Postoperative hyponatremia predicts an increase in mortality and in-hospital complications after cardiac surgery

Background: The association between postoperative hyponatremia (Na < 135 mEq/L) and outcomes after cardiac surgery has not been established. We studied the prevalence of postoperative hyponatremia and its effects on outcomes after cardiac surgery. Study Design: We studied 4,850 patients who underwent cardiac surgery from 2002 to 2008. We used multivariable logistic and Cox regression analysis to study the association between postoperative hyponatremia and mortality, length of hospital stay (LOS), and complications. Results: Postoperative hyponatremia was present in 59%. Hyponatremic patients were older (mean ± SD, 62 ± 13 vs 61 ± 14 years, p = 0.001), had lower left ventricle ejection fraction (mean ± SD, 44% ± 16% vs 48% ± 13%, p < 0.001), higher mean pulmonary artery pressures (mean ± SD, 30 ± 11 vs 27 ± 9 mmHg, p < 0.001), lower glomerular filtration rate (mean ± SD, 72 ± 29 vs 74 ± 27 mg/min/1.73 m², p = 0.01), higher EuroSCORE (median, 15% vs 6%, p < 0.001), higher New York Heart Association class IV (31% vs 26%, p = 0.002), prevalence of COPD (23% vs 14%, p < 0.001), and peripheral vascular disease (16% vs 12%, p < 0.001). Hyponatremia increased overall (24% vs 18.2%, p < 0.001) and late mortality (18.6% vs 13.9%, p < 0.001) and length of stay (LOS; 11 vs 7 days, p < 0.001). Mortality increased with the severity of the hyponatremia. After adjusting for baseline and procedure variables, postoperative hyponatremia was associated with increase in mortality (hazard ratio 1.22, 95% CI 1.06-1.4, p = 0.004), LOS (multiplier 1.34, 95% CI 1.22-1.49, p < 0.001), infectious (odds ratio [OR] 2.32, 95% CI 1.48-3.62, p < 0.001), pulmonary (OR 1.82, 95% CI 1.49-2.21, p < 0.001), and renal failure complications (OR 2.46, 95% CI 1.58-3.81, p < 0.001) and need for dialysis (OR 3.66, 95% CI 1.72-7.79, p = 0.001). Conclusions: Hyponatremia is common after cardiac surgery and is an independent predictor of increased mortality, length of hospital stay, and postoperative complications.