Tachycardia of carotid chemoreceptors originates in apneic asphyxia in dogs.

Using various anesthetics, we investigated the heart rate changes resulting from apneic asphyxia in paralyzed artificially ventilated dogs with 1) low control heart rate (LCHR) and 2) high control heart rate (HCHR). We observed that apneic asphyxia produced by stopping the ventilation pump for 1 min evoked tachycardia in dogs with LCHR and bradycardia in dogs with HCHR. In dogs breathing 100% O2 in the preapneic period, the tachycardia response was almost wholly abolished, whereas the bradycardia response was attenuated. Intracarotid injection of sodium cyanide induced a tachycardia response in dogs with LCHR and a bradycardia response in dogs with HCHR. The results suggest that 1) apneic asphyxia evokes tachycardia or bradycardia response depending on whether the resting cardiac vagal tone is high or low, respectively, and 2) both types of heart rate responses are in part due to stimulation of carotid chemoreceptors.