The role of signal averaged electrocardiography in the investigation of unselected patients with syncope.

The cause of syncope is often not determined, despite extensive investigations, yet it is important to identify the high risk group who may be in danger of sudden death due to ventricular arrhythmias. Recent studies have shown that the signal averaged electrocardiogram (SA ECG) can identify low amplitude signals in the terminal portion of the QRS and ST segment (late potentials) recorded from the body surface, which represent areas of delayed conduction in small areas of diseased myocardium in patients with ventricular tachycardia (VT). Sixty-five consecutive patients presenting to hospital with syncope were prospectively evaluated to determine a cause of syncope; this included quantitative analysis of the terminal QRS complex using SA ECG. A cardiac cause of syncope was assigned to 49% of patients, a non-cardiac cause to 20%, and no cause was found in the remaining 31%. Initial history and physical examination established a diagnosis in 14% of patients. A prior history of heart disease was an important indicator to a cardiac cause for syncope. Continuous electrocardiographic monitoring was diagnostic in 23% and the yield for electrophysiological testing (in a selected subgroup) was 40%. Ancillary cardiac and neurological investigations were of little diagnostic value, although they were useful in defining the severity and extent of clinically suspected conditions. SA ECG identified a late potential in 11 of 13 patients with VT, but was normal in all except three of the remaining patients who were not considered to have VT (sensitivity 85%, specificity 94%). We conclude that high frequency analysis of the signal averaged ECG is a reliable non-invasive indicator of syncope due to VT.