumatic-Venturi, or electric), design (single-circuit system, double-circuit system, rotary piston, linear piston), and control mechanisms (eg, electronic timer or microprocessor).

Inspiratory Phase

During inspiration, ventilators generate tidal volumes by producing gas flow along a pressure gradient. The machine generates either a constant pressure (constant-pressure generators) or constant gas flow rate (constant-flow generators) during inspiration, regardless of changes in lung mechanics (Figure 4-23). Nonconstant generators produce pressures or gas flow rates that vary during the cycle but remain consistent from breath to breath. For instance, a ventilator that generates a flow pattern resembling a half cycle of a sine wave (eg, rotary piston ventilator) would be classified as a nonconstant-flow generator. An increase in airway resistance or a decrease in lung compliance would increase peak inspiratory pressure but would not alter the flow rate generated by this type of ventilator (Figure 4-24).

Transition Phase from Inspiration to Expiration

Termination of the inspiratory phase can be triggered by a preset limit of time (fixed duration), a set inspiratory pressure that must be reached, or a predetermined tidal volume that must be delivered. Time-cycled ventilators allow tidal volume and peak inspiratory pressure to vary depending on lung compliance. Tidal volume is adjusted by setting inspiratory duration and inspiratory flow rate. Pressure-cycled ventilators will not cycle from the inspiratory phase to the expiratory phase until a preset pressure is reached. If a large circuit leak decreases peak pressures significantly, a pressure-cycled ventilator may remain in the inspiratory phase indefinitely. On the other hand, a small leak may not markedly decrease tidal volume, because cycling will be delayed until the pressure limit is met. Volume-cycled ventilators vary inspiratory duration and pressure to deliver a preset volume. In reality, modern ventilators overcome the many shortcomings of classic ventilator designs by incorporating secondary cycling parameters or other limiting mechanisms. For example, time-cycled and volume-cycled ventilators usually incorporate a pressure-limiting feature that terminates inspiration when a preset, adjustable safety pressure limit is reached. Similarly a volume-preset control that limits the excursion of the bellows allows a time-cycled ventilator to function somewhat like a volume-cycled ventilator, depending on the selected ventilator rate and inspiratory flow rate.

Expiratory Phase

The expiratory phase of ventilators normally reduces airway pressure to atmospheric levels or some preset value of positive end-expiratory pressure (PEEP). Exhalation is therefore passive. Flow out of the lungs is determined primarily by airway resistance and lung compliance. Expired gases fill up the bellows; they are then relieved to the scavenging system. PEEP is usually created with an adjustable spring valve mechanism or pneumatic pressurization of the exhalation (spill) valve.

Transition Phase from Expiration to Inspiration

Transition into the next inspiratory phase may be based on a preset time interval or a change in pressure. The behavior of the ventilator during this phase together with the type of cycling from inspiration to expiration determines ventilator mode.

During controlled ventilation, the most basic mode of all ventilators, the next breath always occurs after a preset time interval. Thus tidal volume and rate are fixed in volume-controlled ventilation, whereas peak inspiratory pressure is fixed in pressure-controlled ventilation. Controlled ventilation modes are not designed for spontaneous breathing. In the volume-control mode, the ventilator adjusts gas flow rate and inspiratory time based on the set ventilatory rate and I:E ratio (Figure 4-25A). In the pressure-control mode, inspiratory time is also based on the set ventilator rate and inspiratory-to-expiratory (I:E) ratio, but gas flow is adjusted to maintain a constant inspiratory pressure (Figure 4-25B).

In contrast, intermittent mandatory ventilation (IMV) allows patients to breathe spontaneously between controlled breaths. Synchronized intermittent mandatory ventilation (SIMV) is a further refinement that helps prevent “fighting the ventilator” and “breath stacking”; whenever possible, the ventilator tries to time the mandatory mechanical breaths with the drops in airway pressure below the end-expiratory pressure that occur as the patient initiates a spontaneous breath.

Ventilator Circuit Design

Double-Circuit System Ventilators

In a double-circuit system design, tidal volume is delivered from a bellows assembly that consists of a bellows in a clear rigid plastic enclosure (Figure 4-26). A standing (ascending) bellows is preferred as it readily draws attention to a circuit disconnection by collapsing. Hanging (descending) bellows are rarely used and must not be weighted; older ventilators with weighted hanging bellows continue to fill by gravity despite a disconnection in the breathing circuit.

The bellows in a double-circuit design ventilator takes the place of the breathing bag in the anesthesia circuit. Pressurized oxygen or air from the ventilator power outlet (45-50 psig) is routed to the space between the inside wall of the plastic enclosure and the outside wall of the bellows. Pressurization of the plastic enclosure compresses the pleated bellows inside, forcing the gas inside into the breathing circuit and patient. In contrast, during exhalation, the bellows ascend as pressure inside the plastic enclosure drops and the bellows fill up with the exhaled gas. A ventilator flow control valve regulates drive gas flow into the pressurizing chamber. This valve is controlled by ventilator settings in the control box (Figure 4-26). Ventilators with microprocessors also utilize feedback from flow and pressure sensors. If oxygen is used for pneumatic power it will be consumed at a rate at least equal to minute ventilation. Thus, if oxygen fresh gas flow is 2 L/min and a ventilator is delivering 6 L/min to the circuit, a total of at least 8 L/min of oxygen is being consumed. This should be kept in mind if the hospital’s medical gas system fails and cylinder oxygen is required. Some anesthesia machines reduce oxygen consumption by incorporating a Venturi device that draws in room air to provide air/oxygen pneumatic power. Newer machines may offer the option of using compressed air for pneumatic power. A leak in the ventilator bellows can transmit high gas pressure to the patient’s airway, potentially resulting in pulmonary barotrauma. This may be indicated by a higher than expected rise in inspired oxygen concentration (if oxygen is the sole pressurizing gas). Some machine ventilators have a built-in drive gas regulator that reduces the drive pressure (eg, to 25 psig) for added safety.

Double-circuit design ventilators also incorporate a free breathing valve that allows outside air to enter the rigid drive chamber and the bellows to collapse if the patient generates negative pressure by taking spontaneous breaths during mechanical ventilation.

Piston Ventilators

In a piston design, the ventilator substitutes an electrically driven piston for the bellows (Figure 4-24); the ventilator requires either minimal or no pneumatic (oxygen) power.

Spill Valve

Whenever a ventilator is used on an anesthesia machine, the circle system’s APL valve must be functionally removed or isolated from the circuit. A bag/ventilator switch typically accomplishes this. When the switch is turned to “bag” the ventilator is excluded and spontaneous/manual (bag) ventilation is possible. When it is turned to “ventilator,” the breathing bag and the APL are excluded from the breathing circuit. The APL valve may be automatically excluded in some newer anesthesia machines when the ventilator is turned on. The ventilator contains its own pressure-relief (pop-off) valve, called the spill valve, which is pneumatically closed during inspiration so that positive pressure can be generated (Figure 4-26). During exhalation, the pressurizing gas is vented out and the ventilator spill valve is no longer closed. The ventilator bellows or piston refill during expiration; when the bellows is completely filled, the increase in circle system pressure causes the excess gas to be directed to the scavenging system through the spill valve. Sticking of this valve can result in abnormally elevated airway pressure during exhalation.

Pressure & Volume Monitoring

Peak inspiratory pressure is the highest circuit pressure generated during an inspiratory cycle, and provides an indication of dynamic compliance. Plateau pressure is the pressure measured during an inspiratory pause (a time of no gas flow), and mirrors static compliance. During normal ventilation of a patient without lung disease, peak inspiratory pressure is equal to or only slightly greater than plateau pressure. An increase in both peak inspiratory pressure and plateau pressure implies an increase in tidal volume or a decrease in pulmonary compliance. An increase in peak inspiratory pressure without any change in plateau pressure signals an increase in airway resistance or inspiratory gas flow rate (Table 4-3). Thus, the shape of the breathing-circuit pressure waveform can provide important airway information. Many anesthesia machines graphically display breathing-circuit pressure (Figure 4-28). Airway secretions or kinking of the tracheal tube can be easily ruled out with the use of a suction catheter. Flexible fiberoptic bronchoscopy will usually provide a definitive diagnosis.

Ventilator Alarms

Alarms are an integral part of all modern anesthesia ventilators.

Problems Associated with Anesthesia Ventilators

Ventilator-Fresh Gas Flow Coupling

Thus, increasing fresh gas flow increases tidal volume, minute ventilation, and peak inspiratory pressure. To avoid problems with ventilator-fresh gas flow coupling, airway pressure and exhaled tidal volume must be monitored closely and excessive fresh gas flows must be avoided.

Excessive Positive Pressure

Intermittent or sustained high inspiratory pressures (>30 mm Hg) during positive-pressure ventilation increase the risk of pulmonary barotrauma (eg, pneumothorax) or hemodynamic compromise, or both, during anesthesia. Excessively high pressures may arise from incorrect settings on the ventilator, ventilator malfunction, fresh gas flow coupling (above), or activation of the oxygen flush during the inspiratory phase of the ventilator.

In addition to a high-pressure alarm, all ventilators have a built-in automatic or APL valve. The mechanism of pressure limiting may be as simple as a threshold valve that opens at a certain pressure or electronic sensing that abruptly terminates the ventilator inspiratory phase.

Tidal Volume Discrepancies

The compliance for standard adult breathing circuits is about 5 mL/cm H2O. Thus, if peak inspiratory pressure is 20 cm H2O, about 100 mL of set tidal volume is lost to expanding the circuit. For this reason breathing circuits for pediatric patients are designed to be much stiffer, with compliances as small as 1.5-2.5 mL/cm H2O.

Compression losses, normally about 3%, are due to gas compression within the ventilator bellows and may be dependent on breathing-circuit volume. Thus if tidal volume is 500 mL another 15 mL of the set tidal gas may be lost. Gas sampling for capnography and anesthetic gas measurements represent additional losses in the form of gas leaks unless the sampled gas is returned to the breathing circuit, as occurs in some machines.

Accurate detection of tidal volume discrepancies is dependent on where the spirometer is placed. Sophisticated ventilators measure both inspiratory and expiratory tidal volumes. It is important to note that unless the spirometer is placed at the Y-connector in the breathing circuit, compliance and compression losses will not be apparent.

Several mechanisms have been built into newer anesthesia machines to reduce tidal volume discrepancies. During the initial electronic self-checkout, some machines measure total system compliance and subsequently use this measurement to adjust the excursion of the ventilator bellows or piston; leaks may also be measured but are usually not compensated. The actual method of tidal volume compensation or modulation varies according to manufacturer and model. In one design a flow sensor measures the tidal volume delivered at the inspiratory valve for the first few breaths and adjusts subsequent metered drive gas flow volumes to compensate for tidal volume losses (feedback adjustment). Another design continually measures fresh gas and vaporizer flow and subtracts this amount from the metered drive gas flow (preemptive adjustment). Alternately, machines that use electronic control of gas flow can decouple fresh gas flow from the tidal volume by delivery of fresh gas flow only during exhalation. Lastly, the inspiratory phase of the ventilator-fresh gas flow may be diverted through a decoupling valve into the breathing bag, which is excluded from the circle system during ventilation. During exhalation the decoupling valve opens, allowing the fresh gas that was temporarily stored in the bag to enter the breathing circuit.

To avoid the buildup of pressure, excess gas volume is vented through the APL valve in the breathing circuit and the ventilator spill valve. Both valves should be connected to hoses (transfer tubing) leading to the scavenging interface, which may be inside the machine or an external attachment (Figure 4-29). The pressure immediately downstream to the interface should be kept between 0.5 and +3.5 cm H2O during normal operating conditions. The scavenging interface may be described as either open or closed.

An open interface is open to the outside atmosphere and usually requires no pressure relief valves. In contrast, a closed interface is closed to the outside atmosphere and requires negative- and positive-pressure relief valves that protect the patient from the negative pressure of the vacuum system and positive pressure from an obstruction in the disposal tubing, respectively. The outlet of the scavenging system may be a direct line to the outside via a ventilation duct beyond any point of recirculation (passive scavenging) or a connection to the hospital’s vacuum system (active scavenging). A chamber or reservoir bag accepts waste-gas overflow when the capacity of the vacuum is exceeded. The vacuum control valve on an active system should be adjusted to allow the evacuation of 10-15 L of waste gas per minute. This rate is adequate for periods of high fresh gas flow (ie, induction and emergence) yet minimizes the risk of transmitting negative pressure to the breathing circuit during lower flow conditions (maintenance). Unless used correctly the risk of occupational exposure for health care providers is higher with an open interface. Some machines may come with both active and passive scavenger systems.

Misuse or malfunction of anesthesia gas delivery equipment can cause major morbidity or mortality.

Detection of a Leak

After induction of general anesthesia and intubation of a 70-kg man for elective surgery, a standing bellows ventilator is set to deliver a tidal volume of 500 mL at a rate of 10 breaths/min. Within a few minutes, the anesthesiologist notices that the bellows fails to rise to the top of its clear plastic enclosure during expiration. Shortly thereafter, the disconnect alarm is triggered.

Why has the ventilator bellows fallen and the disconnect alarm sounded?

Fresh gas flow into the breathing circuit is inadequate to maintain the circuit volume required for positive-pressure ventilation. In a situation in which there is no fresh gas flow, the volume in the breathing circuit will slowly fall because of the constant uptake of oxygen by the patient (metabolic oxygen consumption) and absorption of expired CO2. An absence of fresh gas flow could be due to exhaustion of the hospital’s oxygen supply (remember the function of the fail-safe valve) or failure to turn on the anesthesia machine’s flow control valves. These possibilities can be ruled out by examining the oxygen Bourdon pressure gauge and the flowmeters. A more likely explanation is a gas leak that exceeds the rate of fresh gas flow. Leaks are particularly important in closed-circuit anesthesia.

How can the size of the leak be estimated?

When the rate of fresh gas inflow equals the rate of gas outflow, the circuit’s volume will be maintained. Therefore, the size of the leak can be estimated by increasing fresh gas flows until there is no change in the height of the bellows from one expiration to the next. If the bellows collapse despite a high rate of fresh gas inflow, a complete circuit disconnection should be considered. The site of the disconnection must be determined immediately and repaired to prevent hypoxia and hypercapnia. A resuscitation bag can be used to ventilate the patient if there is a delay in correcting the situation.

Where are the most likely locations of a breathing-circuit disconnection or leak?

Frank disconnections occur most frequently between the right-angle connector and the tracheal tube, whereas leaks are most commonly traced to the base plate of the CO2 absorber. In the intubated patient, leaks often occur in the trachea around an uncuffed tracheal tube or an inadequately filled cuff. There are numerous potential sites of disconnection or leak within the anesthesia machine and the breathing circuit, however. Every addition to the breathing circuit, such as a humidifier, increases the likelihood of a leak.

How can these leaks be detected?

Leaks usually occur before the fresh gas outlet (ie, within the anesthesia machine) or after the fresh gas inlet (ie, within the breathing circuit). Large leaks within the anesthesia machine are less common and can be ruled out by a simple test. Pinching the tubing that connects the machine’s fresh gas outlet to the circuit’s fresh gas inlet creates a back pressure that obstructs the forward flow of fresh gas from the anesthesia machine. This is indicated by a drop in the height of the flowmeter floats. When the fresh gas tubing is released, the floats should briskly rebound and settle at their original height. If there is a substantial leak within the machine, obstructing the fresh gas tubing will not result in any back pressure, and the floats will not drop. A more sensitive test for detecting small leaks that occur before the fresh gas outlet involves attaching a suction bulb at the outlet as described in step 5 of Table 4-4. Correcting a leak within the machine usually requires removing it from service.

Leaks within a breathing circuit not connected to a patient are readily detected by closing the APL valve, occluding the Y-piece, and activating the oxygen flush until the circuit reaches a pressure of 20-30 cm H2O. A gradual decline in circuit pressure indicates a leak within the breathing circuit (Table 4-4, step 11).

How are leaks in the breathing circuit located?

Any connection within the breathing circuit is a potential site of a gas leak. A quick survey of the circuit may reveal a loosely attached breathing tube or a cracked oxygen analyzer adaptor. Less obvious causes include detachment of the tubing used by the disconnect alarm to monitor circuit pressures, an open APL valve, or an improperly adjusted scavenging unit. Leaks can usually be identified audibly or by applying a soap solution to suspect connections and looking for bubble formation.

Leaks within the anesthesia machine and breathing circuit are usually detectable if the machine and circuit have undergone an established checkout procedure. For example, steps 5 and 11 of the FDA recommendations (Table 4-4) will reveal most significant leaks.