Electrophysiology of normal sinus node with and without autonomic blockade.

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Abstract
Sinus node (SN) function was analyzed with and without autonomic blockade in 20 normal subjects, ages 15-67 yr (mean 44 .+-. 16 yr). 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 a multiple rates. SACT was analyzed using 2 methods (SM [Strauss method] and NM [Narula method]). 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 ms (mean 761 .+-. 93 ms) (.+-. SD); the maximal CSNRT was 167-325 ms (mean 262 .+-. 46 ms); the SACT by SM was 97-200 ms (mean 154 .+-. 30 ms) and by NM 92-193 ms (mean 148 .+-. 28 ms). After autonomic blockade, the SNCl was 470-732 ms (mean 619 .+-. 71 ms); CSNRT 110-240 ms (mean 167 .+-. 39 ms); SACT by SM was 63-147 ms (mean 106 .+-. 24 ms) and by NM 57-142 ms (mean 100 .+-. 25 ms). The upper limits of normal CSNRT and SACT with SM (based on mean plus 2 SD) are 354 ms and 214 ms during control and 245 ms and 154 ms, respectively, after autonomic blockade. In normal patients, the CSNRT is mostly constituted by SAC and, to a lesser amount, by the depression of SN automaticity, although the latter conclusions are speculative and require experimental verification; and intrinsic abnormality of SN automaticity is suggested if the sinus rate after vagal blockade alone is slower than the predicted intrinsic heart rate. When propranolol is contraindicated, atropine alone may unmask intrinsic SN abnormalities.