Electrophysiology of normal sinus node with and without autonomic blockade

Abstract

Sinus node (SN) function was analyzed with and without autonomic blockade in 20 normal subjects, ages 15-67 years (mean 44 ± 16 years). Various electrophysiologic measures of SN function - SN cycle length (SNCL), SN recovery time (SNRT), corrected SNRT (CSNRT) and sinoatrial conduction time (SACT) - were analyzed. CSNRT was measured after atrial pacing at multiple rates. SACT was analyzed using two methods (SM and NM). After control measurements, autonomic blockade was produced by i.v. propranolol (0.02 mg/kg) and atropine (0.04 mg/kg). Measurements of SNCL, SACT and CSNRT were then repeated. During control, the mean resting SN cycle length was 625-920 msec (mean 761 ± 93 msec) (± SD); the maximal CSNRT was 167-325 msec (mean 262 ± 46 msec); the SACT by SM was 97-200 msec (mean 154 ± 30 msec) and by NNM 92-193 msec (mean 148 ± 28 msec). After autonomic blockade, the SNCL was 470-732 msec (mean 619 ± 71 msec); CSNRT 110-240 msec (mean 167 ± 39 msec); SACT by SM was 63-147 msec (mean 106 ± 24 msec) and by NM 57-142 msec (mean 100 ± 25 msec). These data indicate that (1) the upper limits of normal CSNRT and SACT with SM (based on mean plus 2 SD) are 354 msec and 214 msec during control and 245 msec and 154 msec, respectively, after autonomic blockade; (2) in normal patients, the CSNRT is mostly constituted by SACT and, to a lesser amount, by the depression of SN automaticity, although the latter conclusions are speculative and require experimental verification; and (3) intrinsic abnormality of SN automaticity is suggested if the sinus rate after vagal blockade alone is slower than the predicted intrinsic heart rate. Therefore, when propranolol is contraindicated, atropine alone may unmask intrinsic SN abnormalities.