DOES SUBANESTHETIC ISOFLURANE AFFECT THE VENTILATORY RESPONSE TO ACUTE ISOCAPNIC HYPOXIA IN HEALTHY-VOLUNTEERS

Background: Differences in results studying the effects of subanesthet ic concentrations of volatile agents on the hypoxic ventilatory respon se may be related to the conditions under which the subjects were test ed. In this study we investigated the effects of 0.1 minimum alveolar concentration (MAC) of isoflurane on the hypoxic ventilatory response without and with audiovisual stimulation. Methods: Step decreases in a rterial hemoglobin oxygen saturation from normoxia into hypoxia (arter ial hemoglobin oxygen saturation 80% +/- 2%; duration of hypoxia 5 min ) were performed in ten healthy subjects. We obtained four responses p er subject: one without isoflurane in a darkened, quiet room; one with out isoflurane with audiovisual input (music videos); one in a darkene d room at 0.1 MAC isoflurane; and one at 0.1 MAC isoflurane with audio visual input (subjects were addressed to keep their eyes open). Experi ments were performed against a background of isocapnia (end-tidal carb on dioxide tension 1-1.4 mmHg above initial resting values). Results: The hypoxic responses averaged 0.54 +/- 0.09 l.min(-1).%(-1) (without isoflurane in a darkened, quiet room), 0.27 +/- 0.06 l.min(-1).%(-1) ( in a darkened room at 0.1 MAC isoflurane; P < 0.01), 0.56 +/- 0.13 l.m in(-1).%(-1) (without isoflurane with audiovisual input), and 0.47 +/- 0.13 l.min(-1).%(-1) (at 0.1 MAC isoflurane with audiovisual input). Values are means +/- SE. During 0.1 MAC isoflurane administration, all subjects showed a depressed hypoxic response when not stimulated, whi le with stimulation two subjects had an increased response, four a dec reased response and four an unchanged response compared to control. Co nclusions: We observed an important effect of the study conditions on the effects that 0.1 MAC isoflurane has on the hypoxic ventilatory res ponse. A depressant effect of subanesthetic isoflurane was found only when external stimuli to the subjects were absent. With extraneous aud iovisual stimuli the effect of isoflurane on the response to hypoxia w as more variable. On the average, however, the response then was not d epressed by isoflurane.