Hypocalcemia in critical illness

Reduced plasma calcium (hypocalcemia) is a common feature of critical illness. A recent clinical study sought to document the course of hypocalcemia during the early days of critical illness, its significance for mortality, and the efficacy of calcium supplementation. The study population comprised 1038 patients admitted to the critical care units of a UK tertiary care hospital. 

Laboratory, demographic and clinical data relating to these patients was retrieved retrospectively. Both ionized calcium (iCa) and total (albumin-adjusted) calcium (Adj Ca) measurements were included for study, allowing investigation of the ability of Adj Ca to accurately predict ionized calcium. (The local reference range is 1.1-1.3 mmol/L for iCa and 2.2-2.6 mmol/L for Adj Ca). 

Analysis of the data revealed that on admission 43 % of patients were normocalcemic; 49 % were mildly hypocalcemic (defined as iCa 0.9-1.1 mmol/L); 6.2 % were severely hypocalcemic (defined as iCa <0.9 mmol/L) and 1.7 % were hypercalcemic (iCa >1.3 mmol/L). 

There was no significant difference in mortality between severely hypocalcemic, mildly hypocalcemic and normocalcemic patients (23.3 % vs. 19.4 % vs. 17.4 %, respectively). Length of stay in intensive care was, however, significantly longer for those with severe hypocalcemia (14 vs. 9 vs. 8 days respectively). 

By the fourth day calcium had normalized for many patients with hypocalcemia: only 53 % of patients with severe hypocalcemia on admission had iCa <1.1 at day 4, compared with 35 % of patients with mild hypocalcemia on admission. For patients who were mildly hypocalcemic on admission, there was no significant difference in mortality between those who did and did not subsequently normalize. 

However, for those who were severely hypocalcemic on admission, mortality was significantly higher for those who did not normalize by day 4, compared with those who did (37.5 % vs. 19 %). Among the other significant findings was the observation that calcium supplementation had no effect on survival and failed to have a measurable effect on iCa among those with hypocalcemia. 

Finally, the study confirmed that Adj Ca is a poor predictor of ICa in critically ill patients.