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Summary:Since 1963 the signal averaging technique has been applied to improve the signal to noise ratio in highly amplified EKG registrations. Based on the experiences from the literature and the authors own laboratory, the applications of the signal averaging technique in clinical cardiology are reviewed: extraction and analysis of the fetal EKG and P-wave variations, His bundle electrograms from the body surface (recovery rate 33-100% of cases), ventricular delayed depolarizations within the ST segment of the surface EKG (recovery rate, 40-90% of cases, depending on patient groups investigated), preatrial activity (sinus nodal potentials) from intracardiac (recovery rate, 80-90% of individuals), or surface EKGs (recovery rate, 60% of patients), analysis of frequency components of surface EKG-QRS complexes in patients with previous myocardial infarctions, and detection of low amplitude diastolic signals from surface phonocardiogram (recovery rate, 80% of cases). At present, advantages and limitations of the signal averaging technique may be appraised as follows: ( 1 ) sinus nodal potentials: S-A conduction times may be more reliable than those obtained by the extra-stimulus technique, since with averaging they are recorded during undisturbed sinus rhythm; direct recordings of changing S-A blocks may be impossible due to the summation process; validation of sinus nodal potentials in man necessary, (2) A-V nodal potentials: demonstration of true A-V nodal rhythm rather than His bundle rhythm; possibly direct identification of abnormal pathways in A-V nodal tachycardias; direct recordings of single A-V nodal blocks impossible due to summation process; (3) surface His bundle potentials: follow-up or screening of patients with A-V nodal and particularly His-Purkinje-system blocks; monitoring of antiarrhythmic drug therapy; atrial overlap in one-third of cases; direct identification of higher degree A-V nodal blocks impossible due to summation process (future developments may overcome this problem); (4) ventricular delayed depolarizations: possible identification of patients at high risk of sudden cardiac death; follow-up of therapeutic measures like antiarrhythmic drug therapy or cardiac surgery (bypass grafting, aneurysmectomy); validation of delayed depolarizations from body surface by direct intracardiac and/or epicardial mapping necessary.

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