A study on sinoatrial conduction in the aged.

Sinus node response to premature atrial stimulation (PAS) was studied in 30 aged patients (group I:21 cases without sinus node dysfunction, group II:9 cases with sick sinus syndrome). Sinoatrial conduction time (SACT) was calculated according to the method reported by Strauss et al. As coupling interval of PAS decreased, there noted 5 patterns of sinus node response. In Type A, return cycle length (RC) initially increased (fully compensatory pause) and then became constant; RC progressively increased either along the compensatory reference line (Type B1) or along the less-than-compensatory line (Type B2) without being constant. In 2 other cases, RC showed similar pattern as in Type A up to 32-37% of sinus cycle length where they showed abrupt prolongation below or above the compensatory reference line (Types C and D, respectively). The incidence of abnormal response (Types B1, B2, C, and D) was high both in group I (38.0%) and group II (33.3%). The calculated SACT in group I showed a high correlation with basic sinus cycle length, while no correlation was observed in group II. The possible factors influencing return cycle length and significance of frequent observation of abnormal sinus nodal responses in the aged subjects were discussed.