Direct perfusion of the canine sinus node with various pharmacological agents having negative chronotropic effects commonly leads either to abrupt sinus arrest or to a gradual transition from sinus to atrio–ventricular (A–V) nodal rhythm with progressive shortening of the P–R interval. The reappearance of sinus rhythm is usually preceded by a change in A–V nodal rate and a progressive lengthening of the P–R interval to a stable value. During A–V nodal rhythm, changes in heart rate are observed following injections into the sinus node artery. As perfusion of the sinus node is selective, these cannot be attributed to a direct pharmacological effect of the perfusates on the A–V node. Deliberate suppression of A–V nodal pacemaking dominance reveals the persistence of slow sinus node activity which is unapparent electrocardiographically during A–V nodal rhythm. It would seem that even in the absence of P waves, the sinus node may still influence the rate of the A–V node. These observations are consistent with the hypothesis that the sinus and A–V nodes behave as a system of coupled relaxation oscillators.
Sinus node modification with an ultra high-density electroanatomical mapping system in inappropriate sinus tachycardia
