Effect of Airway Anaesthesia on the Ventilatory and Heart Rate Responses to Isocapnic Progressive Hypoxia

Abstract

The effect of airway anaesthesia by lidocaine inhalation on the hypoxic ventilatory response was examined together with the heart rate response by the isocapnic progressive hypoxia test in human subjects. During the test, end-tidal P_CO2 (PET_CO2) was maintained at the resting level. However, because resting PET_CO2 tends to decrease by airway anaesthesia, we conducted the test at the resting PET_CO2 determined both before (normocapnic) and after lidocaine (hypocapnic). Ventilatory and heart rate response were evaluated as a linear function of oxygen saturation of the arterial blood (Sa_O2). In the "hypocapnic" runs, ventilatory responses tended to be depressed, while the slope of heart rate response-PET_CO2 relationship increased after lidocaine. However, when PET_CO2 was restored to the normocapnic level, ventilation apparently increased from the control, and the augmented slope in the heart rate response disappeared. Although the elevated ventilation in normocapnic hypoxia might be due simply to the increased ventilatory response to CO₂, we suggested that the augmented slope in the heart rate response in hypocapnic hypoxia might be related not only to PET_CO2 level itself but also to the direct effect of airway anaesthesia.