Relationship between plasma calcium and QT interval of electrocardiogram in dairy cows.

Relationships between calcium of blood plasma and measurements of the electrocardiogram QT interval corrected for heart rate and interval from Q to the apex of T corrected for heart rate were investigated in six dairy cows. Calcium was varied by infusing 4.7% solution of ethylenediaminetetra-acetic acid disodium salt to induce hypocalcemia, followed by treatment with calcium borogluconate. The within-cow regressions of the intervals on calcium in plasma were significant in all four electrocardiogram leads (leads I, aVF, SIII, and SaVF). These regressions differed between cows in these same leads. Correlation coefficients between calcium and the intervals were consistently larger than --.94 in lead SIII in all cows. When the results from four of the cows similar in age and breed were analyzed separately, regression coefficients between individual cows were not significantly different. However, intervals adjusted for calcium in plasma differed between cows.