The duration of the Q-U interval and its components in electrocardiograms of normal persons

Abstract 1. 1. In fifty normal men and fifty normal women the synchronous standard limb and precordial leads as well as the heart sounds were registered at rest. The following intervals were measured: the intervals from the beginning of the QRS complex to the end of this complex (QRS), to the origin of the T wave (Q-oT), to the apex of the T wave (Q-aT) to the end of the T wave (Q-T), to the “halfway point of T” (Q-T/2), to the apex of the U wave (Q-aU), and to the end of the U wave (Q-U). The origin of the T wave or the end of the RS-T segment was defined as the point most distant from a straight line connecting the RS-T junction with the apex of T. The relation of the beginning of the second heart sound to the apex and end of T and to the apex of U was also studied. All values were correlated with the heart rate and the age and sex, and in some cases also with the body height. 2. 2. The QRS duration, when measured from the earliest to the latest points in synchronous leads, is greater than when measured in separate limb leads. It increases slightly with decreasing heart rate but shows the greates relation to the body size. Men have a greater QRS duration than women, but in the same height groups the values were equal for both sexes. The upper limit for a height up to 5 feet 6 inches was 0.10 second, that for a height from 5 feet 6 inches to 6 feet was 0.11 second, and that for a height from 6 feet to 6 feet 6 inches was 0.12 second. Of nine persons over 6 feet tall, seven had a QRS exceeding 0.11 second. 3. 3. The Q-T duration increases with decreasing heart rate; at all heart rates women had on the average a 7 per cent longer Q-T duration than men. The components of Q-T (Q-oT and Q-aT) showed the same dependence on the heart rate as the entire Q-T interval. When expressed as a percentage of the Q-T interval expected for the heart rate, Q-oT ranged between 49 and 63 per cent, while Q-aT ranged between 62 and 92 per cent, independently of the heart rate and sex. Of all the components of Q-T, the RS-T segment was the most elastic, showing the greatest differences between the sexes and at high and low heart rates. QRS was the most rigid component. 4. 4. The interval from the end of T to the apex of U was practically independent of the heart rate and sex; it averaged 0.10 second. The Q-aU interval expected for a certain heart rate can be easily determined, therefore, by adding 0.10 second to the Q-T interval expected for this rate. The interval from the apex to the end of U was influenced by the heart rate more than any other interval, but as the determination of the end of U especially at high heart rates was difficult, this observation should be interpreted with caution. 5. 5. The second heart sound began within 0.03 second before or after the end of the T wave, 0.06 to 0.12 after the apex of T and 0.04 to 0.14 second before the apex of the U wave. At lower heart rates it tended to begin up to 0.01 second earlier. 6. 6. The electrocardiogram of women is characterized not only by a longer duration of Q-T and its components Q-oT and Q-aT but also by a longer and more horizontal RS-T segment than that of men. The point when the T wave reaches one-half of its final amplitude is situated nearer to the apex of T in women than in men. These characteristics make it possible, in most normal cases, to distinguish the electrocardiogram of a woman from that of a man.