Plasma sodium concentration in the critically ill

Of all blood chemistry parameters, sodium is one of the most frequently measured among hospitalized patients, including those admitted to critical care units. The assay is generally available on point-of-care analyzers sited in these units. Incidence of abnormality in plasma sodium concentration among critically ill adults on admission as well as its significance for survival are the focus of a recently published retrospective study from Austria. 

With a database of over 150,000 unselected patients admitted to 77 medical and surgical intensive care units during a 10-year period (1998-2007), this study is claimed to be the largest of its kind ever conducted. 

Headline findings are that reduced sodium (hyponatremia) is more common than raised sodium (hypernatremia) and although abnormality in either direction is associated with reduced survival, the risk of not surviving critical illness is greater for those with high sodium than for those with low sodium. 

Of the 151,486 patients studied, 114,170 (75.4 %) had a normal plasma sodium (135-145 mmol/L) on admission, 26,782 (17.7 %) were suffering hyponatremia (sodium <135 mmol/L) and 10,534 (6.9 %) hypernatremia (sodium >145 mmol/L). 

Three grades of severity were defined: borderline, mild and severe. Of the 26,782 hyponatremic patients, 20,842 (77.8 %) had borderline hyponatremia (sodium 130-135 mmol/L); 4,076 (15.2 %) had mild hyponatremia (sodium 125-130 mmol/L); and 1,864 (7.0 %) had severe hyponatremia (sodium <125 mmol/L). 

Of the 10,534 with hypernatremia, 7723 (73.3 %) had borderline hypernatremia (sodium 145-150 mmol/L); 1,846 (17.5 %) had mild hypernatremia (sodium 150-155 mmol/L); and 965(9.2 %) had severe hyponatremia (sodium >155 mmol/L). 

Multiple regression analysis taking into account a range of demographics and severity of illness parameters showed abnormal sodium to be an independent risk factor for mortality that increased with severity of both hypo- and hypernatremia. Adjust odds ratio of death for borderline, mild and severe hyponatremia were 1.3, 1.89 and 1.81. The relevant figures for hypernatremia were 1.48, 2.32 and 3.64. 

The study confirms that abnormal plasma sodium on admission is a poor prognostic sign. Normalization of plasma sodium is a therapeutic aim that may increase the chance of critically ill patients surviving their illness.