Leaky anesthesia machine? How to check the pressure systems

An anesthetist must understand how to use multiple pieces of equipment to perform their job properly, but the main tool of the trade is the anesthesia machine. To deliver oxygen efficiently, remove carbon dioxide and deliver appropriate concentrations of inhalant to the patient, it is paramount that the anesthesia machine be functioning properly.

This article describes how to test your anesthesia machine's pressure systems for potential leaks. Separate articles explain how to check the breathing system and scavenging system.

High-pressure system

The anesthesia machine's high-pressure system is where the pipeline and cylinder gas supplies are attached. It consists of the cylinders, hanger yoke, cylinder pressure gauge and regulator. Oxygen and other medical gases are supplied to the anesthesia machine through a central gas supply or by attaching the cylinder directly to the machine via the hanger yoke.

The larger cylinders typically are used for the central gas supply. They can be attached to the anesthesia machine by a single supply line or by a manifold that connects multiple cylinders at once. The regulator attaches to the cylinder through a gas-specific, threaded diameter index safety system (DISS; Figure 1). Malalignment of the threads or a loose connection can result in a leak.

Figure 1. Regulator attachment to an oxygen cylinder using a DISS connection. (Unless noted, all images are courtesy of Darci Palmer, BS, LVT, VTS [Anesthesia & Analgesia])

The smaller cylinders attach directly to the anesthesia machine via a gas-specific, pin-indexed hanger yoke.

A small washer, called a Bodok seal, must be placed around the yoke to form a tight attachment between the gas cylinder and hanger yoke (Figure 2). If the Bodok seal is not present, a leak will be heard when the cylinder is opened.

Figure 2. A Bodok seal must be present between the hanger yoke and gas cylinder to form a tight connection. Left: A green plastic Bodok seal commonly comes with a new cylinder. Right: Position of the Bodok seal between the hanger yoke and cylinder before it is tightened down (arrow).

For anesthesia machines that are equipped with a double hanger yoke, both yokes should contain check valves, which prevent gas leaking from a cylinder with higher pressure to one with lower pressure. The valves also prevent gas leaks into the room of a vacant hanger yoke.

Hanger yokes in older anesthesia machines may not have built-in check valves. In this instance, a plug can be placed on the empty hanger yoke to prevent retrograde leaks.¹ All cylinders should be tested immediately after being changed by slowly opening the valve in a counterclockwise direction to ensure that no leak is present from the regulator attachment.

Timing is everything

How often should you check the pressure systems in your anesthesia machine? The high-pressure system should be checked whenever an oxygen cylinder is replaced. The intermediate and low-pressure systems need to be checked via negative-pressure and positive-pressure tests on a regular basis-at least monthly or whenever the breathing system is not leak checking properly.

The breathing system in your anesthesia machine should be checked before use with every patient.

Intermediate- and low-pressure systems

The intermediate-pressure system in an anesthesia machine includes the pipeline inlet connections, pipeline pressure gauge (may not be present on all machines, especially portable ones), conduits from pipeline to flowmeter, conduits from regulator to flowmeter, flowmeter assembly and oxygen flush valve.^2,3 Aside from the oxygen flush valve, the intermediate-pressure system is difficult to test for leaks because the components are upstream from the oxygen flow control valve (i.e. flowmeter). A negative-pressure leak test (described below) can be used to assess leaks in the intermediate-pressure system, but the flowmeter must be turned on and the oxygen cylinder turned off. 

The low-pressure system-where the oxygen and volatile anesthetic join together-includes the flow indicator within the flowmeter, vaporizer, low-pressure conduits and common gas outlet. Leaks in the low-pressure system may be difficult to recognize simply by listening. The best way to check for leaks in this system is to perform both negative-pressure and positive-pressure tests.

How to perform a negative-pressure leak test. This test assesses only the flowmeter, flush valve, vaporizer and conduit tubing between the intermediate- and low-pressure systems. It cannot be used to check any components of the breathing system.

1. Make sure the flowmeter and vaporizer are turned off. If the machine has a minimum mandatory flow, it must be turned off or disconnected from the oxygen source.
2. Remove the fresh gas inlet hose from the common gas outlet.
3. Attach a suction bulb to the common gas outlet and squeeze until it is fully collapsed (Figure 3). If the bulb remains collapsed for at least 10 seconds, then no leaks are present. If the bulb inflates, a leak is present. (Note that some anesthesia machines may require the suction bulb to be collapsed manually before attaching it to the common gas outlet.)

Figure 3. Use of a suction bulb at the common gas outlet during the negative-pressure leak test.

To test for internal vaporizer leaks, repeat the process with the vaporizer turned on (to the 0% position, if possible). Due to the capacity of the vaporizer, more than 10 seconds will be needed to assess the suction bulb. This test may not be sensitive enough to detect extremely small leaks within the vaporizer.

To complete the test, ensure that the vaporizer is turned off, remove the suction bulb and replace the fresh gas inlet hose to the common gas outlet.

If the anesthesia machine contains a check valve at the common gas outlet, then a negative-pressure leak test should be performed in conjunction with a positive-pressure leak test (see below). A positive-pressure leak test will not detect a leak of the low-pressure components that are upstream from the check valve.

Fun Fact: The negative-pressure leak test is also known as the universal leak test because it can be used on anesthesia machines with or without a common gas outlet check valve.

How to perform a positive-pressure leak test. This test assesses only the flowmeter, vaporizer and conduit tubing between the intermediate- and low-pressure systems. It is not used to check any components of the breathing system. Perform this test with caution on anesthesia machines that contain a minimum mandatory flow option. Pressure that exceeds the prescribed limits can damage the flowmeter and other low-pressure components.

Figure 4. Use of a pressure gauge at the common gas outlet to perform the positive-pressure leak test. (Photo courtesy of Dr. Mike Barletta)

1. Make sure the flowmeter and vaporizer are in the off position. If the anesthesia machine contains multiple flowmeters, make sure they are all turned off.
2. Connect a pressure gauge to the common gas outlet (Figure 4). A sphygmomanometer can be modified with the appropriate connection for the common gas outlet.
3. Slowly turn on the flowmeter until the pressure gauge reads 30 cm H2O (22 mm Hg). Do not use the oxygen flush valve for this test because the high pressure can damage the delicate anesthesia machine components.
4. Turn the flowmeter off and watch the indicator in the pressure gauge for movement:If the pressure remains constant, no leak is present. If the indicator begins to fall, the leak can be quantified by slowly turning on the flowmeter until the sphygmomanometer indicator stays at 30 cm H₂O. The flow of oxygen that is needed to stop the indicator from moving represents the size of the leak. If less than 50 ml/min is needed to stop the leak, then the machine still passes this test.¹ (Note: If the indicator continues to rise with the flowmeter turned off, this can indicate a leak through the flowmeter flow control valve or oxygen flush valve, as oxygen is continuing to be supplied to the common gas outlet.
5. To test for internal leaks, repeat this test with the vaporizer turned on (to the 0% position, if possible). If the anesthesia machine contains multiple vaporizers, turn on only one vaporizer at a time to 0%.
6. Repeat steps 4 and 5 for each vaporizer. It may be necessary to turn on the flowmeter briefly to re-pressurize the system between vaporizers. The most common location for a vaporizer leak is the filling port.

If a leak is detected during either the negative- or positive-pressure test but it cannot be identified easily, then a trained professional should be called to service the machine. Their specialized professional equipment can be used to locate the exact position of the leak, which will help resolve the issue in a quick and efficient manner.  

References

1. Allen M, Smith L. Equipment checkout and maintenance. In Cooley KG, Johnson RA, Eds: Veterinary Anesthetic and Monitoring Equipment. Hoboken, NJ: John Wiley & Sons;2018:365-375.
2. Mosely C, Shelby A. Small animal anesthetic machines and equipment. In Cooley KG, Johnson RA, Eds: Veterinary Anesthetic and Monitoring Equipment. Hoboken, NJ: John Wiley & Sons;2018:23-34.
3. Shelby, A. Large animal anesthesia machines and equipment. In Cooley KG, Johnson RA, Eds: Veterinary Anesthetic and Monitoring Equipment. Hoboken, NJ: John Wiley & Sons;2018:35-54.
4. Darci Palmer became a veterinary technician specialist in anesthesia and analgesia in 2006. She holds the executive secretary position for the Academy of Veterinary Technicians in Anesthesia and Analgesia. Darci is an instructor for the Veterinary Support Personnel Network (VSPN) and an administrator for the Facebook group Veterinary Anesthesia Nerds.