History
An inability to achieve an adequate peak pressure was identified during routine checking prior to use of a large animal anesthesia machine.a The machine had previously been used without problems. The machine's oxygen line and scavenging system were connected to the hospital's main lines, and the machine was visually inspected for appropriate connections and parts. Soda lime integrity was checked and determined to be appropriate, and the canister was reassembled. A 30-L rubber reservoir bagb was attached to the bag port before pressure checking of the system for leaks. The adjustable pressure-limiting valve (ie, pop-off valve) was closed, the Y-piece was occluded, and the oxygen flush valve was activated to pressurize the system to 30 cm H2O. As the reservoir bag filled up, the pressure rose as expected, until it reached 22 cm H2O, after which the pressure no longer increased, despite the addition of more oxygen to the system through the flow-meter or oxygen flush valve. It was assumed that there was a faulty manometer; however, the problem persisted after the manometer was replaced. The system was pressurized again, and peak pressure reached only 24 to 25 cm H2O, followed by a slight decrease to 22 cm H2O, despite visual distension of the bag. Opening of the pressure-limiting valve caused the pressure in the system to decrease to 0. Because the reservoir bag was visibly distended during pressurization of the system and the pressure was steady over time without infusion of additional oxygen, a leak in the system was considered to be an unlikely cause of the problem. Given that the manometer sat on top of the soda lime canister and the fresh gas inlet was located between the reservoir bag and the canister, it was hypothesized that the machine had an obstruction between the fresh gas inlet and the manometer that prevented the manometer from reflecting the correct pressure in the system. Therefore, the pipelines were manually inspected for evidence of airflow and the soda lime canister was emptied to eliminate the possibility that the mesh screen was clogged. However, no obstruction in airflow could be identified. When the machine was reassembled with an empty soda lime canister and again pressurized, the pressure remained at 22 cm H2O. When the top of the soda lime canister was opened and inspected, the rubber gasket used to seal the canister was seen to be partially occluding the opening to the manometer (Figure 1). The gasket was trimmed, and the system was again checked; however, the problem persisted. Finally, the manometer was once again replaced with a new, pretested manometer and the metallic connector between the canister and the manometer was visually inspected. No abnormalities were found, and the problem persisted.
Photographs of the bottom side of the top cover of the soda lime canister from an anesthetic machine that did not reach an adequate pressure during leak testing. Notice the rubber gasket partially obstructing the port to the manometer (left; arrow) and the appearance of the rubber gasket after trimming (right).
Citation: Journal of the American Veterinary Medical Association 238, 5; 10.2460/javma.238.5.577
Question
What is preventing the anesthesia machine from becoming adequately pressurized?
Answer
Given the inspection and testing that had been done, the most likely cause of inadequate pressurization was a faulty reservoir bag. When the reservoir bag was disconnected and its port was manually occluded during pressurization, pressure in the system increased as expected to 30 cm H2O. The faulty 30-L bag was replaced with a new 15-L rubber bag,c and the system was pressurized again. This time, the peak pressure was 40 cm H2O.
Discussion
A proper breathing system leak test includes the following steps. First, ensure an oxygen source is attached to the machine, the Y-piece is occluded, and the pressure-limiting valve is closed. Next, turn the flow-meter or oxygen flush valve on and allow the reservoir bag to fill until pressure within the system reaches 30 cm H2O. Finally, once this pressure is reached, turn the flowmeter or flush valve off and observe the system for at least 10 seconds to ensure that there is no drop in pressure.1 Performing the leak test correctly helps prevent environmental contamination with anesthetic gases and ensures that the system maintains proper airflow and anesthetic concentration during use. Leak testing is particularly important if positive-pressure ventilation is used because any leaks in the system will contribute to inefficient patient ventilation and expose personnel to anesthetic gases. With low-flow anesthesia, leaks lead to inappropriate delivery of the gas mixture to the patient, resulting in hypoxemia and lighter planes of anesthesia.2
Normally, the reservoir bag operates as a pressure-limiting device in a breathing system, protecting patients from excessive buildup of pressure.3 The peak pressure of an anesthetic system reached with a fully distended reservoir bag is important because it represents the maximum pressure that can develop in the system if the pressure-limiting valve is accidentally left closed. According to current standards from the American Society for Testing of Materials and the International Organization for Standardization, a rubber reservoir bag with a capacity > 1.5 L should have a peak pressure no less than 30 cm H2O and no more than 60 cm H2O when expanded to 4 times its size.4 It is difficult to assess how much gas was added to our faulty reservoir bag; however, even when the bag was extremely distended, the pressure in the system did not exceed 22 cm H2O (Figure 2).
Photograph of the anesthetic machine referred to in Figure 1. Notice that even with the reservoir bag fully extended, pressure within the system was only 22 cm H2O. Yellow arrows indicate possible points of obstruction in the breathing system.
Citation: Journal of the American Veterinary Medical Association 238, 5; 10.2460/javma.238.5.577
Reservoir bags have a characteristic pressure-volume curve. As gas is added to the bag, the pressure does not change until the stated capacity of the bag (ie, 30 L) is reached. Thereafter, the pressure quickly increases until it reaches a peak, followed by a slight drop and a plateau. As additional gas is added to the bag, it distends further, and a second decrease in pressure and plateau are observed before the bag finally ruptures.3,5,6 The relationship between the radius of the bag, the pressure in it, and the tension in the bag's wall is represented by Laplace's law (pressure = 2 × [tension/radius]). Once the bag reaches its stated capacity, its radius and wall tension increase proportionally as gas is added, resulting in a constant or plateau pressure. In other words, as the bag is inflated, it reaches a peak pressure (dynamic state), but after elastic equilibrium is reached, the pressure decreases slightly, then remains constant (static state). The difference between the peak pressure and the pressure at equilibrium depends on the inflow gas rate, with higher flows resulting in a higher difference.3,5
Most likely, the reservoir bag originally attached to the anesthetic machine described in the present report did not reach the pressure specified by the standard because of excessive wall compliance, which limited the increase in wall tension. With this reservoir bag, the peak pressure during the leak test was only 22 cm H2O, meaning that any leaks that could have occurred at higher pressures could not be detected. Considering that in horses, pressures obtained in the anesthetic system during mechanical ventilation can easily exceed 20 cm H2O, such leaks would have impaired ventilation. In contrast, this would not have precluded the anesthetist from achieving higher airway pressures when manually ventilating the patient, given that when the bag is compressed, its radius decreases and higher pressures are generated.
Physical characteristics of reservoir bags play an important role in their compliance. Parmley et al5 evaluated the pressure-time-volume relationship of several reservoir bags. According to their study, plastic bags, in contrast to rubber bags, can pose a substantial hazard during anesthesia because pressures as high as 260 cm H2O can be reached when plastic bags are fully inflated. In contrast, the plateau pressure for rubber bags (40 to 45 cm H2O) is much lower. Importantly, when rubber bags are hyperinflated and overstretched several times, the plateau pressure tends to decrease, as demonstrated by Parmley et al.6 In their study, new rubber reservoir bags reached plateau pressures 7 to 10 cm H2O higher than plateau pressures in previously stretched bags. However, unlike the case for the anesthesia bag described in the present report, plateau pressures for new and previously stretched bags were still in compliance with current standards.
More recently, Blanshard and Milne7 investigated the pressure-limiting function of several latex-free reservoir bags, compared with the function of standard latex rubber bags. Surprisingly, only one brand of latex-free bag performed similarly to the latex-containing bags, whereas the other brands had moderately to dangerously high plateau pressures. Excessive airway pressure can lead to air embolism, pneumothorax, pneumomediastinum, subcutaneous emphysema, cardiovascular depression, and hemodynamic collapse.8
There are many potential reasons why an anesthetic system will not reach an adequate pressure during leak testing. Inappropriate inflow of gas into the system as a result of low pressure in the main pipeline or a faulty oxygen flush valve could give the anesthetist the false impression that gas was flowing into the system, even though the gas volume was not really increasing. However, this was not the case for the machine described in the present report because the reservoir bag visually increased in volume when the oxygen flush valve was activated. Excessive outflow of gases from the system resulting from leaks in the connections, a faulty pressure-limit valve, or faulty backup for the pressure-limiting valve could also account for low peak pressure in an anesthetic system. However, when leaks are present, the pressure typically rises initially and steadily falls over time, which did not occur with our anesthetic machine. Initially, we suspected that a faulty manometer was the reason for the fact that pressure did not exceed 22 cm H2O. However, the problem persisted even when the manometer was twice replaced.
As described, it was also thought that an obstruction to gas flow in the metallic tubing connecting the reservoir bag and the bottom of the soda lime canister could have been responsible for the low pressure displayed by the manometer. To test this hypothesis, the metallic tubing was manually inspected for airflow. Oxygen was felt to be reaching the bottom of the canister when the oxygen flush valve was activated, eliminating the possibility of an obstruction prior to this point. A downstream obstruction was also considered a possibility, so the soda lime was discarded, and the machine was pressurized with an empty canister in place. Because the problem persisted, other locations of a possible obstruction were considered. The rubber gasket in the canister top was partially occluding the opening to the manometer; however, after it was trimmed to clear the opening, the problem persisted. Lastly, clogging of the metallic connector leading to the manometer as a result of soda lime granules lodging there was considered, but no obstruction was observed during visual inspection.
The American Society for Testing of Materials has been responsible for setting standards for anesthetic equipment since 1983,9 and it is expected that manufacturers are complying with the current standards. However, not all manufactures of veterinary equipment follow these standards. Adequate basic knowledge of anesthetic machine function and testing is mandatory prior to any anesthetic episode. The equipment must be properly checked to prevent avoidable anesthetic complications. The problem described in the present report prevented the operator from detecting anesthetic system leaks, a frequent critical incident in anesthetic practice.10
Large animal anesthesia machine, Mallard Medical Inc, Redding, Calif.
30-L latex reservoir bag, Surgivet, Smiths Medical North America, Waukesha, Wis.
15-L latex reservoir bag, Surgivet, Smiths Medical North America, Waukesha, Wis.
References
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- 4.↑
International Organization for Standardization. Anaesthetic reservoir bags. ISO 5362. 4th ed. Geneva: International Organization for Standardization, 2006;3.
- 5.↑
Parmley JB, Tahir AH, Adriani J. Disposable plastic breathing bags and tubes: hazards for inhalation therapy and anesthesia. JAMA 1971; 217:1842–1844.
- 6.↑
Parmley JB, Tahir AH, Dascomb HE, et al. Disposable versus reusable rebreathing circuits: advantages, disadvantages, hazards and bacteriologic studies. Anesth Analg 1972; 51:888–894.
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Blanshard HJ, Milne MR. Latex-free reservoir bags: exchanging one potential hazard for another. Anaesthesia 2004; 59:177–179.
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Newton NI, Adams AP. Excessive airway pressure during anesthesia: hazards, effects and prevention. Anaesthesia 1978; 33:689–699.
- 10.↑
Cooper JB, Newbower RS, Kitz RJ. An analysis of major errors and equipment failures in anesthesia management: considerations for prevention and detection. Anesthesiology 1984; 60:34–42.
