Serum ionised calcium and the risk of acute respiratory failure in hospitalised patients: a single-centre cohort study in the USA

OBJECTIVES: The objective of this study was to evaluate the risk of acute respiratory failure in all hospitalised patients based on admission serum ionised calcium. DESIGN: A retrospective cohort study. SETTING: A tertiary referral hospital in Rochester, Minnesota, USA. PARTICIPANTS: All hospitalised patients who had serum ionised calcium measurement within 24 hours of hospital admission from January 2009 to December 2013. Patients who were mechanically ventilated at admission were excluded. PREDICTORS: Admission serum ionised calcium levels was stratified into six groups: ≤4.39, 4.40–4.59, 4.60–4.79, 4.80–4.99, 5.00–5.19 and ≥5.20 mg/dL. PRIMARY OUTCOME MEASURE: The primary outcome was the development of acute respiratory failure requiring mechanical ventilation during hospitalisation. Logistic regression analysis was fit to assess the independent risk of acute respiratory failure based on various admission serum ionised calcium, using serum ionised calcium of 5.00–5.19 mg/dL as the reference group. RESULTS: Of 25 709 eligible patients, with the mean serum ionised calcium of 4.8±0.4 mg/dL, acute respiratory failure requiring mechanical ventilation occurred in 2563 patients (10%). The incidence of acute respiratory failure was lowest when admission serum ionised calcium was 5.00–5.19 mg/dL, with the progressively increased risk of acute respiratory failure with decreased serum ionised calcium. In multivariate analysis with adjustment for potential confounders, the increased risk of acute respiratory failure requiring mechanical ventilation was significantly associated with admission serum ionised calcium of ≤4.39 (OR 2.52; 95% CI 2.12 to 3.00), 4.40–4.59 (OR 1.76; 95% CI 1.49 to 2.07) and 4.60–4.79 mg/dL (OR 1.48; 95% CI 1.27 to 1.72), compared with serum ionised calcium of 5.00–5.19 mg/dL. The risk of acute respiratory failure was not significantly increased when serum ionised calcium was at least 4.80 mg/dL. CONCLUSION: The increased risk of acute respiratory failure requiring mechanical ventilation was observed when admission serum ionised calcium was lower than 4.80 mg/dL in hospitalised patients.