Leak Test for the Low-Pressure Circuit

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. Leaks in the low-pressure circuit can cause hypoxia or awareness by the patient.^{[88] [164]} Flow tubes, the most delicate pneumatic component of the machine, can crack or break. A typical three-gas anesthesia machine has 16 O-ring gaskets in the low-pressure circuit. Leaks can occur at the interface between the glass flow tube 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 the patient to become aware while under anesthesia.^{[88] [165]}

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. For example, most Datex-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.

Several mishaps have resulted from application of the wrong leak test to a machine.^{[38] [86] [166]} It is therefore mandatory to perform the appropriate low-pressure leak test before every case. To do this, it is essential to understand the exact location and operating principles of the Datex-Ohmeda check valve. Most Datex-Ohmeda anesthesia machines have a machine outlet check valve located in the low-pressure circuit ( Table 9-4 ). The check

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Figure 9-30 Machine outlet check valve. The check valve is located downstream from the vaporizers and upstream from the oxygen flush valve. It is open (left) in the absence of backpressure. Gas flow from the manifold moves the rubber flapper valve off its seat and allows gas to proceed freely to the common outlet. The valve closes (right) when backpressure is exerted on it. (Adapted from Bowie E, Huffman LM: The Anesthesia Machine: Essentials for Understanding. Madison, WI, Ohmeda, a Division of BOC Health Care, 1985.)

Generally, machines without check valves can be tested using a positive-pressure leak test, and machines with 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 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 leak tests for low-pressure circuits are described in the following sections.

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Oxygen Flushing in a Positive-Pressure Leak Test

Historically, older anesthesia machines did not have check valves in the low-pressure circuit. It was common practice to pressurize the breathing circuit and the low-pressure circuit with the oxygen flush valve to test for internal anesthesia machine leaks. Because many modern Datex-Ohmeda machines have check valves in the low-pressure circuit, application of a positive-pressure leak test to these machines can be misleading or even dangerous ( Fig. 9-31 ). Inappropriate use of the oxygen flush valve to evaluate the low-pressure circuit for leaks can lead to a false sense of security despite the presence of huge leaks.^{[38] [53] [86] [166]} Positive pressure from the patient's circuit closes the check valve, and the value on the airway pressure gauge does not decline. The system appears to be tight, but only the circuitry downstream from the check valve is leak free.^[32] A vulnerable area exists from the check valve back to the flow control valves because this area is not tested by a positive-pressure leak test.

1993 Food and Drug Administration Universal Negative-Pressure Leak Test

The 1993 FDA universal negative-pressure leak test^[163] (see Appendix 1 , step 5) was so named because, at that time, it could be used to check all contemporary anesthesia machines, regardless of the presence or absence of check valves in the low-pressure circuit. It remains effective for many anesthesia workstations and can be applied to

Figure 9-31 Inappropriate use of the oxygen flush valve to check the low-pressure circuit of an Ohmeda machine equipped with a check valve. The area within the rectangle (dotted line) is not checked by the inappropriate use of the oxygen flush valve, and the components located within this area are precisely the ones most subject to breakage and leaks. Positive pressure within the patient's circuit closes the check valve, and the value on the airway pressure gauge does not decline despite leaks in the low-pressure circuit. (Adapted from Andrews JJ, Brockwell RC: Delivery systems for inhaled anesthetics. In Barash PG, Cullen BF, Stoelting RK [eds]: Clinical Anesthesia, 4th ed. New York, Lippincott-Raven, 2000, pp 567–594.)

The 1993 FDA checklist is based on the Datex-Ohmeda negative-pressure leak test ( Fig. 9-32 ). It 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 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.^[164] It helps eliminate the present confusion regarding exactly which check should be performed on specific machines. The universal test is

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Figure 9-32 The U.S. Food and Drug Administration negative-pressure leak test. A negative-pressure leak-testing device is attached directly to the machine outlet (left). Squeezing the bulb creates a vacuum in the low-pressure circuit and opens the check valve. When a leak is present in the low-pressure circuit, room air is entrained through the leak, and the suction bulb inflates (right). (Adapted from Andrews JJ: Understanding anesthesia machines. In 1988 Review Course Lectures. Cleveland, International Anesthesia Research Society, 1988, p. 78.)