IONIZED 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 [Ca 2 + ], in plasma from healthy volunteers and p atients undergoing cardiopulmonary bypass (CPB). These measurements we re carried out because there have been few reliable measurements of th ese values in healthy volunteers and no direct measurements in this pa tient group. Patients and methods-Dip cast ion selective electrodes we re used to measure Mg2+, Ca2+, and H+ in plasma at 37-degrees-C. These values were correlated with total metal concentrations, [Mg] and [Cal , plasma sodium [Na], and albumin concentrations found by standard tec hniques. Blood samples were taken from the patient group immediately b efore and after CPB and a further sample 24 hours later. Results-In he althy volunteers the [Mg] was 0.86 (0.12) mM and [Mg2+], was 0.48 (0.0 6) mM, and the corresponding value for [Ca] was 2.34 (0-06) and for [C a2+] 1.01 (0.13) mM. Values for [Mg], [Ca], and [Ca2+] have been repor ted 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 on e hour after CPB and was slightly raised after 24 hours. The [Mg2+], h owever, was significantly reduced after 24 hours. Both [Ca] and [Ca2+] were slightly reduced after 24 hours but when adjusted for plasma alb umin concentrations they were unchanged over this period. Conclusions- The most important finding is that around 24 hours after CPB the plasm a [Mg2+] is SignifiCantly reduced, with no change to the total [Mg]. C orresponding changes to [Ca] and [Ca2+] were much smaller. This suppos es the presence of an Mg2+ binding ligand of unknown origin in the pla sma that may contribute to the cardiac arrhythmias that occur in some patients at this time after CPB.