Analysis of alternans in late potentials. Correlations between epicardial and body surface recordings.

The methodical performance of the signal-averaged electrocardiogram is strongly influenced by the beat-to-beat reproducibility of late potentials (LPs). Especially at higher heart rates, epicardial recordings from infarct regions show progressive beat-to-beat prolongations with alternating conduction block. To analyze the influence of beat-to-beat-alternans of LPs on the signal-averaging process, epicardial and body surface recordings were studied at different heart rates and extrastimulation. Epicardial and body surface recordings were obtained from dogs with 4-day postligation of the left anterior descending coronary artery (Harris model). Body surface potentials were averaged in different modes to a final noise level of 0.3 microV (rms) and digitally bandpass filtered (40-250 Hz). Modulation of the heart rate was performed by atrial or His-bundle pacing and by atrial premature extrastimulation. Pacing up to heart rates close to 180 beats/min produced no significant changes in the duration of LPs in epicardial and averaged body surface recordings; however, at higher pacing rates, considerable prolongation of LPs with different patterns in the epicardial leads was observed. In these cases, averaging of all beats revealed only a slight prolongation of LPs, as seen from the body surface. Selective averaging of beats with prolonged epicardial LPs showed the prolongation or absence of LPs, as seen in the epicardial recordings. Similar observations were made using an atrial extrastimulation technique, whereby the average of the premature beats was compared to the average of all normal sinus beats. Selective beat averaging of body surface potentials can unmask the prolongation of LPs due to atrial pacing or extrastimulation, as seen in recordings from the infarcted epicardium. The evidence of this modulation of LPs may improve the positive predictive value of the signal-averaged electrocardiogram.