Significance of increased plasma sodium for the critically ill patient

Increased plasma sodium concentration (hypernatremia) acquired after admission to intensive care increases the risk that critically ill patients will not survive their illness. That is the headline finding of a recently published study of ~200,000 critically patients cared for in 344 intensive care units across the US during a 19-month study period (January 2008 - September 2010). 

For the purposes of this study ICU-acquired hypernatremia was defined as plasma sodium >149 mmol/L developing at least 48 hours after admission to ICU. Criteria for inclusion in the study included: plasma sodium <149 mmol/L on admission to ICU and for the first 48 hours following admission; not having an admission diagnosis of diabetic ketoacidosis or hyperosmolar state; and not having a plasma glucose >400 mg/dL. Of 207,702 eligible study patients, 8896 (4.3 %) developed ICU-acquired hypernatremia. 

The mean time to first sodium >149 mmol/L was 6.5 days and hypernatremia persisted for, on average, 2 days. Unadjusted hospital mortality was 32 % among those who developed ICU-acquired hypernatremia compared with 11 % for those whose plasma sodium remained within normal limits. Length of stay in ICU was also significantly higher for those who acquired hypernatremia (13.7 ± 9.7 days versus 5.1 ± 4.6 days). 

Multivariate analysis revealed that acquired hypernatremia was independently associated with a 40 % increased risk of hospital mortality and a 28 % increase in length of stay in ICU. Further analysis revealed that relative risk of hospital mortality increased with increasing severity of hyponatremia, although mortality risk was unaffected by duration of hypernatremia. 

In discussion of their study results the authors concur with others that acquired hypernatremia is often iatrogenic (i.e. preventable) and increased monitoring and early correction of rising plasma sodium concentration may have the effect of reducing hospital mortality among the critically ill and reducing the length of time survivors need remain in ICU.