THE Penlon Sigma Elite vaporizer (Penlon Ltd., Radley Road, Abingdon, UK) has been widely distributed throughout the United States to meet the consumer demand for delivery of sevoflurane. We present a case describing a Penlon Sigma Elite Vaporizer leak that was not detected using the 1993 Food and Drug Administration (FDA) Anesthesia Apparatus Checkout Recommendations [1]for anesthesia gas delivery machines. To our knowledge, this is the first case report describing a potential problem with the Penlon Sigma Elite Vaporizer when used on an Ohmeda Modulus II anesthesia machine (Madison, WI).

Case Report

A 49-yr-old, 68-kg man with a laryngeal stenosis was scheduled for direct laryngoscopy, esophagoscopy, and biopsy with carbon dioxide laser correction. Before this case, two cases were performed that day uneventfully using the same Ohmeda Modulus II anesthesia machine. It was equipped with three vaporizers, including a Penlon sevoflurane Sigma Elite, an Ohmeda isoflurane Tec 4, and an Ohmeda desflurane Tec 6. The vaporizers were mounted on a Selectatec manifold (Ohmeda), and a circle breathing system was used. The anesthesia machine was checked before the first case of the day by a clinical anesthesia year 1 anesthesiology resident using the FDA 1993 Anesthesia Apparatus Checkout Recommendations. [1]No leaks were detected. Between case 2 and case 3, the anesthesiology resident refilled the sevoflurane vaporizer using a keyed filler. Before induction of anesthesia for case 3, the circle system was tested for leaks using a positive-pressure leak test, with all of the vaporizers in the “off” position.

The anesthetic plan included an inhalation induction at a flow rate of 6 1/min using nitrous oxide, oxygen, and sevoflurane. At 5 min, despite incremental increases to a dial setting of 5%, the inspired concentration of sevoflurane had not increased as rapidly as expected. The Datex Ultima (Tewksbury, MA) multigas analyzer indicated an inspired sevoflurane concentration of only 0.7%, instead of the expected 3-5%.

There was no problem with bag-mask ventilation throughout the induction. However, a sevoflurane odor was detected in the ambient air. Because the patient had a heavy beard, the source of the odor was initially assumed to be a leak between the face mask and the beard. It was then observed that the clamp screw that tightens the filler plug located in the 3-o'clock position of the Penlon Sigma Elite sevoflurane vaporizer was screwed fully out counter clockwise, or in the “open” position instead of in the normal “closed” clockwise position (Figure 1, left). The filler control knob located in the 12-o'clock position was correctly screwed clockwise to the closed position. After it was discovered that the screw clamp knob was in the open position, it was screwed back into the closed position (Figure 1, right). The patient had no awareness or recall and did well.

After the case was over, the leak was reproduced, and a Datex Ultima multigas analyzer was used to quantitate the concentration of sevoflurane escaping from the leak. The vaporizer dial setting was 5% sevoflurane, and flow from the anesthesia machine was 6 l oxygen/min. The filler plug was removed entirely, and the distal end of the Datex sampling line was inserted into the square fill port. The sevoflurane concentration indicated by the Datex analyzer was 15%, and the oxygen concentration was 79%. Because the upper end of the measuring range for sevoflurane for the Datex analyzer is 15%, it was necessary to calculate the actual sevoflurane concentration. The only two gases that could escape through the leak were oxygen and sevoflurane. Because the oxygen concentration was measured to be 79%, the calculated sevoflurane concentration was 21%(100%- 79%= 21%). This value corresponds to the saturated vapor pressure of sevoflurane at 20 [degree sign]C at one atmosphere (160 mmHg [divided by] 760 mmHg = 21% sevoflurane).


Factors that contributed to the problem encountered include:(1) unfamiliarity with the product;(2) vaporizer design;(3) anesthesia machine design;(4) use of a non-Ohmeda product on an Ohmeda machine; and (5) limitations of the 1993 FDA Anesthesia Apparatus Checkout Recommendations.

The design of the Penlon Sigma Elite Vaporizer is different from Ohmeda vaporizers, and unfamiliarity with design differences can become a contributory factor in the occurrence of critical incidents. According to reports by Cooper et al., [2,3]equipment design and insufficient familiarity with equipment were indictable in many categories of human error. This case shows the continued need for evaluation and training in the proper use of new equipment, and it reveals some of the potential shortcomings in equipment designs when different manufacturers' products are paired. The anesthesia care providers delivering anesthesia during this case were unfamiliar with the Penlon Sigma Elite Vaporizer. The anesthesia attending had not been adequately trained in its use, and the CA-1 resident was new to the service. In academic and private practice settings, anesthesia care providers occasionally encounter equipment with which they are not familiar. To avoid problems such as this one, it is of paramount importance to have ongoing in-service education of new equipment, particularly if coverage is provided by a large number of anesthesia care providers on a rotational basis.

The design of the Penlon Sigma Elite Vaporizer is different from Ohmeda vaporizers. The major difference is that liquid anesthetic does not leak from the Penlon vaporizer, even when the filler plug is completely removed. On keyed, filled Ohmeda vaporizers, if the handle (which is equivalent in function to the Penlon agent-specific filler plug clamp screw) is moved to the open position, liquid inhaled anesthetic leaks out the keyed filler port. Thus, on Ohmeda vaporizers, the leak is obvious, but it is not so apparent on the Penlon vaporizer. Only one revolution of the Penlon clamp screw located in the 3-o'clock position (Figure 1B) differentiates fully open versus fully closed. This subtle difference is very difficult to appreciate without very careful and close observation.

Design features of the Ohmeda Modulus II anesthesia machine also contributed to the problem. This vaporizer leak is particularly inconspicuous, especially when the Penlon Sigma Elite Vaporizer is mounted on a Modulus II machine, which has a check valve at the common gas outlet. In this case, a positive-pressure leak test failed to detect the vaporizer leak entirely because the low pressure circuit is isolated from the breathing circuit by the check valve. [4,5]The negative-pressure leak check recommended by Ohmeda (or by the FDA) would have detected the leak, however. In 1993, Meister and Becker [6]reported a case of a fresh gas flow leak through a Drager Vapor 19.1 vaporizer (North American Drager, Telford, PA) with a key-index fill port. They recommended a positive-pressure leak test with each vaporizer turned to the on position. Because North American Drager machines do not have a check valve in the low-pressure circuit, this recommendation would detect a large (but not necessarily a small) low-pressure system leak caused by an open fill port. However, in our case, a positive-pressure leak test would not have detected the internal vaporizer leak even if the vaporizer was turned on because the Ohmeda check valve divorces the breathing circuit from the low-pressure circuit. [4,5]The negative-pressure leak check recommended by Ohmeda or by the FDA would have detected the leak only if the vaporizer was turned on.

Use of a non-Ohmeda product on an Ohmeda machine can defeat safety features intended by the manufacturer. The Ohmeda vaporizer is designed for specific use with the Ohmeda Selectatec manifold. When an Ohmeda vaporizer is mounted on the Ohmeda manifold, the vaporizers are totally out of circuit when turned to the off position. [7]Thus, when the Ohmeda vaporizers are turned to the off position, no gas flows through the bypass chamber of the vaporizer because the vaporizer is isolated from the manifold. This design offers two theoretical safety advantages. First the chance of cross-contamination between vaporizers is minimized. This safety feature helps minimize trace gas exposure during a malignant hyperpyrexia case. Second, internal vaporizer leaks of unused vaporizers do not influence normal operation of the vaporizer in use. The Penlon Sigma Elite, however, has two permanent pins centrally located in the inlet and the outlet of the vaporizer (Figure 2). These pins automatically open the vaporizer port valves when the vaporizer is mounted to the manifold. Thus, when the operator attaches the Penlon Sigma Elite to the Ohmeda manifold, flow is automatically routed through the head of the vaporizer, even when the vaporizer is in the off position. Thus, this Penlon design defeats Ohmeda's safety features described previously.

When using the Penlon Sigma Elite Vaporizer, clinicians are encouraged to make certain that both the filler control knob and the clamp screw are screwed in firmly to the closed position using a clockwise motion. This case also shows the importance of measuring inspired and exhaled concentrations of inhaled anesthetics. Had we administered this anesthetic with only capnography, the patient would have been at risk for light anesthesia and awareness. Even though the dial setting for sevoflurane was set at 3-5%, the inspired concentration shown by the anesthetic agent analyzer was only 0.7% sevoflurane.

Strict adherence to 1993 FDA Anesthesia Apparatus Checkout Recommendations [1]may have contributed to the problem. We followed the guidelines very specifically. That is, we did a thorough machine checkout before case 1 in the morning, and the anesthesia machine and vaporizers were leak free. For cases 2 and 3, we followed the FDA guidelines as well. When one scrutinizes the FDA guidelines, section 5 states, “perform leak check of machine low pressure system.” Number 5 has by it an asterisk, and the asterisk states, “If an anesthetic provider uses the same machine in successive cases, these steps need not be repeated, or they can be abbreviated after the initial checkout.” Therefore, before case 3, we simply checked the breathing circuit for leaks, and we did not check the low-pressure circuit for leaks. When it is practical, anesthesia care providers should check the low-pressure system for leaks using a negative-pressure leak test after vaporizers are refilled. Also, when practical, the low-pressure system should be checked for leaks after vaporizers are switched out.

The authors thank Richard J. Cain of Southern Biomedical, Incorporated for his technical assistance.


United States Food and Drug Administration: Anesthesia Apparatus Checkout Recommendations. Rockville, MD, Food and Drug Administration, 1993
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