Contribution of pulmonary receptors to the heart rate response to acute hypoxemia in rabbits.

We studied the effect of pulmonary afferent activity on the heart rate response to a progressive, isocapnic decrease in oxygen saturation (SaO2) in anesthetized rabbits. To abolish the effect of rapidly adapting receptors, we used inhaled bupivacaine aerosol, and to abolish the effect of slowly adapting stretch receptor activity, we used sulfur dioxide insufflation. The heart rate (HR) response (delta HR/delta SaO2) under control conditions was 0.39 +/- 1.29 beats/min/% (mean +/- SD, n = 11; values greater than 0 indicate a tachycardiac response to hypoxemia). After sulfur dioxide insufflation, all nine rabbits had a bradycardiac response (-2.02 +/- 1.13 beats/min/%), which was significantly less than control (p less than 0.0001). After bupivacaine inhalation, the heart rate response (0.27 +/- 1.04 beats/min/%) was unchanged from control. There were no significant differences in the percent increase of minute ventilation during hypoxemia in all runs. Our results indicate that in rabbits the receptors responsible for the increase in heart rate during progressive hypoxemia are the slowly adapting receptors.