REPETITIVE RAPID INCREASES IN DESFLURANE CONCENTRATION BLUNT TRANSIENT CARDIOVASCULAR STIMULATION IN HUMANS

Background: Rapid increases in desflurane concentrations above minimum alveolar concentration (MAC) can cause transient (2-4-min) circulator y changes, possibly from stimulation of rapidly-adapting airway recept ors. We hypothesized that the initial increase in concentration would produce greater changes than subsequent increases. Methods: Anesthesia was induced with propofol in nine volunteers (25 +/- 1 yr old, mean /- SE) and maintained with 4% end-tidal desflurane for 32 min. We incr eased the desflurane to 8% (1.1 MAC) in 1 min, maintained this concent ration for 10 min, and then decreased it to 4% for 32 min. We repeated this process twice. After 1 week, 5 subjects were treated similarly e xcept that the second increase in concentration occurred 10 min and (o n a separate occasion) 75 min after the initial increase. Four subject s received the initial increase after 75 rather than 32 min of anesthe sia. In four we applied the repeated sequences in a background of 60% nitrous oxide. When a minimal cardiovascular response followed an incr ease of anesthetic concentration, a 60-s supramaximal 100-Hz tetanic s timulus was applied to an ulnar nerve percutaneously to test for sympa thetic responsiveness. Results: The initial increase in concentration increased heart rate (HR) from 57 +/- 2 to a peak of 119 +/- 7 beats/m in (P < 0.05); mean arterial blood pressure (MAP) from 66 +/- 3 to 119 +/- 5 mmHg (P < 0.05); and plasma epinephrine by > 10-fold (P < 0.05) . The second and third increases in desflurane concentration increased HR and MAP by less than 20% of the initial increases, regardless of t he timing of the later concentration increases. Responses to initial c oncentration increases after 75 min of anesthesia did not differ from those after 32 min. Increases in plasma epinephrine with the second an d third increases in desflurane concentration were attenuated. Subject s who did not responded to the second or third increase in desflurane always responded to tetanic electrical stimulation with substantial in creases in HR and MAP (P < 0.05). Addition of nitrous oxide did not ch ange results except for a smaller increase in MAP (P < 0.05). Ulnar ne rve stimulation increased HR and MAP but not epinephrine or norepineph rine concentrations. Conclusions: An initial rapid increase in desflur ane to 1.1 MAC produces much more stimulation than do subsequent incre ases, regardless of the presence of nitrous oxide. The decreased respo nse is consistent with the hypothesis that stimulation of rapidly-adap ting airway receptors produce the initial response.