Factors influencing the ionization of calcium during major surgical procedures

The existence of a clinically feasible calcium electrode makes it possible to obtain rapid, accurate levels of ionized calcium. It is now possible to study the actual ionization of calcium under normal and abnormal physiologic conditions. The present investigation was directed at changes in ionized calcium during major surgical procedures. The total series of 125 patients was divided into three groups according to the type of plasma volume expander: group 1, whole blood alone; group 2, whole blood plus exogenous albumin, and group 3, albumin alone. Ionized calcium levels dropped significantly, p less than 0.001, in all three groups. Although albumin alone produced a decrease in ionized calcium, the addition of albumin to whole blood did not result in a greater decline than that experienced with whole blood alone. Chelation with the citrate ion of bank blood preservative was the major factor responsible for the decrease in ionized calcium. There was no statistically significant relationship between the extent of the decrease, the total volume of blood, the volume of blood per kilogram of the rate of transfusion in milliliters per kilogram per minute. Although the ionized calcium level fell initially, it increased while blood administration continued. In view of these facts, it is difficult to estimate the acutal level of ionized calcium at any point during the operation. Twenty patients in the series had ionized calcium levels below 1.25 milliequivalents per liter, range of 0.51 to 1.24 milliequivalents per liter. With the possible exception of one patient, no adverse cardiovascular effects could be attributed to the low levels of ionized calcium. The results in this series confirm our previous conclusion that the administration of exogenous calcium is not necessary during massive transfusion, with the possible exception of bypass open heart procedures and exchange transfusions in children.