Anesthesia machines have evolved from simple, pneumatic devices to sophisticated, computer-based, fully integrated anesthesia systems. A few years ago, a rudimentary background in pneumatics sufficed, but today an understanding of pneumatics, electronics, and even computer science is useful. Even though it is more difficult for the anesthesiologist to achieve a thorough understanding of modern anesthesia machines, it is essential to the safe practice of anesthesia. The anesthesiologist must be aware of design differences among manufacturers so that appropriate preoperative checks can be performed. The best time to troubleshoot an anesthesia machine is preoperatively, using appropriate preoperative checks.
An anesthesia system consists of the various components that communicate with each other during the administration of inhalation anesthesia.(1) System components include the anesthesia machine, the vaporizers, the anesthetic circuit, the ventilator, and the scavenging system. A thorough understanding of these parts is essential to the safe practice of anesthesia. This manuscript discusses the normal operation, function, and integration of major machine components, and it illustrates some problems and hazards associated with each. Finally, it describes appropriate preoperative checks to test the low-pressure circuit for leaks.
A generic two-gas anesthesia machine is shown in Figure 1. Both oxygen and nitrous oxide have two supply sources, consisting of a pipeline supply source and a cylinder supply source. The pipeline supply source is the primary gas source for the anesthesia machine. The hospital piping system provides gases to the machine at approximately 50 pounds per square inch gauge (PSIG), which is the normal working pressure of most machines. The cylinder supply source serves as a backup if the pipeline fails. The oxygen cylinder source is regulated from 2,200 to approximately 45 PSIG, and the nitrous oxide cylinder source is regulated from 745 to approximately 45 PSIG.(2-18)
A safety device traditionally referred to as the fail-safe valve is located downstream from the nitrous oxide supply source. It serves as an interface between the oxygen and nitrous oxide supply sources. This valve shuts off, or proportionally decreases, the supply of nitrous oxide (and other gases) if the oxygen supply pressure decreases. Contemporary machines have an alarm device to monitor the oxygen supply pressure. An alarm is actuated at a predetermined oxygen pressure, such as 30 PSIG.(2,3,13,18)
Most Ohmeda machines have a second-stage oxygen regulator located downstream from the oxygen supply source. It is adjusted to a precise pressure level, such as 14 PSIG.(3,6-12) This regulator supplies a constant pressure to the oxygen flow control valve regardless of fluctuating oxygen pipeline pressures. For example, the flow from the oxygen flow control valve will be constant if the oxygen supply pressure is greater than 14 PSIG.
The flow control valves are an important anatomical landmark because they divide the anesthesia machine into two parts. The high-pressure circuit is that part of the machine which is upstream from the flow control valves, and the low-pressure circuit is that part of the machine which is downstream from the flow control valves. The operator regulates flow entering the low-pressure circuit by adjusting the flow control valves. The oxygen and nitrous oxide flow control valves are linked mechanically or pneumatically by a proportioning system to help prevent delivery of a hypoxic mixture. The flow travels through a common manifold and may be directed to a calibrated variable-bypass vaporizer. Precise amounts of inhaled anesthetic can be added, depending on the vaporizer setting. The total fresh gas flow travels toward the common gas outlet.(2,3)
Many Ohmeda machines have a machine outlet check valve between the vaporizers and the common gas outlet.(3-10) Its purpose is to prevent backflow into the vaporizer, therefore minimizing the effects of downstream intermittent pressure fluctuations on inhaled anesthetic concentration. The presence or absence of a check valve profoundly influences which preoperative leak test is indicated. The oxygen flush connection joins the mixed-gas pipeline between the one-way check valve and the machine outlet. Thus, the oxygen flush, when activated, has a "straight shot" to the common outlet.(2,3)
Manufacturers have equipped newer machines with proportioning systems in an attempt to prevent delivery of a hypoxic mixture. Nitrous oxide and oxygen are interfaced either mechanically or pneumatically so that the minimum oxygen concentration at the common outlet is 25 percent.
Contemporary Ohmeda machines use the Link-25 system. The heart of the system is the mechanical integration of the nitrous oxide and oxygen flow control valves. It allows independent adjustment of either valve, yet automatically intercedes to maintain a minimum 25 percent oxygen concentration with a maximum nitrous oxide/oxygen flow ratio of 3:1. The Link-25 automatically increases oxygen flow to prevent delivery of a hypoxic mixture.(6-12)
Figure 2 shows the Ohmeda Modulus II Link-25 system. The nitrous oxide and oxygen flow control valves are identical. A 14-tooth sprocket is attached to the nitrous oxide flow control valve, and a 28-tooth sprocket is attached to the oxygen flow control valve. A chain physically links the sprockets. When the nitrous oxide flow control valve is turned through two revolutions, or 28 teeth, the oxygen flow control valve will revolve once because of the 2:1 gear ratio. The final 3:1 flow ratio results because the nitrous oxide flow control valve is supplied by approximately 26 PSIG, whereas the oxygen flow control valve is supplied by 14 PSIG. Thus, the combination of the mechanical and pneumatic aspects of the system yields the final oxygen concentration.(7,8) North American Dräger's proportioning system, the Oxygen Ratio Monitor Controller (ORMC), is used on the North American Dräger Narkomed 2A, 2B, 3, and 4.(13-18) It is a pneumatic oxygen / nitrous oxide interlock system designed to maintain a fresh gas oxygen concentration of at least 25 +/- 3 percent. The device controls the fresh gas oxygen concentration to levels substantially higher than 25 percent at oxygen flowrates less than 1 L/min. The ORMC limits nitrous oxide flow to prevent delivery of a hypoxic mixture.(13-17) This system is unlike the Ohmeda Link-25, which actively increases oxygen flow.
A schematic of the ORMC is shown in Figure 3. It is composed of an oxygen chamber, a nitrous oxide chamber, and a nitrous oxide slave control valve; all are interconnected by a mobile horizontal shaft. The pneumatic input into the device is from the oxygen and the nitrous oxide flowmeters. These flowmeters are unique because they have specific resistors located downstream from the flow control valves. These resistors create back pressures directed to the oxygen and nitrous oxide chambers. The value of the oxygen flowtube resistor is three to four times that of the nitrous oxide flowtube resistor, and the relative value of these resistors determines the value of the controlled fresh gas oxygen concentration. The back pressure in the oxygen and nitrous oxide chambers pushes against rubber diaphragms attached to the mobile horizontal shaft. Movement of the shaft regulates the nitrous oxide slave control valve, which feeds the nitrous oxide flow control valve.(1,14,18)
If the oxygen pressure is proportionally higher than the nitrous oxide pressure, the nitrous oxide slave control valve opens more widely, allowing more nitrous oxide to flow. As the nitrous oxide flow is increased manually, the nitrous oxide pressure forces the shaft toward the oxygen chamber. The valve opening becomes more restrictive and limits the nitrous oxide flow to the flowmeter. Figure 3 illustrates the action of a single ORMC under different sets of circumstances. In the upper configuration, the back pressure exerted on the oxygen diaphragm is greater than that exerted on the nitrous oxide diaphragm. This causes the horizontal shaft to move to the left, opening the nitrous oxide slave control valve. Nitrous oxide is then able to proceed to its flow control valve and out through the flowmeter. In the lower configuration, the nitrous oxide slave control valve is closed because of inadequate oxygen back pressure.(1,14,18)
Proportioning systems are not foolproof. Machines equipped with proportioning systems still can deliver a hypoxic mixture under the following conditions.
Both the Link-25 and the ORMC will be fooled if a gas other than oxygen is present in the oxygen pipeline. In the Link-25 system, the nitrous oxide and oxygen flow control valves will continue to be mechanically linked, and a hypoxic mixture will proceed to the common outlet. The oxygen rubber diaphragm of the ORMC will recognize adequate "oxygen" pressure, and flow of both the wrong gas plus nitrous oxide will result. The oxygen analyzer is the only machine monitor that will detect this condition in both systems.
Normal operation of the Ohmeda Link-25 and the North American Dräger ORMC is contingent upon pneumatic and mechanical integrity. Pneumatic integrity in the Ohmeda system depends on properly functioning second-stage regulators. A nitrous oxide / oxygen ratio other than 3:1 will result if the regulators are not precise. The chain connecting the two sprockets must be intact. A 97 percent nitrous oxide concentration can occur if the chain is cut or broken.(19) In the North American Dräger system, a functional Oxygen Failure Protection Device (OFPD) is necessary to supply appropriate pressure to the ORMC. The mechanical aspects of the ORMC, such as the rubber diaphragms, the flowtube resistors, and the nitrous oxide slave control valve, must likewise be intact.
Administration of a third inert gas, such as helium, nitrogen, or carbon dioxide, can cause a hypoxic mixture because contemporary proportioning systems link only nitrous oxide and oxygen.(6-17) Use of an oxygen analyzer is mandatory if the operator uses a third inert gas.
The ORMC and the Link-25 function at the level of the flow control valves. A leak downstream from these devices such as a broken oxygen flow tube (Figure 4) can cause delivery of a hypoxic mixture. Oxygen escapes through the leak, and the predominant gas delivered at the common outlet is nitrous oxide. The oxygen analyzer is the only machine safety device that can detect the problem.1
The least well understood preoperative check is the low-pressure circuit leak test. Several mishaps have resulted from application of the wrong leak test to the wrong machine.(20-22) Leaks in the low-pressure circuit can cause hypoxia or patient awareness under anesthesia.
The low-pressure leak test checks the integrity of the anesthesia machine from the flow control valves to the common 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. Flowtubes are the most delicate pneumatic component of the machine, and they 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 flowtube and the manifold, and at the O-ring junction between the vaporizer and its manifold. Loose filler caps on vaporizers are a common source of leaks, and these leaks can cause patient awareness.(23) Therefore, it is mandatory to perform the appropriate low-pressure leak test before every case.
Several different methods have been used to check the low-pressure circuit for leaks. They include the common gas outlet occlusion test, the oxygen flush leak 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 new 1993 FDA universal negative-pressure leak test (see below), 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 most Ohmeda machines have a check valve near the common gas outlet, whereas North American Dräger machines do not. The presence or absence of the check valve profoundly influences which preoperative check is indicated. For example, application of the inappropriate oxygen flush leak test to a machine equipped with a check valve can lead to a false sense of security despite the presence of huge leaks (Figure 5).(24) Positive pressure from the patient circuit closes the check valve, and the value on the airway pressure gauge does not decline. The system appears to be tight, but in actuality, only the circuitry downstream from the check valve is leak-free. Thus, a vulnerable area, not tested by the positive-pressure leak test, exists. This area extends from the check valve back to the flow control valves.
The 1993 FDA Anesthesia Apparatus Checkout Recommendations suggests a universal negative-pressure leak test (Figure 6).(25) It works on all contemporary anesthesia machines regardless of the presence or absence of the check valve. The FDA check is based upon the Ohmeda negative-pressure leak test. The FDA check is performed using a negative-pressure leak testing device, which is a simple suction bulb. The machine master switch, the flow-control valves, and vaporizers are turned off. The suction bulb is attached to the common fresh gas outlet and squeezed repeatedly until it is fully collapsed. This action creates a vacuum in the low-pressure circuitry. The machine is leak-free if the hand bulb remains collapsed for at least 10 seconds. A leak is present if the bulb reinflates during this time period. The test is repeated with each vaporizer individually turned to the ON position because internal vaporizer leaks can be detected only with the vaporizer turned ON.
The FDA universal negative-pressure low-pressure circuit leak test has several advantages. It will help eliminate the present confusion regarding exactly which check should be performed on specific machines. The universal test is quick and simple to perform. It has an obvious end point, and it isolates the problem. For example, if the bulb reinflates in less than 10 seconds, a leak is present somewhere in the low-pressure circuit. Therefore, it differentiates between breathing-circuit leaks and leaks in the low-pressure circuit. The universal negative-pressure leak test is the most sensitive of all contemporary leak tests because it is not volume dependent; that is, it does not involve a compliant breathing bag or corrugated hoses. It can detect leaks as small as 30 cc/min. Finally, the operator does not need a detailed or in-depth knowledge of proprietary design differences. If the operator performs the universal test correctly, the leak will be detected.
Rapid advancements in the anesthesia industry make it increasingly difficult for the anesthesiologist to keep up with anesthesia machine technology. Nevertheless, a thorough understanding of the machine is mandatory for the safe practice of anesthesia. Machines are equipped with dozens of safety features, yet none of them are foolproof. The anesthesiologist still must check the machine preoperatively using appropriate checkout procedures.
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