Monitoring of ionized calcium during human hepatic transplantation. Critical values and their relevance to cardiac and hemodynamic management.

During human hepatic transplantation, ionized calcium (ICa) measured in whole blood with an ion-selective electrode varied greatly from ICa predicted from total calcium (TCa), protein, albumin, and pH, by means of recently published nomograms. Measurement of ICa was necessary because the interaction of citrate in transfused blood and calcium chloride (administered to offset citrate binding) caused large variations in TCa. During hepatic transplantation, ICa and electrolyte measurements were obtained at approximately 15-minute intervals or more frequently if indicated by changing cardiac status. In one patient, hemodynamic instability was accompanied by a large decrease in ICa, which then was followed by cardiac arrest aggravated by myocardial depression from inadequate ICa. Cardiovascular phenomena associated with ionized hypocalcemia suggest that the critical value for ICa should be no more than 0.4 mmol/L (1.6 mg/dL) below the reference range mean. The authors propose critical limits for ICa and discuss their significance in clinical management of tetany, hypotension, arrythmias, and cardiac arrest.