Normal parathyroid hormone responses to hypocalcemia during cardiopulmonary bypass

To determine whether the calcium-magnesium-parathyroid hormone-calcitriol (vitamin D) axis responds appropriate to the hypocalcemia that routinely follows initiation of cardiopulmonary bypass (CPB), we measured blood ionized calcium (Ca(I)), total calcium (Ca(T)), total magnesium (Mg(T)), ultrafilterable magnesium (Mg(I)), total protein, intact parathyroid hormone (PTH), and calcitriol concentrations at eight defined time points in 28 patients undergoing elective cardiac surgery. With the onset of CPB, Ca(I) decreased from 1.14 ± 0.02 to 0.91 ± 0.03 mM, P < 0.05) (n = 17), and then gradually returned to a normal value by the time of separation from CPB (0.98 ± 0.01 mM). Ca(T), Mg(I), Mg(T), and total protein concentrations declined significantly upon initiation of CPB and remained depressed thereafter. PTH initially decreased upon initiation of CPB (from 50 ± 8 to 24 ± 9 pg/ml, n = 9, P < 0.05), remained inappropriately decreased during the early phases of CPB, and then gradully increased to maximal concentrations in response to hypocalcemia (103 ± 15 pg/ml) before emergence. Calcitriol concentrations (n = 8) were unchanged during surgery. Based on these initial results, which suggested an association between hypomagnesemia and the slow PTH response to hypocalcemia, measurements were repeated in 10 additional patients, to whom magnesium (Mg) (1 g MgSO₄ in two separate intravenous doses) was administered. Mg administration neither altered the PTH response to ionized hypocalcemia nor hastened the return of Ca(I) to normal. We conclude that Mg(T), Mg(I), and Ca(I) concentrations remain depressed at the time of separation from CPB, but that routine supplemental administration of neither calcium (Ca) nor Mg is required for the restoration of normal Ca(I) values after CPB.