Effect of Torso Resistivity Variation on the Electrocardiograms of Children, Using a Grid Lead System

Abstract

Most clinical ECG leads determine cardiac currents from a few surface potential samples, without quantitating torso geometry and structure, by empirical-intuitive methods. The present report is part of a broader study asking whether the known limitations of ECG can be reduced by multi-electrode grid leads which sample extensively, measure torso geometry and structure, and use clearly defined biomathematics. One torso characteristic not measured in clinical ECG is torso resistivity ρ, while past studies of the cardiac dipole moment replaced measurement of ρ by assuming that it is 480 ohms-cm. In essence this amounts to using Ohm's law: Current = voltage/resistance without consideration of the resistance term. The present work attempts to measure each patient's ρ as part of the ECG recording procedure, by one of two methods: Dipole moment and ρ are separately determined and the measured dipole moment (Mm) is later corrected by computer, or the ECG recorder is calibrated for the patient's ρ, resulting in an ECG directly read as corrected dipole moment, ma-cm. The effect of ρ variation on the ECG was assessed in 51 children by comparing instantaneous and maximal Mm and its components when ρ was an arbitrary 480 ohms-cm (Mm 480) or individually measured (Mmi). Measured ρ was less than 480 ohms-cm in all subjects and decreased with age. The differences between Mm 480 and Mmi were typically half to a third of the mean Mm 480 and about half of the Mm 96 percentile. Correction of the ECG for individual ρ should result in revaluation of estimates of the heart's total force, as did area correction in previous reports of this series.