An Alternative Pressure Test

An anesthesia machine is a series of components and tubing designed to take high pressure oxygen to low pressure, mix it with a volatile anesthetic drug, and deliver it safely to a patient, all without exposing any team members to anesthetic or waste gases. There are many ways for complicated machines to fail, and just like with a car, there are regular maintenance and checks that can and should be performed to ensure the machine is operating safely and effectively.

The American College of Veterinary Anesthesia and Analgesia provides recommendations for evaluating anesthetic equipment in their 2013 position statement on the control of waste anesthetic gas. The recommendations include that “anesthesia machines and ventilators, breathing systems, and scavenging systems should be checked for leaks prior to each use.” There are many manufacturers of anesthetic equipment and no common standard for these tests that would comprehensively test every version. Therefore, it is wise for each hospital to be aware of the make of the anesthetic machines and vaporizers in use as well as the specific manufacturer recommendations for leak testing and regular maintenance.

Performing daily anesthesia machine check-outs serves several purposes. This series of events ensures that: the machine is in good working order with sufficient oxygen and inhalant anesthetic for the upcoming procedure, CO₂ absorbent granules are not exhausted in rebreathing systems, the expected amount of anesthetic gas and oxygen will safely reach the patient, and the anesthetic team will not be exposed to anesthetic or waste gases.

The risk levels for inhalant anesthetics currently used in veterinary medicine have not been established. The exposure limits of 2 ppm for halogenated agents published by the National Institute of Occupational Safety and Health were established before isoflurane and sevoflurane were in common use and are likely not applicable to these agents. Personnel exposed to modern volatile anesthetics have risk of adverse effects. These include short-term symptoms of dizziness, headaches, fatigue, and nausea. The effects of chronic exposure to modern volatile anesthetics are less certain. Some studies show the potential of slight increased risk for reproductive concerns such as miscarriage for pregnant women exposed to isoflurane. Therefore, OSHA provides non-obligatory guidelines for minimizing workplace exposure out of caution, including regularly checking anesthetic machines for leaks. Keep in mind that if a team member can smell the inhaled anesthetic, the level of exposure is likely well over recommended limits, as one study found that the concentration of isoflurane that 90% of participants could detect by smell was 250 ppm.

When establishing a thorough maintenance checklist, be sure to start at the oxygen source of the high-pressure system and extend all the way to the patient connection. A full system check should be performed daily; the breathing head should be checked for leaks after any changes, such as CO₂ absorbent granule replacement, and prior to each anesthetic event. The breathing circuit and bag should also be checked for any possible leaks before a patient is placed under anesthesia. However, many continuing education sources focus solely on leak testing the machine and breathing circuit together prior to an anesthetic event. Even with more comprehensive guidelines for anesthesia machine checking in human medicine, a very large majority of operators in one study only performed partial checks and a small minority performed any checks between cases.

A simple way to help avoid an anesthetic problem during a procedure is to perform a pressure test or leak test. The Positive Pressure Leak Test of an anesthesia machine is taught to veterinarians and technicians alike during their formal education and is repeated countless times in continuing education resources and videos. The form of this test is consistent throughout several sources, including the 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats. The leak test should be performed prior to anesthetizing each patient and after changing any of the breathing circuit components, such as changing the reservoir bag or replacing the CO₂ absorbent granules. With a breathing tube and reservoir bag attached to the anesthesia machine, the operator closes the adjustable pressure-limiting (APL) valve, also known as a pop-off valve, increases the oxygen flow until the manometer reads 30 cm H₂O, then turns off the oxygen flow and monitors the manometer; it should not decrease and if it does, a leak is present.

If a leak is noted, it should be quantified. Increase the flowmeter until the manometer reads 30 cm H₂O and does not decrease. The flow of oxygen into the system matches the flow of gas out of the system at this point. According to many veterinary anesthesia resources, an acceptable leak is 300 mL/min or less, although some experts and anesthesia machine manufacturers restrict permissible leaks to 200 mL/min or less.

At the end of this test, pressure should be released by opening the APL valve, rather than by removing the hand occluding the breathing circuit. This serves several purposes: this tests that the scavenging system is appropriately removing gas from the system, ensures that the APL valve is not accidentally left closed while a patient is connected to the breathing circuit, and avoids a sudden pressure decrease in the system that could lead to breathing circuit contamination with CO₂ absorbent granules. Patient deaths have been reported secondary to accidental closure of the APL valve during anesthesia and getting in the habit of re-opening the APL valve after closing it can be life-saving.

Figure 1: The Pressure Test Kit contains (a) a long piece of tubing to connect between the inspiratory and expiratory one-way valve ports, and (b) a short piece of tubing with a red plug at one end to connect to the breathing bag port

Cynthia Burns, the owner of Fix-It Lady: Medical Service and Repair, LLC, suggests an alternative to this Positive Pressure Leak Test. She divides her test into three parts to individually test the breathing head of the anesthesia machine, patient breathing circuit, and the reservoir bag. This allows the operator to find the source of a leak more easily and to find issues with the anesthetic machine more rapidly. In order to complete this test, she provides a pressure test kit (Figure 1), although the alternative pressure test can still be performed without this special kit. See below for the steps to her test.

Positive Pressure or Leak Test

Pressure Testing the Anesthesia Machine

Figure 2: For pressure testing the breathing head without a Pressure Test Kit, F-circuit tubing is connected from the inspiratory to the expiratory one-way valve ports and a hand is used to occlude the breathing bag port

• Using either the piece of tubing from the Pressure Test Kit or an F Breathing Circuit expiratory tube, occlude both one-way valve ports by applying one end of the tube to the inspiratory port and the other to the expiratory port.
• Occlude the rebreathing bag port with the palm of your hand (Figure 2)
• Close the pop-off valve
• Using the flowmeter, bring pressure up to 30 cm H2O, then turn the flowmeter off. Important note: Do NOT use the oxygen flush valve, as this can severely damage the anesthesia machine or manometer
• Observe the manometer for 30 seconds
• If there is no drop in the manometer, the breathing head is safe to use. If the manometer drops, there is a leak in the system
• To determine the size of the leak, slowly increase the pressure in the breathing head to 30 cm H₂O. Once the manometer is staying steady, look at the flowmeter; that is the size of the leak. A leak over 200 mL/min needs to be corrected
• Open the pop-off valve to release the pressure. The manometer should slowly decrease to 0 cm H₂O

Pressure Testing the Reservoir Bag

Figure 3: For pressure testing the rebreathing bag without a Pressure Test Kit, F-circuit tubing is connected from the inspiratory to the expiratory one-way valve ports and the rebreathing bag is connected to the rebreathing bag port

• Keep the one-way valves occluded as in the first portion of the test
• Place the reservoir bag on the rebreathing bag port (Figure 3)
• Close the pop-off valve
• Since the bag is more flexible, use the oxygen flush valve until the manometer reaches 20 cm H₂O. Then use the flowmeter to bring pressure up to 30 cm H₂O. Turn the flowmeter off
• Observe the manometer for 30 seconds
• If the pressure reading on the manometer begins to drop, check the breathing bag for any holes or tears. (Remember: the pressure will also decrease if a leak was detected, but not resolved, in the breathing head.)
• Open the pop-off valve to release the pressure

Pressure Testing the Patient Breathing Circuit Hoses: Circle System

Figure 4: For pressure testing the breathing circuit without a Pressure Test Kit, the Y-circuit is connected to the inspiratory and expiratory one-way valve ports, F-circuit tubing is connected to the patient end of the breathing circuit, and the other end of the F-circuit tubing is connected to the rebreathing bag port

• Remove the tubing between the inspiratory and expiratory one-way valves as well as the reservoir bag
• Connect the breathing tube to the one-way valve ports and the F-circuit to the end that would connect to the endotracheal tube. Connect the other end of the F-circuit tubing to the rebreathing bag port (Figure 4)
• Close the pop-off valve
• Using the flowmeter, bring pressure up to 30 cm H₂O, then turn the flowmeter off. Important note: Do NOT use the oxygen flush valve, as this can severely damage the anesthesia machine or manometer
• Observe the manometer for 30 seconds
• If the pressure reading on the manometer begins to drop, there is a leak in the breathing tube and it is time to be replaced. (Remember: the pressure will also decrease if a leak was detected, but not resolved, in the breathing head.)
• Open the pop-off valve to release the pressure.

This more thorough pressure test takes only a few minutes to perform and allows veterinary anesthetists to more easily find leaks. The first step of checking the anesthesia machine breathing head can find issues such as a broken flowmeter needle valve, which will allow more oxygen to leak into an anesthesia machine, effectively diluting the anesthetic gas delivered to the patient. Cynthia Burns has also noted several instances of the standard positive pressure leak test showing no leaks present, but while individually testing the breathing tubes or breathing head, she discovered a leak greater than one L/min. Many different components of the anesthesia machine can be faulty. The combination of increased flow into the system can mask increased flow out of the system while the pressure remains static. Adding a few extra checks to routine anesthesia machine check-outs can find issues earlier, decrease risk to personnel, and improve patient safety.

Click here for a complete list of references cited in this article. This content is sponsored by Jurox and Think Anesthesia, an educational platform providing veterinary professionals with both on-demand and live training material, primarily focused on anesthesia and analgesia.