Observations on the Change of Ventricular Systole

In a previous communication (1) we have briefly described the electrocardiographic changes of the QT interval during exercise in normal subjects. On the average the corrected QT interval or K was 7% greater than that observed during rest, and with the onset of recovery it diminished by 9%o within the first two minutes and gradually returned to the original value at rest. There was no significant alteration of QRS complex, ST segment or T wave associated with the change of QT interval. The significance of QT interval in various conditions has been recently studied. In normal persons it varies with age, sex and heart rate (2, 3). Shortening of QT interval occurs in conditions where the blood calcium is high (i.e., hyperparathyroidism [4, 5]) and after digitalis administration (6, 7). It is prolonged in many conditions including dilatation and hypertrophy of the heart (8), cardiac failure (9), myocardial ischemia and infarction (2, 3), various types of myocarditis (notably rheumatic [2, 10] and diphtheritic [2] ), disturbance of electrolytes (hypopotassemia [11], and hypocalcemia [ 12, 13]), and quinidine poisoning (13). The effect of exercise on the electrocardiogram in the diagnosis of coronary insufficiency by the application of Master's two-step test has been extensively studied and is well known (14-17). The essential abnormal changes include an ST depression of more than 0.5 mm. and a reversal of T waves. But no emphasis was made on the change of QT interval. Hartwell and his associates (18) have likewise studied the effect of exercise on normal human electrocardiograms. The tracings were taken before, during, and after exercise with the