Relation between ionized calcium concentration and ventricular pump performance in the dog under hemodynamically controlled conditions

The effect of plasma ionized calcium concentration on left ventricular function was studied in the canine heart on right heart bypass. Stroke volume, mean arterial pressure and heart rate were controlled. Plasma ionized calcium was lowered to 0.58 ± 0.01 mM by citrate infusion and raised to 1.70 ± 0.01 mM by calcium chloride infusion in random order in each dog. Left ventricular function at each of these ionized calcium levels was compared with that in an immediately preceding normocalcemic period. At a constant stroke work (16.9 ± 0.2 g-m), sustained hypercalcemia was associated with a small decrease in left ventricular end-diastolic pressure (1.7 ± 0.7 cm H₂O, p <0.05) despite a marked increase in peak left ventricular dP/dt (first derivative of ventricular pressure) averaging 34 percent (p <0.001). Coronary blood flow, tension-time index and myocardial oxygen consumption were not significantly altered. Stroke work determined at a left ventricular end-diastolic pressure of 14 cm H₂O, by interpolation in left ventricular function curves, was 11 ± 4.4 percent above that at control normocalcemia (p <0.05). At a constant stroke work (16.9 ± 0.2 g-m), sustained hypocalcemia was associated with a marked depression of left ventricular function as demonstrated by a substantial increase (from 4.9 ± 0.3 to 12.7 ± 1.1 cm H₂O, p <0.0001) in left ventricular end-diastolic pressure (p <0.0001), decreased mean systolic ejection rate (p <0.01) and decreased peak left ventricular dP/dt (p <0.0001). Coronary blood flow increased (p <0.05) whereas myocardial oxygen consumption did not change significantly. A marked displacement of left ventricular function curves to the right (compared with curves obtained during normocalcemia) was observed, and stroke work determined at a left ventricular end-diastolic pressure of 14 cm H₂O was 52 ± 5.4 percent below control level (p <0.001). It appears that hypercalcemia, when initiated from a normal control level, provides only a small enhancement of ventricular pump performance (as indexed by the stroke work-left ventricular end-diastolic pressure relation) despite a marked increase in peak left ventricular dP/dt, whereas marked improvement of left ventricular performance may be expected when calcium infusion is initiated from an ionized calcium level that is below normal.