Comparative study of two methods of estimating sinoatrial conduction time in man

Programmed premature atrial stimulation has been widely used to estimate sinoatrial conduction time in man. A proposed new approach uses continuous atrial pacing just above the spontaneous cycle length. Sinoatrial conduction time is represented by the difference between the first cycle after pacing and the spontaneous cycle length, assuming that sinus nodal automaticity is undisturbed by continuous atrial pacing.

Both techniques were compared in 23 consecutive patients. Mean (± standard deviation) sinoatrial conduction time was 113 ± 27 msec estimated with the premature stimulus technique and 96 ± 48 msec when estimated with the continuous pacing technique. In about 30 percent of cases the two values corresponded well with each other. In the remaining patients sinoatrial conduction time estimated with the premature stimulus technique was longer than the time estimated with continuous atrial pacing. Additionally, the latter was estimated at two different rates of pacing in which the cycle length was 30 and 60 msec, respectively, shorter than the previous cycle length. The estimate then increased to 119 ± 39 and 136 ± 40 msec, respectively. Sinoatrial conduction time estimated with continuous atrial pacing did not depend on spontaneous cycle length and did not correlate with sinus nodal recovery time. The cycles after the first pause were slightly longer than the spontaneous cycle length.

The results suggest that data from the two techniques cannot be easily compared and that premature atrial stimulation may exert a more depressive effect on sinus nodal automaticity than continuous atrial pacing. The observed differences in results may also be due to a more pronounced delay of retrograde conduction during premature atrial stimulation than during continuous atrial pacing. It is also possible that continuous atrial pacing leads to some overdrive exciting effect on the sinus node, although the opposite effect is suggested by the response of the cycles after the first postpacing cycle. A final conclusion regarding the validity of each technique cannot be reached on the basis of these clinical data.