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The changing face of dysnatremia
Summarized from Lansink-Hartgring A, Hessels L, Weigel J et al. Long-term changes in dysnatremia incidence in the ICU: a shift from hyponatremia to hypernatremia. Annals of Intensive Care 2016; 6: 22
Abnormal sodium concentration (dysnatremia), which is a relatively common feature of critical illness – affecting 10-20 % of intensive care patients – is associated with adverse outcome for this patient group.
Past epidemiological study has allowed the widely held view that reduced serum sodium (hyponatremia, i.e. sodium <135 mmol/L) is more common among the critically ill, than increased serum sodium (hypernatremia, i.e. sodium >145 mmol/L).
Such a view is now challenged by a recently published study analyzing long-term trends in the incidence of dysnatremia among the critically ill.
Results suggest that over the past 20 years there has been a progressive rise in the incidence of hypernatremia, accompanied by a progressive fall in the incidence of hyponatremia.
The authors of this study are intensive care physicians working at two large (44-bed and 48-bed) university hospital intensive care units in the Netherlands.
They retrieved all plasma sodium results relating to 80,571 patients who had been admitted to their units over a 21-year period (1992-2012).
A total of 913,272 serum sodium results were analyzed, first by categorizing each result to one of the following 10 concentration ranges (mmol/L), four hyponatremic: <120, 120-124, 125-129, 130-134; two normonatremic: 135-139 and 140-145; and four hypernatremic: 146-150, 150-155, 155-160, and >160.
The 21-year study period was divided into 5 epochs: 1992-1996; 1997-2000; 2001-2004; 2005-2008, 2009-2012.
This permitted identification of % incidence of serum sodium results within each concentration range for each of the 5 epochs.
Results revealed that for all four hyponatremic ranges, % incidence successively decreased during each of the 5 epochs.
By contrast, for all four hypernatremic ranges, % incidence increased during the 5 epochs.
So for example, the incidence of hyponatremia (defined as serum sodium <130 mmol/L) decreased from 47 % in the first epoch (1992-1996) to 25 % in the last epoch (2009-2012).
During the same time period incidence of hypernatremia (defined as sodium >150 mmol/L) increased from 13 % to 24 %.
When analyzed separately, results for both units showed the same trend of reducing incidence of hyponatremia and increasing incidence of hypernatremia.
The case mix of patients admitted to the two ICUs remained broadly the same across the 21-year study period, and with the sole exception of patients recovering from organ transplantation, the same trend towards hypernatremia was evident in all patient types (medical, surgical trauma).
The association between dysnatremia and mortality was also examined.
A familiar U-shaped relationship between serum sodium concentration and mortality was confirmed, so that highest mortality was evident in the two groups with most severe dysnatremia, i.e. those with serum sodium <120 mmol/L, and those with serum sodium >160 mmol/L.
Mortality rates for each of the 10 serum sodium categories remained broadly unchanged across the 21-year study period.
Extreme hypernatremia (serum sodium >160 mmol/L) was associated with slightly higher mortality than that associated with extreme hyponatremia (serum sodium <120 mmol/L): 50 % versus 40 %.
In discussion of their study, the authors speculate on the cause(s) of the apparent change in the nature of dysnatremia over the past 20 years.
They highlight their finding that in most cases, hypernatremia develops in the days after admission to intensive care.
If, as results indicate, hypernatremia is most often an IC-acquired condition, then the increased incidence of hypernatremia is most likely due to changes in the treatment of the critically ill over the past 20 years.
The authors propose two treatment changes that they consider might be significant: less liberal use of iv fluids in combination with increased use of diuretic treatment; and increased use of steroids (hydrocortisone).
They observe that since hypernatremia is often treatment related (iatrogenic), it is to some extent potentially preventable.
They echo a recent suggestion that the incidence of hypernatremia within an intensive care unit could be used as a quality indicator of critical care.
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