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© 2000 International Anesthesia Research Society
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Abstract |
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Implications: Smaller-than-expected concentrations of sevoflurane were delivered by Sevotec 5® vaporizers (Datex-Ohmeda, Madison, WI) at low-fill states, 8% dialed concentrations, and high fresh gas flows. Clinically, this would lead to prolonged induction times. The risk of complications from prolonged inhaled induction could be increased when using a vaporizer with these characteristics.
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Acommon pediatric inhaled induction technique involves high flow rates and 8% dialed sevoflurane (1). In our clinical experience, we have not always observed the expected delivered sevoflurane concentration when using the Sevotec 5® vaporizer (Datex-Ohmeda, Madison, WI). A smaller-than-expected delivered concentration would lead to longer induction times, resulting in increased risk of laryngospasm, coughing, and/or vomiting on induction as a result of a prolonged second stage of anesthesia. We examined the relationship between actual delivered sevoflurane concentration, fresh gas flows (FGF), and vaporizer fill state using a Sevotec 5® vaporizer with the dial set at 8%.
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Methods |
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We studied five calibrated Sevotec 5® vaporizers (manufactured in 1998). The vaporizers were tested on separate Ohmeda Modulus® (Datex-Ohmeda) anesthesia machines with disposable circle circuits. The end of the circuit was sealed with the adjustable pressure limiting valve in the open position. The vaporizers were filled to the 100-mL mark (not all Sevotec 5® vaporizers in circulation have a 100-mL mark). The vaporizer dial was set at 8%, and the delivered sevoflurane concentration was analyzed (Poet IQTM; Criticare Systems, Inc., Waukesha, WI) at 5-s intervals at the common gas outlet at total FGFs (FIO2 = 1.0) between 6 and 10 liters per minute (LPM). Measurements ceased when each vaporizer was empty as defined by the external indicator ("
"). A linear visual scale placed next to the sight glass indicator established predefined equidistant fill points (A, B, C, D, and Empty) between the 100-mL mark (Point A) and the empty mark. Careful attention was given to avoiding parallax error by assessing the fill state at eye level. Ambient temperature was measured. A binomial distribution test was used to show the minimum percentage of similar vaporizers that would show the same characteristics with a probability of 0.95.
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Ambient temperature ranged from 20.5° to 22.7°C. Delivered sevoflurane decreased to <2% at FGFs of 10 LPM in all vaporizers. In each case, the decrease occurred after Point D but well before the empty mark on the vaporizer (Figure 1). None of the vaporizers showed any change in delivered sevoflurane concentration at FGF of 6 or 7 LPM (Figure 1). The FGFs at which each vaporizer first showed a precipitous decrease in output are presented in Table 1.
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By using a binomial distribution test and assuming an infinite number of vaporizers, the probability that at least 35% of similar model vaporizers will show a similar precipitous decrease in delivered concentration under similar conditions (10 LPM) is 0.95.
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Discussion |
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The use of over-pressurization for anesthetic induction is common with sevoflurane. None of the Sevotec 5® vaporizers studied delivered the expected large concentrations of sevoflurane when the following three conditions coexisted: 1) the dial was set at 8%, 2) the vaporizer had a low-fill state, and 3) total FGF was
10 LPM. These findings have clinical significance in that a smaller concentration than expected would be delivered to a patient, and therefore induction would be prolonged. Clinical experience shows that prolonged inductions lead to increased complication rates. Complications related to prolonged inductions include, but are not limited to, coughing, vomiting, and/or laryngospasm. Although we do not recommend the use of a vaporizer with a low-fill state, we recognize that this likely occurs frequently, for a variety of reasons. Under conditions likely to be used during an inhaled induction of a pediatric patient, (8% dialed concentration and high flow rates), the vaporizers we studied did not operate to specification when the fill state was low, but not empty. This is not consistent with the manufacturer’s statement that "The vaporizer is designed to function according to specification as long as there is agent visible above the minimum level" (2,3).
In 1999, Datex-Ohmeda released a medical device advisory notice (3), which described two clinically unusual situations when low output can occur with the Sevotec 5®. Low output occurred when "the vaporizers were refilled to just above the minimum fill level after having been emptied during use at high flow rates (>8 LPM) and high dial settings (near 8%)," or "when the vaporizers were used after having sat for an extended period of time at or near the minimum fill level" (3). Our data show a relationship between FGF rate, a low-fill state, and 8% dialed sevoflurane concentration. The vaporizers used in our study were neither refilled just above minimum nor allowed to stand for extended periods at low-fill states. Our data suggest that the refilling method or the duration of time a vaporizers is left at a low-fill state may not be key factors in this phenomenon.
It is well established that high FGF results in smaller steady-state concentrations of delivered drug (2). The phenomenon we describe is separate and distinct from this. The cause of our findings is unclear. Although the sight glass display may not accurately reflect the fill state of the vaporizer at low levels when rapid emptying occurs, measurements of remaining fluid (mass of fluid remaining at the empty mark) in two vaporizers showed no significant mass difference between the 6 and 10 LPM tests. The variability of the flow at which a drop-off occurs (Table 1) suggests that flow is not the only variable. We hypothesize that the phenomenon we describe is not an "emptier-than-expected" vaporizer, but rather relates to the individual internal relationship between flow characteristics at high FGF, low liquid fill state, and an 8% dial setting. The sevoflurane liquid temperature undergoes cooling with continuing vaporization. This may play a role in the function of the vaporizer; however, it was not measured, as it would require significant modification of the vaporizer. Calibration error was very unlikely to be a factor in this study.
Priming a circuit with low, rather than high flows would avoid the problem we describe. Many practitioners do not prime their circuits, but rather use high flows to achieve desired sevoflurane concentrations rapidly. In this situation, end-tidal gas analysis would have limited usefulness in alerting the practitioner of a problem, although, inspired gas analysis could alert the practitioner of smaller than expected delivered sevoflurane concentrations. The medical advisory provides some suggestions to avoid the problem in clinical practice (3).
Clinicians should be aware of the phenomenon we have described and should follow the recommendations of the Datex-Ohmeda advisory. Careful attention to inspired sevoflurane concentrations and the use of lower flows (
7 LPM) will also likely avoid the problem. Affected Sevotec 5® vaporizers will be relabeled when they are sent in for periodic servicing (3). We recommend that practitioners contact Datex-Ohmeda for further information.
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Acknowledgments |
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We would like to acknowledge the assistance of Wendell Stevens, MD.
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Footnotes |
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Funding received from the Research and Education Society, Department of Anesthesiology, Oregon Health Sciences University.
This work was presented, in part, at the Society for Pediatric Anesthesia meeting, Fort Myers, FL, February 24–27, 2000.
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References |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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- Epstein RH, Stein AL, Marr AT, et al. High concentrations versus incremental induction of anesthesia with sevoflurane in children: a comparison of induction times, vital signs and complications. J Clin Anesth 1998; 10: 41–5.[Web of Science][Medline]
- Datex-Ohmeda. TEC5 Continuous Flow Vaporizer: operation and maintenance manual, part no. 1105 0100 000. Madison, WI: Datex-Ohmeda, August 1999:7,25.
- Medical device advisory notice, subject: Datex-Ohmeda Sevotec 5 Anesthesia Vaporizers (for use with sevoflurane). Madison, WI: Datex-Ohmeda, June 11, 1999.
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