The cyanide gasp and spontaneous deep breaths.

Stimulation of the carotid body chemoreceptors with cyanide in anaesthetized rabbits usually causes a deep breath or gasp, but only if the vagus nerves are intact. This gasp has several similarities with spontaneous deep breaths in eupnoea. In paralysed rabbits, artificially ventilated, chemoreceptor stimulation induces an augmented discharge in the phrenic nerve equivalent to a gasp. In spontaneously breathing rabbits spontaneous deep breaths are more frequent with hypoxia than with normoxia. The results are interpreted in relation to (i) positive feedback from the lungs and (ii) summation of chemoreceptor and tonic vagal drive causing augmented deep breaths.