The function of each veterinary anesthesia machine should be checked regularly. The following is how to evaluate your machine breathing system, which should be tested before each use.
It is necessary to test your anesthesia machine for leaks to ensure that the machine is functioning properly when in use. This article discusses how to check the respiratory system of a veterinary anesthesia machine. A separate article explains how to check the pressure system and scavenging system.
The respiratory system contains all the components needed to deliver the anesthetic gas mixture to the patient. Before each use, visually inspect the parts of the respiratory system to ensure that they are not damaged. Because it is the most common source of leaks from anesthesia machines (see sidebar), it is absolutely necessary to perform a leak test on the respiratory system before each use.
The rebreathing circuit is connected to the inhalation and exhalation check valve (check valve), pop-up valve (adjustable pressure limiting valve), reservoir bag, pressure gauge, intake valve (not available on all machines) and CO2 Absorbent tank. The most common type of rebreathing circuit used in veterinarians is the circulatory system, which is designed so that the gas flows in only one direction. The breathing hose configuration can be a pair of hoses connected with a Y-shaped piece (Y-shaped piece), or a coaxial design with the inhalation hose inside the exhalation hose (general F).
Connect one breathing tube to the inhalation check valve, connect the other to the exhalation check valve, and then connect the patient-sized reservoir bag to the mouth of the bag. Alternatively, each component of the rebreathing circuit can be tested individually using the following steps:
Figure 1A. Test the components of the respiratory system without using hoses or reservoir bags. (Vetamac test kit) (Photo courtesy of Michelle McConnell, LVT, VTS [Anaesthesia and Analgesia])
Figure 1B. Test the breathing tube with a plug at the port of the reservoir bag. (Vetamac test kit) (Photo courtesy of Michelle McConnell, LVT, VTS [Anaesthesia and Analgesia])
Figure 1C. Test the reservoir bag with plugs at the inhalation and exhalation check valves. (Vetamac test kit) (Photo courtesy of Michelle McConnell, LVT, VTS [Anaesthesia and Analgesia])
Close the pop-up valve and close the patient end of the circuit with your thumb or palm. Do not use pop-up blocking valves for pressure checks. These valves are designed to leak after reaching a certain pressure, so they may hinder the true assessment of leak-free breathing systems.
Fill the system with oxygen by opening the flow meter or pressing the oxygen purge valve until a pressure of 30 cm H2O is reached on the pressure gauge. Once this pressure is reached, turn off the flowmeter. If you use the exhaust pipe mentioned in the alternative method of step 1, do not use the oxygen flush valve. Sudden high pressure may damage the delicate internal components of the anesthesia machine.
If there are no leaks in the breathing system, the pressure should remain constant for at least 15 seconds (Figure 2).
Figure 2. Pressure check of the rebreathing system (Wye dual hose configuration), the pressure gauge is kept at 30 cm H2O. (Photo courtesy of Darci Palmer, BS, LVT, VTS [Anaesthesia and Analgesia])
Slowly open the pop-up valve and observe the pressure release of the storage bag. This ensures that the scavenging system and pop-up valve work properly. Do not simply remove your hand from the patient port. A sudden drop in pressure may damage some parts of the anesthesia machine. It may also cause absorbent dust to enter the breathing tube and may come into contact with the patient’s airway.
The inhalation and exhalation check valves work together to ensure that the gas moves in only one direction throughout the respiratory system. They are made of round, lightweight materials, usually called discs, placed inside a transparent dome so you can see them moving. The one-way valve is placed on the anesthesia machine in a horizontal or vertical position. Failure of these valves can cause excessive CO2 rebreathing, which is harmful to the patient when using the anesthesia machine. Therefore, before each use of the anesthesia machine, the ability of the one-way valve should be evaluated.
There are many ways to test the check valve, but the one I am most familiar with is the pressure drop method, as described below.
A complete suction check valve will prevent backflow of gas to the machine. If there is no leakage, the bag will remain inflated (Figure 3).
Figure 3. Assessing the integrity of the suction check valve. If there is no leakage, the reservoir bag will remain inflated. (Photo courtesy of Darci Palmer, BS, LVT, VTS [Anaesthesia and Analgesia])
A complete exhalation check valve should prevent air from flowing out of the machine. If there is no leakage, the bag should remain inflated (Figure 4).
Figure 4. Assessing the integrity of the exhalation check valve. If there is no leakage, the reservoir bag will remain inflated. (Photo courtesy of Darci Palmer, BS, LVT, VTS [Anaesthesia and Analgesia])
How to find the leak. When performing a pressure check on an anesthesia machine, soapy water can help determine the source of the leak. Follow the gas flow through the anesthesia machine and spray soapy water in all locations that may be a source of leaks. If there is a leak, soapy water will start to bubble from the machine (Figure 5).
A refrigerant leak detector (purchased from Amazon for less than $30) can be used to detect halogenated hydrocarbon vapors. The device does not quantify the concentration or parts per million of the inhalant, but it is more sensitive than a basic “sniff” test when it comes to exposure to leaks that exist downstream of the evaporator.
Figure 5. Soapy water sprayed on the CO2 absorbent tank will produce bubbles, indicating that the rubber seal of the tank is leaking. (Photo courtesy of Darci Palmer, BS, LVT, VTS [Anaesthesia and Analgesia])
Steps to perform pressure check on the re-breathing circuit (general F hose configuration). Universal F has an inhalation hose (coaxial configuration) inside the exhalation hose, so only one hose is connected to the patient, but at the machine end, the hoses are separated, so each hose is connected to its corresponding unit. To the valve. Follow the same procedure outlined above to check the pressure of the Wye dual hose configuration. In addition, the inner tube test method should be the same as the Bain coaxial circuit (see below).
Non-repetitive breathing circuits are often used for smaller patients to help minimize breathing resistance during spontaneous ventilation. These circuits do not use chemical absorbents to remove CO2, but rely on high fresh gas flow rates to flush the exhaled CO2 containing gas out of the system. Therefore, the components of the non-repetitive breathing circuit are not too complicated. The two non-repetitive breathing circuits commonly used in veterinary medicine are the Bain coaxial circuit and the Jackson Rees circuit.
Pressure check of non-repetitive breathing circuit (Bain coaxial using Bain block). The Bain coaxial circuit is usually used in combination with a Bain block that can be installed on an anesthesia machine. This allows the circuit to use the reservoir port, pressure gauge, and pop-up valve.
Follow steps 2 to 5 outlined above to check the rebreathing circuit. Please note that even if the pressure remains constant, there is no guarantee that the inner tube of the coaxial circuit will not leak. There are two methods for evaluating inner tubes: blocking test and oxygen flushing test.
Use a pencil eraser or syringe plunger to close the inner tube on the patient end for no more than 2 to 5 seconds.
Depending on the diameter of the inner tube, not all types of coaxial circuits may be blocked. The inner tube should be carefully checked before each use to ensure that it is properly connected to the patient and both ends of the machine. If there is a problem with the integrity of the inner tube, the circuit should be discarded. The failure of the inner tube will greatly increase the mechanical dead space, which may lead to a large amount of CO2 rebreathing.
Activate the oxygen flush valve and observe the reservoir bag. If the inner tube is intact, the reservoir bag should be slightly deflated (Venturi effect).
If the inner tube separates from the machine end of the circuit, the reservoir bag may be inflated rather than deflated during this test.
Pressure check of non-repetitive breathing circuit (Jackson Rees). The same procedure outlined above for the circular (Wye dual hose configuration) rebreathing circuit can be used to perform pressure checks on the Jackson Rees non-rebreathing circuit. The pop-up valve may be a button pressed on the liquid storage bag or a valve that moves between open and closed positions. The standard Jackson Rees circuit does not use a pressure gauge. Therefore, to perform a pressure check on the circuit, the reservoir bag should be overfilled for at least 15 to 30 seconds to see if there are any leaks. The pop-up valve should be opened to relieve pressure in the circuit, rather than removing the hand from the patient port. This will test the normal function of the pop-up valve. A disposable pressure gauge can be purchased and used on the Jackson Rees circuit (Figure 6). The pressure gauge can be used to check the pressure of the Jackson Rees circuit in the same way as other breathing circuits.
Figure 6. The disposable pressure gauge on the Jackson Rees non-rebreathing circuit. (SafeSigh Pressure Gauge-Vetamac) (Photo courtesy of Michelle McConnell, LVT, VTS [Anaesthesia and Analgesia])
Allen M, Smith L. Equipment inspection and maintenance. In Cooley KG, Johnson RA, Eds: Veterinary Animation and Monitoring Equipment. Hoboken, New Jersey: John Wiley & Sons; 2018: 365-375.
Darci Palmer became an anesthesia and analgesic veterinary technologist in 2006. She serves as the executive secretary of the Veterinary Technical College of Anesthesia and Analgesia. Darci is an instructor of the Veterinary Support Personnel Network (VSPN) and an administrator of the Facebook group Veterinary Anesthesia Nerds.
Post time: Nov-15-2021