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Ensuring that all parts of your veterinary anesthesia machine are in good working order requires both preventive care and routine maintenance. Here's how to detect leaks in your machine's 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.
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.
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.
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.
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.
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.
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.
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.