Ventilatory Responses to Acute and Sustained Hypoxia During Isoflurane Anesthesia

Abstract

In the awake state, isocapnic hypoxic ventilatory responses (HVRs) are biphasic, with an acute response within 5 min of hypoxic stimulation followed by a less pronounced sustained response.In this study, we investigated the influence of isoflurane anesthesia (endtidal concentration 1.1 kPa) on acute and sustained isocapnic HVRs in eight healthy women at pulse oximetry arterial saturations of 75%-80%. The aims were to determine whether HVR (20 min of hypoxia) during anesthesia was biphasic and to quantify ventilatory responses. Pneumotachography and in-line infrared capnometry were used. A biphasic HVR was found both in awake and anesthetized patients. Of the subjects, six had decreased and two had increased acute and sustained isocapnic HVRs in the anesthetized, compared with the awake state, which resulted in an approximately 50% reduction in both acute and sustained HVRs. In addition, the ventilatory response pattern was altered by anesthesia. Awake HVR was accomplished by increased tidal volumes while respiratory rates were unchanged. The opposite occurred during anesthesia. The underlying mechanisms for this biological action of inhaled anesthetics remains to be elucidated. In conclusion, this study clearly demonstrates the persistence of hypoxic ventilatory sensitivity during clinical anesthesia. Implications: We studied the ventilatory effects of 20 min of breathing air with low oxygen content (hypoxic) in eight women, before and during anesthesia with inhaled isoflurane. We demonstrated a persistent but blunted hypoxic ventilatory response during clinical anesthesia. (Anesth Analg 1998;86:403-9)