CLINICAL NOTES IN DIAGNOSTIC CARDIOLOGY Refer to: Cabeen WR Jr, Roberts NK, Child JS: Syncope and the sick sinus syndrome (Clinical Notes in Diagnostic Cardiology). West J Med 129:452-455, Nov 1978
Syncope and the Sick Sinus Syndrome WILLIAM R. CABEEN, JR, MD NIGEL K. ROBERTS, MD JOHN S. CHILD, MD Los Angeles SYNCOPE can be a troublesome and relatively common patient presentation with a variety of possible causes. Many cases are due to cardiac disease that results in periods of decreased cerebral blood flow. Prominent among the cardiac causes is the so-called sick sinus syndrome, an entity the spectrum and prevalence of which have become progressively more apparent in the past decade.' The diagnosis of sick sinus syndrome includes a number of manifestations or mechanisms of abnormal sinus node function such as spontaneous decrease or failure of sinus automaticity (or both), exaggerated suppression of sinus automaticity by rapid or premature atrial activity and sino-atrial conduction block."12 In addition, carotid sinus hypersensitivity, which may exist independently of intrinsic sinus node disease, may result in sinus node dysfunction due to vagal overactivity or sympathetic underactivity.' These mechanisms can appear as sinus bradycardia, sinus arrest, sino-atrial exit block or alternating bradycardia and tachycardia or vice versa, the so-called tachybrady syndrome. While occasionally the mechanism is apparent on routine electrocardiography, usually it is not, or the abnormality noted (usually sinus bradycardia) is an incomplete expression of the patient's sinus node dysfunction. Therefore, further evaluation is required to understand and manage the patient appropriately. A case we have recently seen is typical and illustrates the approach to and findings from such a patient. He was a 69-year-old man who presented after the second of two episodes of syncope lastFrom the Departments of Medicine and Pediatrics, Division of Cardiology, University of California, Los Angeles, School of Medicine. Reprint requests to: William R. Cabeen, Jr., MD, Division of Cardiology, Department of Medicine, UCLA School of Medicine, 10833 Le Conte Ave., Los Angeles, CA 90024.
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ing several seconds. He gave a history of intermittent lightheadedness during the past six months and exertional angina during the past nine years which recently had occurred only after meals followed by exercise. The angina was relieved promptly by nitroglycerin and was not a disabling symptom. Results of physical examination were normal except for the finding of periods of irregular pulse. An initial resting 12-lead electrocardiogram (ECG) (Figure 1, 8:16 AM) showed atrial fibrillation with a ventricular response rate of 130 beats per minute and prominent ST segment depression. A short time later a repeat ECG showed sinus bradycardia, with a rate of 57 beats per minute, and improved but still abnormal ST-T waves (Figure 1, 10:30 AM). Because of the history and these findings, additional studies were carried out to investigate a possible cardiac cause of the patient's syncope. A 24-hour Holter tape ECG monitoring study was done. It showed sinus rhythm and periods of atrial flutter and fibrillation during which the patient was asymptomatic. However, when the atrial flutter spontaneously terminated there were long pauses before cardiac rhythm resumed, due to failure of return of sinus rhythm or an appropriate lower escape rhythm (Figure 2). During these pauses the patient felt transient lightheadedness. These findings identified the patient's symptoms as being due to excessive overdrive suppression of his sinus node by atrial tachyarrhythmias and allowed appropriate diagnosis and treatment (see below). When long-term ECG recording does not show such findings and other causes of syncope have been excluded, patients can be further evaluated by (1) rapid atrial pacing followed by termination and measurement of sinus return time or "sinus node recovery time" and (2) programmed single atrial premature stimuli to evaluate sinus response. In a similar case-the patient was 65 years old, also with syncope and intermittent atrial flutter-the Holter 24-hour ECG recording showed only sinus bradycardia at a rate of 50 beats per minute. Pacing studies were therefore done. Atrial pacing resulted in atrial flutter which was terminated by rapid overdrive atrial pacing. Following
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