Bedside evaluation of sinus bradycardia: usefulness of atropine test in discriminating organic from autonomic involvement of sinus automaticity

Sick sinus syndrome

Sinus-node dysfunction is common in the elderly and, in most cases, does not cause any symptoms. Despite the high number of laboratory investigations, most diagnoses of sinus-node dysfunction are made by 12-lead electrocardiography, which shows severe sinus bradycardia, sinus arrest, or sinoatrial block. Continuous electrocardiographic monitoring, exercise testing, and electrophysiologic investigations (including pharmacologic interventions to cause complete autonomic blockade) are sometimes useful in detecting transient or latent sinus-node abnormalities. The term sick sinus syndrome should be reserved for patients with symptomatic sinus-node dysfunction. Sick sinus syndrome has a protean presentation with variable degrees of clinical severity. Symptoms are often intermittent, changeable, and unpredictable. Because these symptoms can be observed in several other diseases, none are specific to sick sinus syndrome. Owing to the nonspecific nature of its symptoms, sick sinus syndrome can be diagnosed only when clear electrocardiographic signs corroborate symptoms. In the absence of a demonstrable link between signs and symptoms, a diagnosis can be presumed only when signs of severe sinus dysfunction are present and when every other possible cause of symptoms has been excluded carefully. Sinus-node dysfunction frequently is associated with diseases of the autonomic nervous system, and autonomic reflexes play a major role in the genesis of syncope. Survival does not seem to be affected by sick sinus syndrome. Atrioventricular block, chronic atrial fibrillation, and systemic embolism are major pathologic conditions that affect the outcome of the syndrome. Treatment should be aimed at controlling morbidity and relieving symptoms. Cardiac pacing is the most powerful therapy; physiologic pacing (atrial or dual-chamber) has been shown definitively to be superior to ventricular pacing.

Atropine administration in experimental electromechanical dissociation

Atropine can have a place during cardiopulmonary resuscitation (CPR) in the management of asystole, where parasympathetic influence might be excessive. However, the beneficial effects of atropine in electromechanical dissociation (EMD) have not been clearly demonstrated. The authors studied the effects of atropine in combination with epinephrine on an experimental model of EMD in the closed-chested dog. In 15 pentobarbital-anesthetized, mechanically ventilated dogs (mean weight 20 kg), EMD was induced by ventricular fibrillation followed by an external countershock, and was observed for 2 minutes before CPR was started. After 5 minutes of chest compression using a CPR thumper, either atropine 0.5 mg or D5W was administered, and the same injection was repeated every 5 minutes until recovery. Epinephrine 1 mg was administered in alternans. Each dog was submitted to two successive episodes of CPR, using either atropine or D5W, in a randomized order. Of a total of 28 CPRs, five were successful with chest compression alone. In the treatment groups, 10 of 11 were successful with atropine, but only eight of 12 with D5W (P < .01). The duration of CPR was also significantly shorter when atropine was used (9 minutes 56 seconds +/- 14 seconds versus 12 minutes 08 seconds +/- 43 seconds, P < .001). During the recovery period, atropine-treated animals had higher arterial pressure, heart rate, cardiac output and stroke volume. On this experimental model, the administration of high doses of atropine together with epinephrine enhances the recovery from EMD and results in a better cardiac function during recovery.

Parasympathetic overactivity and its evaluation in patients with sinus nodal dysfunction

In 17 controls, and 17 patients with sinus nodal dysfunction, an electrophysiologic study was made of sinus nodal function and atrioventricular nodal conduction in the basal state. The study was then repeated in all patients after atropine. The heart rate, mean sinus cycle length, variations of sinus cycle length, sinus node recovery times, sinuatrial conduction time, AH interval, and atrioventricular nodal Wenckebach threshold were significantly different in patients from those of controls. All these parameters changed significantly in patients after atropine, and were comparable to those of controls except for the atrioventricular nodal Wenckebach threshold. Atropine failed to increase the heart rate beyond 90 beats per minute in 10 of 17 patients (sensitivity of 59%) or by at least 30% above the resting heart rate only in 4 of them (sensitivity of 24%). The variations of sinus cycle length, and their standardized value, could detect sinus nodal dysfunction with sensitivities of 59 and 47%, respectively. From our results, we conclude that there is parasympathetic overactivity in patients with sinus nodal dysfunction. Because of their very low sensitivities, the atropine test and variations of sinus cycle length were not useful in identifying sinus nodal dysfunction noninvasively. The normal response of the heart rate to atropine does not exclude sinus nodal dysfunction, but atropine may help to differentiate abnormalities intrinsic and extrinsic to the sinus node during the electrophysiologic study.

Evidence for a central site of action to explain the negative chronotropic effect of atropine: studies on the human transplanted heart

The chronotropic response to atropine is biphasic; low doses cause slowing of the sinus rate and high doses cause acceleration. Although it is accepted that atropine functions as a competitive antagonist at high doses, the mechanism of the negative chronotropic response at low doses is controversial. Specifically, it is unclear whether the effect is mediated centrally or peripherally. Since at the time of cardiac replacement all central nervous system connections to the heart are severed, the transplanted heart is a unique model for separating these effects. Graded doses of atropine sulfate (0.5, 1.0, 2.0, 4.0, 8.0 and 40.0 micrograms/kg body weight) were administered to 12 human heart transplant recipients to test the hypothesis that the bradycardiac effect of low dose atropine is centrally mediated. The baseline sinus cycle lengths of the decentralized donor and innervated native sinus nodes were 694 +/- 20 and 733 +/- 27 ms, respectively. At the 0.5 and 1.0 microgram/kg doses, the cycle lengths of the native sinus node increased by 29.1 +/- 13.5 and 23.1 +/- 14.2 ms, respectively. At the 2.0 micrograms/kg dose the sinus cycle length again shortened to control. At the maximal dose of atropine the sinus cycle length shortened by 138.3 +/- 29.7 ms compared with control. In contrast, the decentralized donor sinus node exhibited a flat dose response to atropine. High dose atropine (40 micrograms/kg) caused no change in the donor heart's atrial effective refractory period, corrected sinus node recovery time, or sinoatrial conduction time measured by either the Strauss or the Narula method.(ABSTRACT TRUNCATED AT 250 WORDS)