I RELATION TO HEART RATE AND TO CLINICAL CONDITIONS ASSOCIATED WITH CHRONIC HYPERTENSION
Variations in the length of the different phases of the human electrocardiogram have long been observed, and satisfactory explanations have been offered for many of them by various writers. The length of the entire ventricular complex is known to vary in different individuals and in the same individual at different times but the explanation of this phenomenon has received comparatively scant attention at the hands of cardiologists.The normal ventricular complex, the Q-R-S-T group, is usually divided into subgroups, the Q-R-S interval and the S-T interval. Prolongation of the Q-R-S interval is due to defects in certain portions of the conducting tissues. This fact has been established by animal experiment and by carefully controlled clinical observation. There are, however, certain cases in which the Q-R-S interval is normal but the Q-R-S-T interval exceeds the accepted time limit.
FENN GK. STUDIES IN THE VARIATION OF THE LENGTH OF THE Q-R-S-T INTERVAL. Arch Intern Med (Chic). 1922;29(4):441-448. doi:10.1001/archinte.1922.00110040040004