Effect of pacing cycle length and autonomic blockade on sinus node refractoriness.

Sinus node (SN) refractoriness can be measured indirectly by observing the return responses after the introduction of progressively earlier atrial premature beats. The SN effective refractory period (ERP) is defined as the longest premature interval resulting in an interpolated atrial return response. In the present study, SNERP was analyzed in 71 subjects--51 control persons and 20 patients with evidence of SN dysfunction. SNERP could be measured in 40 of 51 control subjects and was shown to prolong at shorter basic pacing cycle lengths. At a basic cycle length of 600 ms, SNERP was 330 +/- 40 ms, whereas at 500 ms it was 350 +/- 50 ms (p less than 0.05). At a basic cycle length of 600 ms, SNERP was measured in 31 control subjects and 7 patients with SN dysfunction. The values of 330 +/- 40 and 520 +/- 20 ms, respectively, in these 2 groups suggested that this method can be used to differentiate patients with SN dysfunction (p less than 0.001). In 12 control subjects, SNERP was measured before and after partial autonomic blockade with propranolol and atropine. SNERP shortened from 360 +/- 40 to 320 +/- 40 ms (p less than 0.05). It shortened with atropine and prolonged with propranolol. Thus, SNERP prolongs with a shorter basic pacing cycle length and is affected by autonomic manipulation, in a fashion analogous to the atrioventricular node.