The Relationship Between Duration of Q-T Interval and Plasma Ionized Calcium Concentration

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Abstract
The duration of the Q-T intervals (QTI) of the ECG and some indices of hemodynamic performance were evaluated during steady-state abnormalities of ionized Ca2+ homeostasis in 16 anesthetized closed-chest dogs. Ca2+ was maintained .apprx. 60% above or below normal. Progressive changes in QTI occurred for as long as 15 min after onset of steady-state changes in Ca2+. With institution of a hyper- or hypocalcemic plateau, the duration of the corrected intervals between the Q-wave deflection and the origin of the T-wave deflection (Q-oTc) and the Q wave and the end of the T-wave deflection (Q-Tc) were altered. In view of the progressive changes in QTI observed during the first 15 min of steady-state alterations in Ca2+, the magnitude of a disturbance in Ca2+ balance must be evaluated by direct Ca2+ measurement. Apparently changes in Ca2+ are assoicated with changes in both ventricular and peripheral vascular function. Hypocalcemia was associated with a step increase in left ventricular filling pressure (LVFP) from 6.8 .+-. 0.6 to 9.7 .+-. 0.7 torr, mean .+-. SEM [standard error of the mean], a step decrease in mean arterial blood pressure (MAP) from 108 .+-. 12 to 78 .+-. 15 torr, and a transient increase in cardiac output (CO) from 4.8 .+-. 0.2 to 5.7 .+-. 0.2 l/min, which was limited to the first 5 min of observation. With hypercalcemia, changes in CO and LVFP were insignificant and MAP increased from 112 .+-. 6 to 123 .+-. 8 torr.