We thank Drs. Byhahn and Westphal for the attentive reading they have accorded our article. 1We agree with their comment that our data represent air contamination at the air conditioning exhaust grill, not the concentration around the medical workers in the operating rooms. Because of the air conditioning system present (laminar flow system), the concentration of contaminant anesthetic gas near the anesthesiologists is bound to be greater than at the periphery of the operating rooms.
We did not measure regional differences in contaminant anesthetic gases in our operating rooms, but Wood et al. 2reported that the nitrous oxide exposure of anesthesiologists was five to six times higher than the level at the air conditioning exhaust grill. On this basis, the actual exposure of individuals in the operating rooms might well be greater than we reported. We should again like to emphasize the frequent occurrence of anesthetic gas contamination in operating rooms in routine practice.
To estimate the dilution of contaminant anesthetic gases in operating rooms, Byhahn and Westphal investigated the difference in nitrous oxide concentration between sites close to the wall-mounted gas outlet sockets and sites in the middle of the operating room. They reported a 40-fold difference between these two locations, but such a large difference will not be applicable to our investigation. In operating rooms, air flow is usually from the center of the ceiling to the periphery near the floor. In their study, the source of nitrous oxide contamination was at the wall, so little nitrous oxide would have been able to reach the center of the room against the air flow. In the clinical situation, Wood et al. 2reported a regional difference in nitrous oxide concentration in operating rooms of fivefold to sixfold.
Byhahn and Westphal claim that our data for the concentration of nitrous oxide at the exhaust grill are disproportionately high, 3and they attribute this to a possible undetected massive leakage of nitrous oxide from the central gas system or malfunction of air-conditioning or scavenging devices. For the purposes of our study, we actually checked the baseline concentrations of nitrous oxide in our operating rooms, and the average concentration was 3.0 ppm. There was no massive leakage of nitrous oxide from the central gas system or malfunction of air-conditioning or scavenging devices. Furthermore, the values for contaminant nitrous oxide in clinical practice reported by other investigators are in the same range as those reported in our article. For example, Wood et al. 2reported that the mean concentration of nitrous oxide at the exhaust grill during pediatric anesthesia was 87 ppm with the use of scavenging system. Kant et al. 4reported a mean time-weighed concentration of nitrous oxide in the periphery of their operating room of 41.2 ppm, whereas Davenport et al. 5reported that the mean concentration of nitrous oxide at the periphery of the operating room was 136 ppm without scavenging precautions, but this value was reduced to 13 ppm with an active scavenging system. To judge from these reports, we do not think that our values are disproportionately high.
In our study, we did not attempt to measure the actual concentrations of anesthetic gases near the medical workers in the operating rooms; instead, we assessed the frequency with which contamination occurred, and we established that nitrous oxide contamination was common during routine circumstances. The other point we would like to emphasize is the important role of scavenging systems in preventing anesthetic gas contamination.