Hyponatremia in hospitalised patients with heart failure in internal medicine: Analysis of the Spanish national minimum basic data set (MBDS) (2005-2011).

Objective: Hyponatremia is the most common electrolyte disorder seen in clinical practice. Numerous studies have reported increased inhospital mortality associated to this condition, which is also an independent predictor of comorbidity in patients admitted with heart failure (HF). The objective of this study is to assess the incidence, average length of stay, associated comorbidities, readmissions and mortality caused by hyponatremia in admissions for acute heart failure from the Spanish national minimum basic data set (MBDS).

Materials And Methods: Data from the Spanish national minimum basic data set (MBDS) of discharged patients who were initially diagnosed with heart failure (HF) from all internal medicine (IM) departments of Spanish National Health System (SNS) hospitals between 2005 and 2011 were analysed (ICD-9: 428; DRGs 127 and 544). A descriptive data analysis was conducted comparing the diagnosis codes and administrative variables of heart failure patients with and without hyponatremia. The chi-square test was used for qualitative variables and the Student's t test for quantitative variables. A bivariate analysis was used to detect statistical differences in the mortality of both groups, as well as mean age, Charlson index, average length of stay and readmissions. A multivariate logistic regression analysis was performed, taking intrahospital mortality and hospital readmissions as dependent variables, and age, gender, comorbidity according to the Charlson index and hyponatremia as independent variables.

Results: A total of 504,860 patients with acute heart failure were identified, of whom 11,095 (2.2%) presented with HNa. A gradual year-on-year increase of hyponatremia codification (both primary and secondary diagnosis) was observed at discharge throughout the study period (from 1.6% in 2005 to 2.8% in 2011; p<0.0001). Overall mortality due to any cause in patients with hyponatremia was 17% (1937 patients) versus 11% in non-hyponatremic patients (53,820 patients). The probability of readmission for patients with hyponatremia was 22% versus 17% in the non-hyponatremic group. Hyponatremia was associated to a higher rate of mortality during hospitalisation for acute heart failure with an odds ratio (OR) of 1.58, 95% CI, 1.50-1.66 (p<0.05). Hyponatremia maintained statistical significance in the regression model after adjusting for gender, OR 0.919 (95% CI 0.902-0.936); age, OR 1.061 (95% CI 1.060-1.062); and Charlson index, OR 1.388 (95% CI 1.361-1.461).

Conclusions: Hyponatremia is associated to an increased rate of mortality and readmission in patients admitted for acute heart failure in SNS hospitals. Our study identified a statistically significant association between hyponatremia and increased intrahospital mortality independent of age, gender and the Charlson comorbidity index. During the defined follow-up period the discharge reports showed an increased codification of hyponatremia.