Nitrous Oxide Diffusion and the Second Gas Effect on Emergence from Anesthesia

Abstract

Background: Rapid elimination of nitrous oxide from the lungs at the end of inhalational anesthesia dilutes alveolar oxygen, producing "diffusion hypoxia." A similar dilutional effect on accompanying volatile anesthetic agent has not been evaluated and may impact the speed of emergence. Methods: Twenty patients undergoing surgery were randomly assigned to receive an anesthetic maintenance gas mixture of sevoflurane adjusted to bispectral index, in airoxygen (control group) versus a 2:1 mixture of nitrous oxideoxygen (nitrous oxide group). After surgery, baseline arterial and tidal gas samples were taken. Patients were ventilated with oxygen, and arterial and tidal gas sampling was repeated at 2 and 5 min. Arterial sampling was repeated 30 min after surgery. Sevoflurane partial pressure was measured in blood by the double headspace equilibration technique and in tidal gas using a calibrated infrared gas analyzer. Time to eye opening and time extubation were recorded. The primary endpoint was the reduction in sevoflurane partial pressures in blood at 2 and 5 min. Results: Relative to baseline, arterial sevoflurane partial pressure was 39% higher at 5 min in the control group (P Ͻ 0.04) versus the nitrous oxide group. At 30 min the difference was not statistically significant. Time to eye opening (8.7 vs. 10.1 min) and time to extubation (11.0 vs.13.2 min) were shorter in the nitrous oxide group versus the control group (P Ͻ 0.04). Conclusions: Elimination of nitrous oxide at the end of anesthesia produces a clinically significant acceleration in the

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“…One recent study found arterial plasma partial pressure of sevoflurane 30 min after cessation of sevoflurane administration to be 20% of original arterial partial pressure (Table 2 in reference 16) - this result is consistent with the finding in our simulation (0.38/2 = 19% of original F VRG ) at 30 min in the sevoflurane group with mild hypoventilation (Figure 2) [16]. …”

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“…1 We agree that entire dose-response curve analysis does permit a more robust statistical comparison than ED 50 and ED 95 single-point comparisons. Dr. Pace's reanalysis found ED 50 and ED 95 values similar to our original calculations.…”

Rapid Recovery from Ambulatory Surgery: The New Paradigm in Ambulatory Anesthesia

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