Ionised magnesium and calcium in plasma from healthy volunteers and patients undergoing cardiopulmonary bypass

OBJECTIVES--To measure the concentration of ionised magnesium, [Mg2+], and ionised calcium [Ca2+], in plasma from healthy volunteers and patients undergoing cardiopulmonary bypass (CPB). These measurements were carried out because there have been few reliable measurements of these values in healthy volunteers and no direct measurements in this patient group. PATIENTS AND METHODS--Dip cast ion selective electrodes were used to measure Mg2+, Ca2+, and H+ in plasma at 37 degrees C. These values were correlated with total metal concentrations, [Mg] and [Ca], plasma sodium [Na], and albumin concentrations found by standard techniques. Blood samples were taken from the patient group immediately before and after CPB and a further sample 24 hours later. RESULTS--In healthy volunteers the [Mg] was 0.86 (0.12) mM and [Mg2+], was 0.48 (0.06) mM, and the corresponding value for [Ca] was 2.34 (0.06) and for [Ca2+] 1.01 (0.13) mM. Values for [Mg], [Ca], and [Ca2+] have been reported by others and those quoted here are similar. In the CPB group the preoperative [Mg] was lower than the normal group but did not alter one hour after CPB and was slightly raised after 24 hours. The [Mg2+], however, was significantly reduced after 24 hours. Both [Ca] and [Ca2+] were slightly reduced after 24 hours but when adjusted for plasma albumin concentrations they were unchanged over this period. CONCLUSIONS--The most important finding is that around 24 hours after CPB the plasma [Mg2+] is significantly reduced, with no change to the total [Mg]. Corresponding changes to [Ca] and [Ca2+] were much smaller. This supposes the presence of an Mg2+ binding ligand of unknown origin in the plasma that may contribute to the cardiac arrhythmias that occur in some patients at this time after CPB.