Abstracts of Posters Presented at the International Anesthesia Research Society; 72nd Clinical and Scientific Congress; Orlando, FL; March 7-11, 1998: Anesthesia/OR Economics
Introduction: In many countries, sevoflurane is replacing halothane as the agent of choice for mask induction of general anesthesia in pediatric patients. Because sevoflurane is much more expensive than halothane, it is important to consider strategies to minimize its cost of administration. Since the cost of volatile gas administration is directly proportional to gas flow, reducing flow is a simple method to decrease expenses. We routinely perform 8% sevoflurane inductions at high flows then, for brief cases, reduce the rate to [approximate]1 L/min for the remainder of surgery. This study was designed to measure the actual flow rates and sevoflurane concentrations under these conditions and calculate the cost of sevoflurane administration.
Methods: With IRB approval and parental consent, we monitored children undergoing laser treatment of port wine stains under general anesthesia with a laryngeal mask airway. Gas flows and agent concentrations were continuously measured using a flow transducer inserted between the common gas outlet of the anesthesia machine and the anesthesia circuit and recorded to disk. The author supervised all cases, which were performed by anesthesia residents. The induction interval was defined as the first 6 minutes following initiation of sevoflurane; this included priming the anesthesia circuit, if this procedure was done. The maintenance interval was defined as minute 7 to the end of the case. The sevoflurane cost was calculated by converting the total anesthetic gas volume delivered to ml of liquid agent using a price of $0.72/ml. Data are presented as the mean +/- SD.
Results: Six patients age 2-8 years were studied. Cases ranged in duration from 18 to 39 min (median 30 min). The mean gas flow during induction was 2.7 +/- 0.5 L/min at 2.5 +/- 0.3 MAC. During maintenance, the gas flow was 0.9 +/- 0.1 L/min at 1.2 +/- 0.2 MAC. The mean MAC-hr exposure in these patients was 0.6 +/- 0.1. These flow rates and concentrations correspond to an induction cost of $4.70 +/- $0.53 and a maintenance cost of $5.16 +/- $0.99/hr. The total sevoflurane cost per case was $6.45 +/- $0.72 ([approximate]28 cases/bottle). A representative low flow sevoflurane anesthetic is presented in Figure 1.
Discussion: Lack of data under low flow conditions and uncertainty about the toxic threshold of compound A in humans led the FDA to warn agaisnt sevoflurane flows <2 L/min in the original labeling of the drug. Based on more recent data, discussions are currently underway at the FDA to change to MAC-hour based criteria at flows less than 2 L/min. In pediatric patients exposed to 5.6 MAC-h of sevoflurane at 2 L/min flows in a circle system, the mean compound A concentration was 5.4 +/- 4.4 ppm (maximum 15 ppm); none of these patients had any evidence of renal toxicity . These values are considerably less than the toxic threshold for compound A in any of the animal studies. Since our patients' exposure to sevoflurane during port wine stain surgery is approximately 10% of that in the Frink study, we feel it is safe to administer (2) L/m flows in this patient population.
1. Frink et al. Anesthesiology 1996;84:566