Beat-to-beat high-resolution electrocardiogram: Technical and clinical aspects☆

T HERE ARE important low-level electrocardiographic (ECG) potentials that are too small to be detected on the body surface by routine measurement techniques. These include the potentials produced by the HisPurkinje system and by slow and inhomogeneous conduction in diseased ventricular myocardium (usually called latepotentials). Identification of the His-Purkinje potential can localize the site of atrioventricular-conduction disorders, and the detection of late potentials may identify patients at high risk for malignant ventricular tachyarrhythmias. These potentials are small because the activation front is slow and fractionated, or the mass of tissue undergoing depolarization is small, or both. The problem in identifying these potentials is that the signal is smaller than the electrical noise produced by various sources. Different techniques have been used to improve the signal-to-noise (S/N) ratio. A commonly used technique is ensemble or temporal averaging (usually referred to as signaZ averaging).‘-” Temporal averaging is a process whereby fixed intervals of a noisy signal are aligned temporally with respect to a reference point and then summed. A signal-averaging system stores the information of a designated interval of the ECG and sums the information received from successive intervals. Division of the sum of stored information by the number of cycles yields an ensemble average. In this average, the components of the information arising from the noise sources diminish because they are random, thus resulting in an improved S/N ratio. In fact, the averaging process reduces to a simple summation, with the signal present in the summations building up linearly owing to the coherent timings with which the samples were taken. The noise, if it is random, adds up in a rootmean-square sense with the net result that after X signal repetitions have been combined, the S/N ratio has been improved by 4X. There are two major limitations for recording the His-Purkinje signals and late potentials by the temporal (ensemble) averaging technique.4.5 I. Alignment will not be able to deal with dynamic (beat-to-beat) changes in the signal (the His-ventricle interval) during sinus rhythm, and the QRS cannot be aligned during complex cardiac arrhythmias. 2. Atria1 potentials can obscure the His signal, unless the atrial-His interval is long. Therefore, the clinical advantage of identifying the His-Purkinje signal on a beat-tobeat basis is clear because the dynamic change of the temporal relation between the atria1 and ventricular potentials is usually meaningful. Late potentials may vary from beat to beat. Electrophysiologic observations in the canine postinfarction model of reentry suggest that spontaneous reentrant arrhythmias may be associated with a Wenckebach-like conduction pattern in a potentially reentrant pathway.f’-9 Recording of late potentials on a beat-to-beat basis has the potential of directly identifying reentrant “malignant” versus focal “benign” ventricular rhythms. 4,5 A variety of techniques have been used to improve the S/N ratio and to record His-Purkinje potentials of late potentials on a beat-to-beat basis.10-‘h These techniques and their clinical application are discussed in this article.