[Sinus node automaticity and sinoatrial conduction].

In 64 patients so-called sino-atrial conduction time (SACT) was calculated by means of the extrastimulus technique. The normal value of this parameter was 82 plus or minus 18.7 ms (20 patients). Plotting SACT versus heart rate in all patients no correlation could be found. However, in 4 patients with spontaneous variations of the sinus cycle length a short SACT was calculated after long spontaneous intervals and vice verse. These results may indicate that SACT ist frequency-dependent. However, it is not clear whether the behaviour of the postextra-systolic pauses in these patients is due to changes of SACT or due to spontaneous variations of sinus node automaticity. The measurement of the first spontaneous interval (A3-A4) after the postextrasystolic pause revealed a prolongation of 30 ms (mean value) in comparison to the basic cycle length (A1-A1) after resetting the sinus node. If the stimulus was elicited late in diastole without premature depolarisation of the pacemaker cells, no significant difference between A3-A4 and A1-A1 intervals could be found. These results indicate that sinus node automaticity may be influenced by the premature stimulus. Therefore, the calculation of SACT is burdened by unresolved methodological problems.