Sinus nodal responses to atrial extrastimuli in patients without apparent sinus node disease.

In 36 patients without sinus node disease scanning with an atrial extrastimulus (A2) was performed during sinus rhythm with the sinus cycle length measured in milliseconds. Zones of nonreset due to interference, reset, interpolation and sinus echoes were defined by noting the timing of the first response after A2. Zones were defined in terms of their longest and shortest A1-A2 coupling intervals (in milliseconds). A zone of nonreset was found in 12 of 12 patients in whom A2 was delivered late. The mean cycle length in these 12 patients was 779 msec, with a mean zone of nonreset of 779 to 585 msec (25 percent of cycle length). All 36 patients (100 percent) had a zone of reset. The mean cycle length in these 36 patients was 803 msec with a zone of reset from 692 to 319 msec (46 percent of cycle length). Seven of 36 patients (19 percent) had a zone of interpolation. The mean cycle length in these seven patients was 754 msec, with a mean zone of interpolation of 344 to 279 (9 percent of cycle length). Four of 36 patients (11 percent) had a zone of sinus echoes. The mean cycle length in these four patients was 870 msec, with a mean zone of echoes from 350 to 313 msec (4 percent of cycle length). Calculated sinoatrial conduction time ranged from 40 to 153 msec (mean +/- standard deviation 92 +/- 30 msec). Shortening of the cycle length with atrial pacing increased the number of patients with zones of interpolation and echoes. In conclusion, zones of nonreset and reset are found in all patients with normal sinus nodal function, whereas zones of interpolation and echoes are much less common. Sinoatrial conduction time is surprisingly long in patients without apparent sinus node disease.