The oxygen analyzer is one of the most important monitors on the anesthesia workstation. It is the only machine safety device


that evaluates the integrity of the low-pressure circuit in an ongoing fashion. Other machine safety devices, such as the oxygen failure cutoff (“fail-safe”) valve, the oxygen supply failure alarm, and the proportioning system, are all upstream from the flow control valves. The only machine monitor that detects problems downstream from the flow control valves is the oxygen analyzer. Calibration of this monitor is described in Appendix A (Anesthesia Apparatus Checkout Recommendations, 1993, Step 9). The actual procedure for calibrating the oxygen analyzer has remained reasonably similar over the recent generations of the anesthesia workstations (Guideline for Designing Pre-Anesthesia Checkout Procedures, 2008, Item 10 in Appendix B). Generally, the oxygen concentration-sensing element (usually a fuel cell on traditional machines) must be exposed to room air (at sea level) for calibration to 21%. This may require manually setting a dial on older machines, but on newer ones, it usually only involves temporary removal of the sensor, selecting and then confirming that the oxygen calibration is to be performed from a set of menus on the workstation’s display screen, and finally reinstalling the sensor. The function of the low oxygen concentration alarm should be verified by setting the alarm to trigger above the current oxygen reading. Newer workstations have automatic oxygen sensor calibration.

The low-pressure circuit leak test checks the integrity of the anesthesia machine from the flow control valves to the common gas outlet. It evaluates the portion of the machine that is downstream from all safety devices except the oxygen analyzer. The components located within this area are precisely the ones most subject to breakage and leaks. Leaks in the low-pressure circuit can cause hypoxia or patient awareness.¹⁸,¹⁹ Flow tubes, the most delicate pneumatic component of the machine, can crack or break. A typical three-gas anesthesia machine has 16 O-rings in the low-pressure circuit. Leaks can occur at the interface between the glass flow tubes.²⁰ and the manifold, and at the O-ring junctions between the vaporizer and its manifold. Loose filler caps on vaporizers are a common source of leaks, and these leaks can
lead to delivery of sub-anesthetic doses of inhaled agents, causing patient awareness during general anesthesia.¹⁸,²¹

Several different methods have been used to check the low-pressure circuit for leaks. They include the oxygen flush test, the common gas outlet occlusion test, the traditional positive-pressure leak test, the North American Dräger positive-pressure leak test, the Ohmeda 8000 internal positive-pressure leak test, the Ohmeda negative-pressure leak test, the 1993 FDA universal negative-pressure leak test, and others. One reason for the large number of methods is that the internal design of various machines differs considerably. The most notable example is that many GE Healthcare/Datex-Ohmeda (hereafter referred to as Datex-Ohmeda) machines/workstations have a check valve near the common gas outlet, whereas Dräger Medical workstations do not. The presence or absence of the outlet check valve profoundly influences which pre-use check is indicated.

Several mishaps have resulted from application of the wrong leak test to the wrong machine.²²,²³,²⁴,²⁵ Therefore, it is mandatory to perform the appropriate low-pressure leak test each day. To do this, it is essential to understand the exact location and operating principles of the Datex-Ohmeda check valve. Many Datex-Ohmeda anesthesia workstations have a machine outlet check valve located in the low-pressure circuit (Table 24-1). The check valve is located downstream from the vaporizers and upstream from the oxygen flush valve (Fig. 24-4). It is open in the absence of back pressure. Gas flow from the manifold moves the rubber flapper valve off its seat and allows gas to proceed freely to the common gas outlet. The valve closes when back pressure is exerted.¹²,²⁶ Back pressure sufficient to close the check valve may occur with the following conditions: oxygen flushing, peak breathing circuit pressures generated during positive-pressure ventilation, or use of a positive-pressure leak test.

Generally speaking, the low-pressure circuit of anesthesia workstations without an outlet check valve can be tested using a positive-pressure leak test, and machines with outlet check valves must be tested using a negative-pressure leak test. When performing a positive-pressure leak test, the operator generates positive pressure in the low-pressure circuit using flow from the anesthesia machine or from a positive-pressure squeeze bulb to detect a leak. When performing a negative-pressure leak test, the operator creates negative pressure in the low-pressure circuit using a suction bulb to detect leaks. Two different low-pressure circuit leak tests are described below.

The circle system tests (Appendix B, Items 12–13) evaluate the integrity of the circle breathing system, which spans from the machine common gas outlet to the Y-piece (Fig. 24-7). The test has two components: (1) breathing system pressure and leak testing and (2) verification that gas flows properly through the breathing circuit during both inspiration and exhalation. To thoroughly check the circle system for leaks, valve integrity, and obstruction, both tests must be performed preoperatively. The ASA 2008 recommendations call for performing the breathing system test and leak test before starting each case, such that pressure can be developed in the system during both manual/bag and automatic/mechanical ventilation. Automated leak testing routines are implemented in modern workstations; system compliance is also calculated and used to adjust volume delivery during mechanical ventilation (Appendix B, Item 12). Because pressure and leak testing cannot identify all obstructions in the breathing circuit or confirm the function of the inspiratory and expiratory unidirectional valves, a test lung or second reservoir bag connected at the Y-piece can be used to confirm circuit integrity and function. Visual inspection of the unidirectional valves should be performed daily, though, because subtle damage to these valves is difficult to determine. Older 1993 FDA checkout procedures to identify valve incompetence that may not be visually obvious can be implemented, but are typically too complex for daily testing (Appendix B, Item 13).³⁰,³¹

In the 1993 FDA Anesthesia Apparatus Checkout Recommendations, a leak test is performed by closing the APL (or pop-off) valve, occluding the Y-piece, and pressurizing the circuit to 30 cm water pressure using the oxygen flush valve. The value on the pressure gauge will not decrease if the circle system is leak-free, but this does not assure unidirectional valve integrity or function. The value on the pressure gauge will read 30 cm H₂O even if the unidirectional valves are stuck shut or are incompetent. In addition, a flow test checks the integrity of the unidirectional valves, and it detects obstruction in the circle system. It can be performed by removing the Y-piece from the circle system and breathing through the two corrugated hoses individually. The unidirectional valve leaflets should be present and should move appropriately. The operator should be able to inhale but not be able to exhale through the inspiratory limb. The operator should be able to exhale but not inhale through the expiratory limb. The flow test can also be performed by using the ventilator and a reservoir bag connected to the “Y” piece as described in the 1993 FDA
Anesthesia Apparatus Checkout Recommendations (Appendix A, Steps 11–12).¹³

Many new anesthesia workstations now incorporate technology that allows the machine to either automatically or manually guide the user through a series of self-tests to check for functionality of electronic, mechanical, and pneumatic components. Tested components commonly include the gas supply system, flow control valves, the circle system, ventilator, and integrated vaporizers. The comprehensiveness of these self-diagnostic tests varies from one model and manufacturer to another. If these tests are to be employed, users must be certain to read and strictly follow all manufacturer recommendations. Although a thorough understanding of what the particular workstation’s self-tests include is very helpful, this information is often difficult to obtain and may vary greatly between devices.

One particularly important point of caution with self-tests should be noted on systems with manifold-mounted vaporizers such as the Dräger Apollo, Dräger Fabius GS and Narkomed 6000 series. A manifold-mounted vaporizer does not become a part of an anesthesia workstation’s low-pressure system until its concentration control dial is turned to the “on” position. Therefore, to detect internal vaporizer leaks in this type of a system, the “leak test” portion of the self-diagnostic must be repeated with each individual vaporizer turned to the “on” position. If this precaution is not taken, large leaks that could potentially result in patient awareness, such as those from a loose filler cap or cracked fill indicator, could go undetected.

Most hospitals today have a central piping system to deliver medical gases including oxygen, nitrous oxide, air, and carbon dioxide to outlets in the operating room. The central piping system must supply the correct gases at the appropriate pressure for the anesthesia workstation to function properly. Unfortunately, this does not always occur. Even as recently as 2002, a large medical center with a huge cryogenic bulk oxygen storage system was not immune to component failures that contributed to a critical oxygen pipeline
supply failure.³⁵ In this case, a faulty joint ruptured at the bottom of the primary cryogenic oxygen storage tank, releasing 8,000 gallons of liquid oxygen to flood the streets in the surrounding area and compromised oxygen delivery to the medical center.

In a 1976 survey of approximately 200 hospitals, 31% reported difficulties with pipeline systems.³⁶ The most common problem was inadequate oxygen pressure, followed by excessive pipeline pressures. The most devastating reported hazard, however, was accidental crossing of oxygen and nitrous oxide pipelines, which has led to many deaths. This problem caused 23 deaths in a newly constructed wing of a general hospital in Sudbury, Ontario, during a 5-month period.³⁷ In 2002, two hypoxic deaths were reported in New Haven, Connecticut. These resulted from a medical gas system failure in which an altered oxygen flowmeter was connected to a wall supply source for nitrous oxide.³⁸

In the event that a pipeline crossover is suspected, the workstation user must immediately take two corrective actions. First, the backup oxygen cylinder should be turned on. Then, the pipeline supply must be disconnected. This second step is mandatory because the machine will preferentially use the (potentially) inappropriate 50 psig pipeline supply source instead of the lower-pressure (45 psig) oxygen cylinder source if the wall supply is not disconnected. Recent publications suggest that many anesthesia providers may not appreciate the importance of or reasons for these actions.³⁹,⁴⁰

The wall outlet connections for pipeline gases are gas-specific. If they are “quick connect” fittings then they are gas-specific within the same manufacturer. For example, a wall oxygen outlet made by Ohmeda will not accept an oxygen connector made by Chemetron, even though the gas is the same. This can create problems if outlets and connectors by more than one manufacturer exist in the same facility.⁴¹ Many institutions seeking to create uniformity are now using nationally standardized Diameter Index Safety System (DISS) threaded connections. The DISS provides threaded, noninterchangeable connections for medical gas lines, which minimizes the risk of misconnection. Regardless of which type of gas-specific connector (DISS or “quick connect”) exists at the wall end of the hose conducting gas to the anesthesia machine, the gas enters the anesthesia machine through DISS inlet connections (see Fig. 24-4; arrows). A pressure gauge measures the pipeline gas pressure when the machine is connected to a pipeline supply. A check valve is located downstream from the inlet. It prevents reverse flow of gases from the machine to the pipeline or the atmosphere.

Anesthesia workstations have E-cylinders for use when a pipeline supply source is not available or if the pipeline system fails. Anesthesia providers can easily become complacent and falsely assume that backup gas cylinders are, in fact, present on the anesthesia workstation, and further, if present, that they contain an adequate supply of compressed gas. The pre-use checklist should contain steps that confirm both.

Medical gases supplied in E-cylinders are attached to the anesthesia machine via the hanger yoke assembly. The hanger yoke assembly orients and supports the cylinder, provides a gas-tight seal, and ensures a unidirectional flow of gases into the machine.³³ Each hanger yoke is equipped with the Pin Index Safety System (PISS). The PISS is a safeguard introduced to eliminate cylinder interchanging and the possibility of accidentally placing the incorrect gas on a yoke designed to accommodate another gas. Two metal pins on the yoke assembly are arranged so that that they project into corresponding holes in the cylinder valve. Each gas or combination of gases has a specific and unique pin arrangement.⁴²,⁴³ A failure of the pin index system, and medical staff to properly identify E-cylinder contents, was the cause of an intraoperative fire during laparoscopy.⁴⁴ A mixture of oxygen and CO₂ was utilized rather than 100% CO₂.

Once the cylinders are turned on, compressed gases may pass from their respective high-pressure cylinder sources into the anesthesia machine (see Fig. 24-4). A check valve is located downstream from each cylinder if a double-yoke assembly is used. This check valve serves several functions. First, it minimizes gas transfer from a cylinder at high pressure to one with a lower pressure. Second, it allows an empty cylinder to be exchanged for a full one while gas flow continues from the other cylinder into the machine with minimal loss of gas or supply pressure. Third, it minimizes leakage from an open cylinder to the atmosphere if one cylinder is absent.³²,³³ A cylinder supply pressure gauge is located downstream from the check valves. The gauge will indicate the pressure in the cylinder having the higher pressure when two reserve cylinders of the same gas are opened at the same time.

Each cylinder supply source has a pressure-reducing valve known as the cylinder pressure regulator. It reduces the high and variable storage pressure present in a cylinder to a lower, more constant pressure suitable for use in the anesthesia machine. The oxygen cylinder pressure regulator reduces the oxygen cylinder pressure from a high of 2,200 psig to approximately 45 psig. The nitrous oxide cylinder pressure regulator receives pressure of up to 745 psig and reduces it to approximately 45 psig.³²,³³

The gas supply cylinder valves should be turned off when not in use, except during the preoperative machine pre-use checkout. If the cylinder supply valves are left open, the reserve cylinder supply can be silently depleted whenever the pressure inside the machine decreases to a value lower than the regulated cylinder pressure. For example, oxygen pressure within the machine can decrease below 45 psig with oxygen flushing or possibly even during the use of a pneumatically driven ventilator, particularly at high inspiratory flow rates. In addition, the pipeline supply pressures of all gases can fall to less than 45 psig if problems exist in the central piping system. If the cylinders are left open when this occurs, they will eventually become depleted and no reserve supply may be available if a complete central pipeline failure were to occur.²⁹,³²

The amount of time that an anesthesia machine can operate from the E-cylinder supply is important knowledge. This is particularly true now that anesthesia is being provided more frequently in office-based and in remote (outside the OR) hospital settings where pipeline oxygen may not be available. Oxygen can exist only in gaseous form at room temperature, and it obeys Boyle’s law which states that for a fixed mass of gas at constant temperature, the product of pressure times volume is constant.⁴⁵ The volume of oxygen available from the cylinder is directly proportional to the cylinder pressure.

An E-cylinder has an internal volume of 4.8 L and when “full” is pressurized to 2,000 psig. Since psig is the pressure measured in excess of atmospheric pressure (14.7 psia, pounds per square inch absolute pressure), the cylinder pressure is 2,014.7 psia. Applying Boyle’s Law:

Therefore, V2, the volume of oxygen in a “full” E-cylinder at 1 atm is

The following equation has been proposed to help estimate the remaining time that oxygen can be delivered at a given flow rate⁴⁶:

For example, an E-cylinder of oxygen with a pressure of 1,000 psig, used at an oxygen flow rate of 5 L/min would be depleted in

It should be noted that this calculation will provide only a gross estimate of remaining time and may not be exact. Furthermore, users should be cautioned that use of a pneumatically driven mechanical ventilator will dramatically increase oxygen utilization rates and decrease the remaining time until the cylinder is depleted. Uses of spontaneous or manual ventilation, with low FGF rates in a circle system with CO₂ absorption, will significantly reduce oxygen consumption from an E-cylinder if this is the only source of oxygen available.⁷,⁸,³⁵ Because electrically powered piston type anesthesia ventilators, such as found in the Dräger Fabius GS and Apollo workstations, do not impact oxygen usage rates they may be preferable to conventional gas-driven ventilators in practice settings where the supply of compressed gas cylinders may be limited.

A new regulator for E-cylinders of oxygen is available that permits controlled delivery of oxygen via a nozzle at flows of ≤25 L/min for patient transport (Fig. 24-8 A,B,C,D and E). The tank regulator also permits delivery of oxygen at 50 psig from a DISS connection (Fig. 26-9). If the oxygen hose from the anesthesia machine is connected to a central source (e.g., at the wall) via a DISS connector, and that central source becomes unavailable, then the machine hose can be easily connected to the tank’s DISS connector and provide a backup supply of oxygen (Fig. 24-9 B). A conventional E cylinder with pin index safety system is shown in Fig. 24-8C,D,E.

Nitrous oxide (N₂O) can be supplied to the anesthesia machine from the pipeline system at a pressure of approximately 50 psig or from a backup E-cylinder in the N₂O hanger yoke. N₂O has a molecular weight of 44 atomic mass units (AMU) and a boiling point of −88°C at 760 mm Hg (14.7 psia) pressure.⁴⁷ The critical temperature (CT) is the highest temperature at which a gas can exist in liquid form. The CT of N₂O is 36.5°C (critical pressure: 1,054 psig), therefore N₂O can exist as a liquid at room temperature (20°C). E-cylinders of N₂O are factory-filled to 90% to 95% capacity with liquid N₂O. Above the liquid in the tank is N₂O vapor. Because the liquid agent is in equilibrium with its vapor or gas phase, the pressure exerted by the gaseous N₂O is its saturated vapor pressure (SVP) at the ambient temperature. At 20°C, the SVP of N₂O is 750 psig.

A full E-tank of N₂O generates approximately 1,600 L of gas at 1 atm pressure at sea level (14.7 psia). As long as some liquid N₂O is present in the tank and the ambient temperature remains at 20°C, the pressure in the N₂O tank will remain at 750 psig, which is the SVP of N₂O at 20°C. The volume of N₂O gas available from a tank therefore cannot be determined by reference to the N₂O tank pressure gauge. It is determined by weighing the tank and subtracting the weight of the empty tank (tare weight) to determine the weight of the contained N₂O.

Once all the liquid N₂O has been used and the tank contains only gas, Boyle’s law (i.e., P1 × V1 = P2 × V2) may be applied. When the tank pressure is ∼750 psig (or 764.7 psia) from gas only, and the internal volume of the E-cylinder is 4.8 L, the
volume of N₂O available at a pressure of 1 atm (i.e., 760 mm Hg or 14.7 psia) is 250 L. At this point the N₂O tank is 250/1,600, or ∼16%, full. From then on, as N₂O continues to be utilized, the value on the tank pressure gauge will fall.

Nitrous oxide from the tank supply enters the N₂O hanger yoke at pressures of up to 750 psig (at 20°C) and then passes through a regulator that reduces this pressure to 40 to 45 psig (Fig. 38-2). The PISS is designed to ensure that only a N₂O tank may hang in a N₂O hanger yoke. As with oxygen, a check valve in each yoke prevents the back leakage of N₂O if no tank is hanging in the yoke.

The N₂O pipeline is supplied from a bulk storage container of liquid N₂O or from banks of large N₂O tanks, usually H cylinders. (Each H cylinder of N₂O evolves 16,000 L of gas at atmospheric pressure.) The pressure in the N₂O pipeline is regulated to approximately 50 psig to supply the outlets in the operating room. Having entered the anesthesia machine intermediate-pressure system, N₂O must flow past the “fail-safe” valve to reach the N₂O flow-control.

Having entered the anesthesia machine’s intermediate-pressure system from the pipeline supply at ∼50 psig, or from the tank supply at 45 psig, oxygen can take several paths:

The 2000 ASTM F1850-00 standard states, “The anesthesia gas supply device shall be designed so that whenever oxygen supply pressure is reduced to below the manufacturer specified minimum, the delivered oxygen concentration shall not decrease below 19% at the common gas outlet.”⁴⁸ Contemporary anesthesia machines have a number of safety devices that act together in a cascade manner to minimize the risk of delivery of a hypoxic gas mixture as oxygen pressure decreases. Several of these devices are described in the following sections.

The flowmeter assembly (Fig. 24-12) precisely controls and measures gas flow to the common gas outlet. With traditional glass flowmeter assemblies, the flow control needle valve regulates the amount of flow that enters a tapered, transparent flow tube known as a Thorpe tube. The tube is tapered such that it has a small cross-sectional area at its lower (low flow) end, and a larger cross-sectional area at its upper (high flow) end. A mobile indicator float inside the flow tube indicates the amount of flow passing through the associated flow control valve. The quantity of flow is indicated on a scale associated with the flow tube.³²,³³ Some newer anesthesia workstations have now replaced the conventional glass flow tubes with electronic flow sensors that measure the flow of the individual gases. The flow rate data are then presented in numerical format, graphical format, or a combination of the two. The integration of these “electronic flowmeters” is an essential step in the evolution of the anesthesia workstation if it is to become fully integrated with anesthesia data-capturing systems, such as computerized anesthesia record keepers (or AIMS: anesthesia information management systems).

Newer anesthesia workstations such as the GE-Datex-Ohmeda S/5 ADU, the Dräger Fabius GS, and the Dräger Apollo (among others) have conventional flow control knobs and flow control valves, but have electronic flow sensors and digital displays rather than glass flow tubes (Fig. 24-17). The output from the flow control valve is represented graphically and/or numerically in liters per minute on the workstation’s integrated user interface. These systems are dependent on electrical power to provide a precise display of gas flows. However, even when electrical power is totally interrupted, since the flow control valves themselves are mechanical (i.e., non-electronic), the set gas flows will continue uninterrupted. Since these machines do not have individual flow tubes that physically quantitate the flow of each gas, a small conventional pneumatic “fresh gas” or “total flow” indicator is also provided that
gives the user an estimate of the total quantity of fresh gas flowing from all gas flow-control valves to the anesthesia workstation’s common gas outlet, and is functional even in the event of a total power failure (Fig. 24-18).

In the GE Datex Aisys Carestation, the traditional needle valve gas flow controls and color-coded control knobs are replaced by an electronic control system that uses a gas mixer. In the GE Aisys Carestation, the second gas, either N₂O or air is first selected, followed by the desired inspired oxygen concentration (FIO₂) and total FGF. Total flow and FIO₂ selections are made by pressing soft keys on the control panel, adjusting the settings using a “com wheel,” and then pressing the com wheel to “confirm.”

In the Aisys Carestation, the controls to increase or decrease flows (or agent concentration) represent a departure from the traditional. The traditional needle valve gas flow controls were designed by mechanical engineers so that one turns the flow control knob counterclockwise to increase flow (by opening the valve wider). The same applies to increasing agent concentration on a variable bypass vaporizer. The Aisys Carestation controls are designed by electrical engineers where the standard is to increase the output by rotating the dial (com wheel) in a clockwise direction. Thus when learning to use the Aisys Carestation workstation, the operator must adapt to “clockwise to increase” and remember to confirm new settings, otherwise they are not implemented. In the event the gas mixer fails, the Aisys Carestation will switch to a backup system that permits delivery of oxygen to the breathing system via an Alternate Oxygen flowmeter, which is a traditional mechanical needle valve and rotameter flow tube.

Manufacturers equip anesthesia workstations with N₂O/O₂ proportioning systems designed to prevent creation and delivery of a hypoxic mixture when nitrous oxide is administered. Nitrous oxide and oxygen are interfaced mechanically and/or pneumatically, or electronically (on the GE Aisys Carestation), so that the minimum oxygen concentration at the common gas outlet is between 23% and 25% depending on the manufacturer.

The oxygen flush valve allows direct communication between the oxygen intermediate-pressure circuit and the low-pressure circuit (see Fig. 24-4). Flow from the oxygen flush valve enters the
low-pressure circuit downstream from the vaporizers and, most importantly, downstream from any outlet check valve, if present. The spring-loaded oxygen flush valve remains closed until the operator opens it by depressing the oxygen flush button. Actuation of the valve delivers 100% oxygen at a flow of 35 to 75 L/min to the breathing circuit.³²

The oxygen flush valve can provide a “high pressure” oxygen source that might be used for jet ventilation under the following circumstances: (1) the anesthesia machine is equipped with a one-way check valve positioned between the vaporizers and the oxygen flush valve; and (2) when a positive-pressure relief valve exists downstream from the vaporizers. The pressure relief valve must be upstream of the outlet check valve. Because the Ohmeda Modulus II has such a one-way check valve and its low-pressure system positive-pressure relief valve is upstream from the outlet check valve, the entire oxygen flow of 35 to 75 L/min is delivered to the common gas outlet at a pressure of 45 to 50 psig. On the other hand, the Ohmeda Modulus II Plus and some Ohmeda Excel machines are not capable of functioning as an appropriate oxygen source for jet ventilation. The Ohmeda Modulus II plus, which does not have the check valve, provides only 7 psig at the common gas outlet because much of the oxygen flows retrograde into the low-pressure circuit and out to atmosphere through an internal relief valve located upstream from the oxygen flush valve. The Ohmeda Excel 210, which does have a one-way check valve, also has a positive-pressure relief valve downstream from the check valve and therefore is unsuitable for jet ventilation. Older North American Dräger machines such as the Narkomed 2A (which also does not have the outlet check valve) produce a pressure of 18 psig at the common gas outlet because oxygen is vented to atmosphere through a pressure relief valve located in the Dräger Vapor vaporizers.⁶¹

It must be emphasized that use of the oxygen flush to drive a jet ventilation system connected at the machine’s common gas outlet is an “off label” use of the machine, and is not recommended by the machine manufacturers. If jet ventilation is required, a purpose-built Sanders type system should be used, connected to a 50 psig oxygen source.

Several hazards have been reported with use of the oxygen flush valve. A defective or damaged valve can stick in the fully open position, resulting in barotrauma.⁶² A valve sticking in a partially open position can result in patient awareness during general anesthesia, because the oxygen flow from the incompetent valve dilutes the inhaled anesthetic.²⁸ Improper use of normally functioning oxygen flush valves also can result in problems. Overzealous intraoperative oxygen flushing can dilute inhaled anesthetics. Oxygen flushing during the inspiratory phase of positive-pressure ventilation can produce barotrauma in patients if the anesthesia machine does not incorporate fresh gas decoupling or an appropriately adjusted inspiratory pressure limiter. Anesthesia systems (Dräger Narkomed 6000 series, Julian, Fabius GS, and Datascope Anestar) with fresh gas decoupling are inherently safer from the standpoint of minimizing the chance of producing barotrauma from inappropriate oxygen flush valve use. These systems physically separate the fresh gas inflow from either the flowmeters or the oxygen flush valve, from the delivered tidal volume presented to the patient’s lungs (see Fresh Gas Decoupling section). With traditional anesthesia breathing circuits, excess volume cannot be vented during the inspiratory phase of mechanical ventilation because the ventilator pressure relief valve is closed and the APL valve is either out-of-circuit or closed.⁶³ An alternative solution to this problem is used in the GE-Datex-Ohmeda S/5 ADU and GE-Aestiva. The breathing systems on these machines utilize an integrated adjustable pressure limiter. If this device is properly adjusted, it functions like the APL (or pop-off) valve to limit the maximum airway pressure to a safe level, thereby reducing the possibility of barotrauma.

Some very old anesthesia systems made use of a freestanding vaporizer downstream from the common gas outlet; on these systems, oxygen flushing could rapidly deliver dangerously large quantities of inhaled anesthetic to the patient. Finally, inappropriate use of the oxygen flush to evaluate the low-pressure circuit for leaks can be misleading, particularly on GE Datex-Ohmeda machines with a one-way check valve at the common gas outlet.²⁷ Since back pressure from the breathing circuit closes the one-way check valve gas-tight, major low-pressure circuit leaks can go undetected with this leak test (see Checking Your Anesthesia Workstation section).