Poikilocapnic hypoxic ventilatory response in humans during 0.85 MAC isoflurane anesthesia

Ventilatory responses to hypoxia (HVR) were investigated using poikilocapnic conditions (i.e. end–tidal CO₂'s allowed to seek it's own level) in 15 cardio–pulmonary healthy patients who were first studied awake and then at 0.85 MAC isoflurane. The influence of hypercapnia (HyperCapnic Ventilatory Response, HCVR) was also elucidated. Pneumotachography, capnography and airway occlusion pressures at 0.1 s (P < u) were used before and during both mild hypoxia (end–tidal O₂ tension 8.7 kPa) and hypercapnia achieved by an inspired CO₂ concentration of 5%. HCVR was attenuated by 60% during anesthesia ( P < 0.01). In the awake state , five of the 15 patients decreased HVR during hypoxia as compared with during normoxia. This resulted in a V_E that on average increased by 0.6 l–min^-1 ( P < 0.05) whereas P_E0₁ was unchanged. In the anesthetized state , no case of decreased HVR was seen and hypoxia induced a mean V_E increase (±s.d.) by 1.0± 0.2 lmin^-1 ( P < 0.001) and a P_E0₁ that on average was improved by 0.63 ± 0.27 cm H₂O ( P < 0.01). It is suggested that when the aim is to evaluate the influence of volatile anesthetic agents on HVR and to quantitate its clinical relevance during and immediately after anesthesia, a poikilocapnic technique should be used. It is concluded that the poikilocapnic HVR to P_F.o₂'s of 8.7 kPa was maintained during 0.85 MAC isoflurane.