The Diagnostic Significance of an Increased Q-T Interval in the Electrocardiogram

Abstract

The principal factor which influences the duration of electrical systole as indicated by the Q-T interval of the electrocardiogram is the heart rate.¹ In studies of the effect of other factors upon the length of electrical systole, corrections, therefore, must be made for the influence of variations in the heart rate. This can be done either by the use of the chart constructed by White and Mudd¹ or by employing Bazett’s² formula:

$\text{systole}=K\sqrt{\text{cycle}.}$ For purposes of clinical application the use of the chart of White and Mudd appears to be more practical than the use of the formula.

Prolongation of the period of electrical systole often is observed in association with such electrocardiographic abnormalities as bundle branch block, ventricular paroxysmal tachycardia, ventricular premature beats and auricular paroxysmal tachycardia.¹ It also has been known for many years that a low serum calcium value may be accompanied by a prolonged Q-T interval,^1,3,4,5 and more recently⁶ attention has been directed to the fact that a reduction in the concentration of the serum potassium may be responsible for lengthening of electrical systole. In two patients who had tetany, White and Mudd¹ observed a prolongation of the Q-T interval beyond the normal when the serum calcium content was extremely low (4 to 6 mg. per 100 cc.). When the serum calcium value rose above 6 mg. per 100 cc., however, the duration of the Q-T interval usually fell within the limits of normal. It would seem, therefore, that although a reduction in the concentration of serum. . .