Isoflurane anaesthesia (0.6 MAC) and hypoxic ventilatory responses in humans

In order to evaluate the difference between poikilo-capnic (no CO2 added to inspired gas) and iso-capnic (CO2 added to keep end-tidal CO2 constant) hypoxic ventilatory responses (HVR) awake and during 0.6 MAC isoflurane anaesthesia, seven cardio-pulmonary healthy patients were investigated. Pneumotachography and capnography were used before and during hypoxia (end-tidal O2 tension approx. 7 kPa). In the awake state, poikilo-capnic hypoxic challenges resulted in an increased HVR as indicated by a VE that on average increased by 1.4 +/- 1.0 (mean +/- s.d.) l.min-1, whereas the iso-capnic hypoxic challenges resulted in a VE increase that was 4.7 +/- 2.3 l.min-1 on average. In the anaesthetized state, the corresponding value during poikilocapnia was 1.3 +/- 0.8 l.min-1 (88% of the awake responses, n.s.) and during iso-capnia 2.3 +/- 1.4 l.min-1 (49% of the awake, P < 0.02). Awake HVR was achieved by greater tidal volumes during poikilocapnia as well as during isocapnic challenges, while respiratory rates were unchanged. In the anaesthetized state, during poikilocapnia, however, HVR was mediated by an increased respiratory rate, (from 17.5 +/- 1.7 breath.min-1 to 20.2 +/- 2.2) and during isocapnia by a combination of increased rate (from 17.1 +/- 1.9 breath.min-1 to 19.1 +/- 1.8) and tidal volume (from 496 +/- 80 to 560 +/- 83 ml). It is concluded that poikilocapnic HVR is maintained at 0.6 MAC isoflurane whereas iso-capnic HVR is depressed by 50%.(ABSTRACT TRUNCATED AT 250 WORDS)