Investigation of post vagai tachycardias by analysis of the dynamics of cardiac period responses

Cardiac period responses after cessation of vagai stimulation were analyzed in the cat and the dog using non-linear curve fittting techniques. The relative undershoot (post vagai tachycardia) was investigated before and after muscarinic blockade with atropine. It was found that after atropine the tachycardia was abolished in the cat but persisted in the dog. However the altered temporal dynamics to that pre-atropine indicated that this tachycardia had a different etiology to that before blockade. It is hypothesized that this tachycardia is due to co-release of vasoactive intestinal polypeptide (VIP) from vagai nerve terminals.