Poikilocapnic hypoxic ventilatory response in humans during 0.85 MAC isoflurane anesthesia

Abstract

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 degree 0.1) 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 VE that on average increased by 0.6 l.min-1 (P < 0.05) whereas P degree 0.1 was unchanged. In the anesthetized state, no case of decreased HVR was seen and hypoxia induced a mean VE increase (+/- s.d.) by 1.0 +/- 0.2 l.min-1 (P < 0.001) and a P degree 0.1 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 PEO2's of 8.7 kPa was maintained during 0.85 MAC isoflurane.

Effect of domperidone on the ventilatory response to transient hyperoxia in patients anaesthetized with isoflurane

“…However, our results and those of Clergue and co-workers 12 suggest that peripheral chemoreceptors are not "silent" at anaesthetic concentrations of isoflurane and halothane, respectively, suggesting that although peripheral chemoreceptor responses are depressed, this is perhaps not to the degree as suggested by previous investigators. 3 Our findings also support recent studies by Lindahl's group [4][5][6] who found persistent ventilatory responses to hypoxia under poikilocapnic conditions in patients anaesthetized with 0.6-0.85 MAC of isoflurane and a reduction of 60-70% in HVR under isocapnic conditions with 0.85-1.1 MAC of isoflurane. Therefore, anaesthetic concentrations of isoflurane do not appear to suppress HVR to the same extent as that found by previous investigators.…”

“…However, recent studies have shown that HVR is not as sensitive to isoflurane and ventilatory response to hypoxia remained in patients anaesthetized with 0.6-1.1 MAC of isoflurane. [4][5][6] Animal studies also provide conflicting data as to whether or not HVR is depressed by volatile anaesthetics. 7 8 The site of action of volatile agents on HVR remains unclear.…”

Sources of Inspiration: A Neurophysiologic Framework for Understanding Anesthetic Effects on Ventilatory Control

“…In other studies where end-tidal CO 2 was not artificially clamped to a constant value (poikilocapnia), 0.85 MAC isoflurane did not significantly depress HVR probably because of the interaction between the central and peripheral ventilatory control mechanisms [92,93]. HCVR is much larger than HVR under most conditions, and so when CO 2 is allowed to rise, the stimulation of ventilation by HCVR may simply render anesthetic depressant effects upon HVR inconsequential.…”