The effectiveness of an open referral electrocardiographic monitoring service in identifying an arrhythmogenic cause for syncope was evaluated. Over 5 years, 7,364 patients of all ages underwent ambulatory 24-hour electrocardiographic (Holter) monitoring using a 2-channel recorder. Of these, 1,512 (20.5%) were referred because of syncope. During monitoring, 15 patients had syncope and 7 of the episodes were related to an arrhythmia, usually ventricular tachycardia. Presyncope was reported in 241 patients, with a related arrhythmia in 24. Thus, an arrhythmia-related symptom that could be diagnostic was present in only 2% of the patients monitored. However, syncope or presyncope without an associated arrhythmia might be considered a negative diagnostic clue and occurred in 225 (15%). High-grade atrioventricular block was present in 15 and ventricular tachycardia in 116; only 6 (5%) reported associated symptoms. An age-related incremental increase in atrial and ventricular arrhythmias was found. In 415 of the 1,004 patients (41%) aged 60 years or more, arrhythmias that are conventionally associated with sinoatrial disease were recorded. Using stringent diagnostic criteria, the sick sinus or tachybradycardia syndrome was present in 33 (3%). Many older patients (70%) were taking drugs that could be arrhythmogenic, hypotensive or both. It is concluded that an open referral 24-hour ambulatory monitoring service rarely results in identifying relevant symptom-related arrhythmias in patients with syncope. It records many asymptomatic arrhythmias that can compound rather than resolve the diagnostic problem in older patients, because the data obtained could lead to unnecessary therapy. An iatrogenic cause for syncope should always be considered.

Diagnostic efficacy of 24-hour electrocardiographic monitoring for syncope.

Twenty patients of mean age 66.2 years, with suspected sinus node dysfunction, underwent extensive electrophysiologic study. Sinus bradycardia (18), sinus pauses (3), and sinoatrial block (1) were identified in their ECGs prior to study. Also 11 patients had some abnormality of atrioventricular nodal and/or intraventricular conduction prior to study. At the time of electrophysiological study, 10/20 patients (50%) had a mean cycle length exceeding 1000 msec, and mean P-V interval exceeded 210 msec in 7/20 (35%). The estimated "sinoatrial conduction time" exceeded 215 msec in 6/16 (38%) patients. The maximum first escape cycle following pacing at six different rates exceeded a value equal 1.3 X the mean value of the control cycle length + 101 msec (slope of regression line + Y intercept + 1 SD) in 13/9 (68%) patients. Nineteen patients received 1 mg atropine intravenously and mean cycle length decreased by 19%, from 891 +/- 175.8 msec to 718 +/- 182.9 msec. Graded infusion of isoproterenol was employed in 19 patients; four patients required an infusion rate greater than 28.3 ng/kg/min to produce a 20% decrease in spontaneous sinus cycle length. These data would indicate that a variety of interventions are required to characterize the disturbance of sinus node automaticiy and sinoatrial conduction in patients with sinus node dysfunction.

Electrophysiologic evaluation of sinus node function in patients with sinus node dysfunction.

▪Objective:To review data on the type, mechanism, and prevalence of the proarrhythmic effect of drugs used to treat atrial fibrillation or flutter. ▪Data Sources:English-language literatur...

Proarrhythmia in Patients Treated for Atrial Fibrillation or Flutter

Part I Digitalis glycosides: Mechanisms and manifestations of toxicity

Role of electrophysiologic testing in the diagnosis and treatment of patients with known and suspected bradycardias and tachycardias

The premature atrial stimulation technique was investigated as a method of measuring sinoatrial conduction time in the rabbit. Fifteen studies were performed in which intracellular recordings were obtained from a sinus node cell and atrial electrical activity was recorded from the crista terminalis by a surface electrogram. An additional ten studies were performed without microelectrode recordings. Atrial premature depolarizations late in the cardiac cycle produced compensatory atrial return cycles, but earlier premature depolarizations produced less than compensatory return cycles. Compensatory return cycles only occurred with atrial premature depolarizations that failed to capture the sinus node cell. The transition from compensatory to less than compensatory return cycles occurred with late atrial premature depolarizations that failed to capture the sinus node. Therefore, the transition from compensatory to less than compensatory atrial return cycles failed to indicate sinus node capture by an atrial premature depolarization. Although these premature depolarizations were too late to capture the sinus node cell, they still shortened the sinus node and atrial return cycles to make the atrial return cycle less than compensatory. This shortening of the sinus node return cycle was due to a shortening of the sinus node action potential by electrotonic interaction between sinus node and adjacent cells during repolarization. The electrotonic effect resulted in shortening of the sinus node action potential and accounted for the poor correlation ( r = 0.64) between estimated and measured values of sinoatrial conduction time. These data indicate that there are significant limitations to the use of the premature atrial stimulation technique for estimating sinoatrial conduction time.

Measurement of Sinoatrial Conduction Time by Premature Atrial Stimulation in the Rabbit

The electrophysiologic effects of intravenously administered propranolol (0.1 mg/kg) on three parameters of sinus node function were examined in ten symptomatic patients with sinus node dysfunction. The patients ranged in age from 26 to 79 years. Symptoms ranged from fatigue to frank syncope. Sinoatrial (SA) block and sinus pauses were observed in one patient; sinus pauses alone were observed in three patients. Five (5/10) patients had intraatrial block; three (3/10) patients had atrioventricular block; four (4/10) patients had an intraventricular conduction disturbance. At the time of electrophysiologic study, two patients had a control spontaneous sinus cycle length that exceeded 1000 msec. Following propranolol, the mean spontaneous cycle length increased by 17.4% (924 to 1085 msec, P less than 0.005) and spontaneous second degree SA block reappeared in the one patient. The maximum escape cycle ranged from 116% to 229% of the prepacing spontaneous cycle length and was considered to be prolonged in two of ten patients. Propranolol had no significant effect on the maximum escape cycle/prepacing cycle length X 100 (%). The estimated sinoatrial conduction time (SACT) was determined in seven patients and ranged in value from 120 to 238 mes. Propranolol increased the mean value of the estimated SACT from 179 to 213 msec, P less than 0.025. Propranolol may cause marked bradyarrhythmias in some patients with sinus node dysfunction, and should be used with caution in these patients.

Electrophysiologic effects of propranolol on sinus node function in patients with sinus node dysfunction.

Digoxin, in a common clinical dose and at a low serum level, brought out severe manifestations of sinus node dysfunction in a patient who had previously undergone successful mitral valve replacement This report presents the results of extensive clinical and electrophysiologic studies of this patient before and after a digoxin challenge In the absence of cardiac glycoside, the only demonstrable abnormalities of sinus node function were mild resting sinus bradycardia and failure to respond to atropine administration Responses to isoproterenol administration, programmed premature atrial stimulation, and overdrive pacing at several cycle lengths were normal Following the administration of intravenous digoxin, 1025 mg/24 hrs, the resting sinus cycle length increased and the response to overdrive pacing became markedly abnormal The latter was followed by sinus pauses in excess of six seconds, even at relatively slow overdrive pacing rates The electrophysiologic and clinical implications of these data are discussed It is suggested that despite previous reports that digitalis preparations are relatively well tolerated by patients with sick sinus syndrome, caution should be used when administering these drugs to this group of patients

Hugh C. Miller

Papers

Measurement of Sinoatrial Conduction Time by Premature Atrial Stimulation in the Rabbit

Electrophysiologic effects of propranolol on sinus node function in patients with sinus node dysfunction.

Digitalis and the sick sinus syndrome. Clinical and electrophysiologic documentation of severe toxic effect on sinus node function.

Surgical Mortality Statistics