Does subanesthetic isoflurane affect the ventilatory response to acute isocapnic hypoxia in healthy volunteers?

BACKGROUND

Differences in results studying the effects of subanesthetic concentrations of volatile agents on the hypoxic ventilatory response may be related to the conditions under which the subjects were tested. 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 arterial hemoglobin oxygen saturation from normoxia into hypoxia (arterial hemoglobin oxygen saturation 80% +/- 2%; duration of hypoxia 5 min) were performed in ten healthy subjects. We obtained four responses per subject: one without isoflurane in a darkened, quiet room; one without isoflurane with audiovisual input (music videos); one in a darkened room at 0.1 MAC isoflurane; and one at 0.1 MAC isoflurane with audiovisual input (subjects were addressed to keep their eyes open). Experiments were performed against a background of isocapnia (end-tidal carbon dioxide tension 1-1.4 mmHg above initial resting values).

RESULTS

The hypoxic responses averaged 0.54 +/- 0.09 1.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 +/- 0.131.min-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, while with stimulation two subjects had an increased response, four a decreased response and four an unchanged response compared to control.

CONCLUSIONS

We observed an important effect of the study conditions on the effects that 0.1 MAC isoflurane has on the hypoxic ventilatory response. A depressant effect of subanesthetic isoflurane was found only when external stimuli to the subjects were absent. With extraneous audiovisual stimuli the effect of isoflurane on the response to hypoxia was more variable. On the average, however, the response then was not depressed by isoflurane.