To analyze the prognosis of the sick sinus syndrome (SSS), we compared the clinical aspects among unpaced, ventricular paced, and physiologically paced patients who were followed over a long period. Unpaced intrinsic SSS was not always progressive and patients did not necessarily require permanent pacing. The incidence of concomitant AV conduction disturbance was 65.6% before pharmacologic autonomic block, (PAB), but this was significantly reduced to 31.7% after PAB. Follow-up study of the physiologically paced groups revealed no development of either new or more than second degree AVB. The VVI group had significantly more complications (68%) than the physiologically paced groups, mainly chronic atrial fibrillation (36%) and thromboembolism (20%). In addition, cardiothoracic ratio (CTR) in the VVI group was significantly greater compared with that in the physiologic groups. Nine deaths occurred during the follow-up period in the pacing groups, including six with VVI and three with physiologic pacing. In the VVI pacing group, heart failure and thromboembolism were most commonly the causes of death, while in the physiologic pacing groups, the causes of death were noncardiac. Although the survival rate in the ventricular paced group was not significantly different from that in the physiologic pacing groups, cardiac deaths were fewer in the latter group. Considering our clinical data, the decision to use ventricular pacing needs to be carefully weighed in patients with sick sinus syndrome, and physiologic pacing is more highly recommended.
1 Negative chrono-and inotropic responses to both carbachol (CCh) into the sinus node artery and electrical stimulation of vagal nerve fibres (ES) were studied in the isolated, blood-perfused canine right atrium, using four muscarinic receptor antagonists, atropine, 4-DAMP, AF-DX 1 16 and pirenzepine.2 ES and CCh evoked negative chrono-and inotropic responses in a frequencydependent manner and in a dose-related manner, respectively.3 Each antagonist inhibited these negative chrono-and inotropic responses in a dose-dependent manner. The ranking order of blocking potency (ID,J was atropine~4-DAMP>AF-DX 1 16>pirenzepine. 4 The ID,, of atropine, 4-DAMP or AF-DX 116 against sinus rate decreases induced by CCh or ES was not significantly different from that against the atrial tension decreases. In contrast, the ID,, of pirenzepine against sinus rate decreases evoked by CCh (17 nrnol) or ES (20 nmol) was significantly smaller than that for the atrial tension decrease (CCh, 200 and ES, 53 nmol, respectively). 5 These results suggest that, in the isolated dog atrium, M2-receptor-mediatedresponses are predominant. However, M 1-receptor activation may also be involved in sinus rate response.
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