[Evaluation of sino-atrial function using the method of extrasystole induced by constant-relative premature impulses....

Atrial pacemaking under conditions of relative constancy (40 or 50% of the preceding cycle) enables us to calculate the immediate sino-atrial conduction time (retrograde and antegrade) (SACT). 17 patients were chosen for their normal sino-atrial function under spontaneous changes of the sinus cycle (SC). In each case, a significant inverse linear relationship was found between SACT and the corresponding SC. The mean correlation slope was -0.36 in 10 patients with no post-pacing depression (PPD). The slope was greater in 7 patients with a PPD (-0.89); if this depression is taken into account when the SACT is calculated, the slope decreases. In 5 patients, atropine (1 mg I.V. reduced the mean value of SC, and shortened (constant relative value) the SACT. The mechanisms for the spontaneous and induced variations in the sinus output are discussed; it may be that there are substitute pacemakers within the cells of the sino-artrial node, which are affected by variations in sympathetic or parasympathetic activity or by pacing. In clinical practice, automatism and conduction with the sinus node should be interpreted as inter-related functions, both under normal conditions and after vagal block.