Abnormal Serum Sodium is Associated With Increased Mortality Among Unselected Cardiac Intensive Care Unit Patients

Abstract

Background Abnormal serum sodium levels have been associated with higher mortality among patients with acute coronary syndromes and heart failure. We sought to describe the association between sodium levels and mortality among unselected cardiac intensive care unit (CICU) patients. Methods and Results We retrospectively reviewed consecutive adult patients admitted to our cardiac intensive care unit from 2007 to 2015. Hyponatremia and hypernatremia were defined as admission serum sodium <135 and >145 mEq/L, respectively. In-hospital mortality was assessed by multivariable regression, and postdischarge mortality was evaluated by Cox proportional-hazards analysis. We included 9676 patients with a mean age of 68±15 years (37.5% females). Hyponatremia occurred in 1706 (17.6%) patients, and hypernatremia occurred in 322 (3.3%) patients; these groups had higher illness severity and a greater number of comorbidities. Risk of hospital mortality was higher with hyponatremia (15.5% versus 7.5%; unadjusted odds ratio, 2.41; 95% CI, 2.06-2.82; P<0.001) or hypernatremia (17.7% versus 8.6%; unadjusted odds ratio, 2.82; 95% CI, 2.09-3.80; P<0.001), with a J-shaped relationship between admission sodium and mortality. After multivariate adjustment, only hyponatremia was significantly associated with in-hospital mortality (adjusted odds ratio, 1.42; 95% CI, 1.14-1.76; P=0.002). Among hospital survivors, risk of postdischarge mortality was higher in patients with hyponatremia (adjusted hazard ratio, 1.28; 95% CI, 1.17-1.41; P<0.001) or hypernatremia (adjusted hazard ratio, 1.36; 95% CI, 1.12-1.64; P=0.002). Conclusions Hyponatremia and hypernatremia on admission to the cardiac intensive care unit are associated with increased unadjusted short- and long-term mortality. Further studies are needed to determine whether correcting abnormal sodium levels can improve outcomes in cardiac intensive care unit patients.

Keywords: cardiac intensive care unit; coronary care unit; hyponatremia; mortality; sodium.

Figure 1

Cardiac intensive care unit (CICU) and in‐hospital mortality as a function of admission sodium in the overall study population (n=9676). P<0.001 for all mortality comparisons between sodium groups.

Figure 2

In‐hospital mortality as a function of admission sodium in patients with (A) acute coronary syndrome (ACS); (B) heart failure (HF); (C) chronic kidney disease (CKD), and (D) acute kidney injury (AKI). P<0.001 for all mortality comparisons between sodium groups by chi‐squared test, except in patients with severe AKI (*P=0.30).

Figure 3

Cardiac intensive care unit (CICU) and in‐hospital mortality as a function of the presence of hyponatremia (minimum sodium <135) or hypernatremia (maximum sodium ≥145) during the CICU stay in the overall study population (n=9494). P<0.001 for all mortality comparisons between sodium groups by chi‐squared test.

Figure 4

Cardiac intensive care unit (CICU) and in‐hospital mortality as a function of (A) lowest minimum sodium and (B) highest maximum sodium during the CICU stay in the overall study population (n=9494). P<0.001 for all mortality comparisons between sodium groups by chi‐squared test.

Figure 5

Kaplan–Meier survival curves demonstrating postdischarge survival among hospital survivors (n=8662) as a function of the presence of hyponatremia (minimum sodium <135) or hypernatremia (maximum sodium ≥145) on (A) cardiac intensive care unit (CICU) admission or (B) during the CICU stay. P<0.001 by log‐rank for all groups compared with patients with normal sodium and P>0.1 for other groups compared with each other.