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To investigate in detail the correlates of dysnatremia, and to estimate its differential prognostic relevance in patients with heart failure with reduced or preserved LVEF.Hyponatraemia has been shown to carry important prognostic information in patients with heart failure with reduced left ventricular ejection fraction (LVEF). However, exact serum sodium cut-off levels are not defined and the implications for heart failure with preserved ejection fraction (HF-pEF) are unclear. The prognostic value of hypernatraemia has not been investigated systematically. Therefore, the aim of this study was to investigate in detail the correlates of dysnatraemia, and to estimate its differential prognostic relevance in patients with heart failure with reduced or preserved LVEF.One thousand consecutive patients with heart failure of any cause and severity from the Würzburg Interdisciplinary Network for Heart Failure registry were included. Non-linear models for the association between serum sodium and mortality risk were calculated using restricted cubic splines and Cox proportional hazard regression. Median follow-up time for survivors was 5.1 years.Independent correlates of dysnatraemia included guideline-recommended medication for chronic heart failure, indicators of renal function, and reverse associations with established cardiac risk factors. Overall mortality was 56%. Both hyponatraemia (n = 72) and hypernatraemia (n = 98) were associated with a significantly increased mortality risk: hazard ratio (HR) 2.10, 95% confidence interval (CI) 1.60–2.77; and HR 1.91, 95% CI 1.49–2.45, respectively. A U-shaped association of serum sodium with mortality risk was found. Prognosis was best for patients with high normal sodium levels, i.e. 140–145 mmol/L.Both hypo- and hypernatraemia indicate a markedly compromised prognosis in heart failure regardless of LVEF. Sodium levels within the reference range carry differential information on survival, with serum levels of 135–139 mmol/L indicating an increased mortality risk.