Effect of a subanesthetic minimum alveolar concentration of isoflurane on two tests of the hypoxic ventilatory response

These experiments were designed to study the effect of 0.1 minimum alveolar concentration isoflurane on the hypoxic ventilatory response as measured by two common methods of hypoxic testing: when normocapnic hypoxia was induced abruptly and when it was induced gradually. We hypothesized that any disparity in results would be due to an isoflurane effect that was manifested differently in the two tests.

After 20 min for uptake and equilibration of 0.1 minimum alveolar concentration end-tidal isoflurane or carrier gas in hyperoxia, isocapnic hypoxia was induced either abruptly over 60-80 s ("step" test) or gradually over 10 min ("ramp" test), followed by 20 min of isocapnic hypoxia at 45 mmHg end-tidal oxygen. Control of the hypoxic and isocapnic stimuli was accomplished accurately by a computer-controlled dynamic end-tidal forcing system. Eight subjects performed each test in the presence and absence of isoflurane.

For both step tests and ramp tests, 0.1 minimum alveolar concentration isoflurane had no effect on minute ventilation during the defined periods of hypoxia. With isoflurane, delta VE45, the acute change in ventilation from hyperoxia to hypoxia, was 97 +/- 20% (mean +/- SEM) of the control response for step tests and 100 +/- 25% of the control response for ramp tests. The step tests produced significantly larger acute hypoxic responses than did the ramp tests, but by the end of 20 min of hypoxia, ventilation was similar for both tests.

Neither method of hypoxic testing demonstrated the level of isoflurane effect reported by others. A comparison of the two methods of hypoxic testing suggests that ramp tests, as commonly performed, do not allow adequate time for full expression of the acute hypoxic ventilatory response. Step tests also better separated the opposing hypoxic effects of carotid body stimulation and central ventilatory depression.