The measurement of the Q-T interval of the electrocardiogram.

The sources of error in determination of the beginning of QRS and the end of T during measurement of the Q-T duration are analyzed. An important error is confusion of an elevated U wave with the T wave, resulting in the diagnosis of a prolonged Q-T. In such cases, some of the precordial leads usually show a notch or kink between T and U which indicates approximately the end of T. If these criteria are used, the true corrected Q-T duration in hypopotassemia without hIypocalcemia is not prolonged, but normal or shortened, corresponding to an earlier appearance of the second heart sound. T HE DURATION of the Q-T interval of the electrocardiogram in its relation to the heart rate (relative or corrected Q-T duration) has been receiving increasing attention during the last decade. It has been found of especially great practical importance in the diagnosis of myocarditis'3' 24, 3 41 and of certain diseases accompanied by electrolyte imbalance (literature quoted in reference 24). It is also of importance in the determination and evaluation of the ventricular gradient.24 The sources of error which may appear in the determination of the corrected Q-T duration are threefold. First, there are difficulties in the exact determination of the points which are to be used for the measurement of the Q-T interval in a given complex. Second, there are difficulties in averaging the Q-T duration of different complexes, which shows physiologic fluctuation eveii at constant heart rates, but especially when the heart rate varies, as for instance in respiratory arrhythmia and auricu-lar fibrillation. A third group of difficulties arises when the actually measured Q-T duration is corrected for the heart rate and, in some cases, for a prolonged QRS duration. The two last mentioned groups of difficulties were ade-From the Divisioii of 1jxl)eieimental Medicine, Uni-but no systematic treatment of this subject was attempted up to the present time. In many cases the difficulties in the definition of the points to be used for measurement are so great that different cardiologists would probably assign widely diverging values to the Q-T interval of the same tracing. The purpose of the present paper is to point out some of these difficulties and to attempt to develop a set of exact rules and methods which could be used in future studies. There can be no divergence of opinion that the Q-T duration should be measured from the beginning of …