Both Hyponatremia and Hypernatremia at ICU Admission Predict Poor Outcome

Abstract & Commentary

By David J. Pierson, MD, Professor, Pulmonary and Critical Care Medicine, Harborview Medical Center, University of Washington, Seattle. Dr. Pierson reports no financial relationships relevant to this field of study. This article originally appeared in the December 2009 issue of Critical Care Alert. It was peer reviewed by William Thompson, MD.

Synopsis: In this study of initial serum sodium values in more than 150,000 adults admitted to ICUs, both hyponatremia (Na < 130 mmol/L) and hypernatremia (Na > 150 mmol/L) were associated with substantially increased ICU and hospital mortality.

Source: Funk GC, et al. Incidence and prognosis of dysnatremias present on ICU admission. Intensive Care Med. 2009 Oct 22; Epub ahead of print.

In this study, using a large database of adult patients admitted to 77 ICUs in Austria from 1998 through 2007, Funk et al examined the association between initial values for serum sodium (either below or above the normal range of 135-145 mmol/L) and the outcomes of ICU and hospital stays. From 176,703 admissions to the ICUs contributing to the database, the authors excluded 8,509 readmissions, 3,292 patients < 18 years of age, and 13,416 patients who lacked a recorded hospital outcome, a valid SAPS II score, or an admission sodium value, but included data on all others. This left 151,486 patients for analysis. Considering the serum sodium value that deviated most from the predicted normal value of 140 mmol/L during the first 24 hours in the ICU, there were 26,782 patients (17.7%) with hyponatremia (Na < 135 mmol/L), 114,170 patients (75.4%) with normonatremia, and 10,534 patients (6.9%) with hypernatremia (Na > 145 mmol/L).

Most patients had initial serum sodium values in the normal range. Hyponatremia was 2.5 times more common than hypernatremia. Most hyponatremic patients (78%) were classified as borderline, with Na = 130-135 mmol/L; while 15% had mild hyponatremia (Na = 125-130 mmol/L) and 7% were severe (Na < 125 mmol/L). Of the patients with initial sodium values that were abnormally high, 73% were borderline (Na = 145-150 mmol/L), 18% were mild (Na = 150-155 mmol/L), and 9% were severe (Na > 155 mmol/L). Statistical evaluation of the seven categories of admission sodium (from severe hyponatremia to severe hypernatremia) revealed a U-shaped curve with respect to hospital mortality. That is, with normonatremia taken as 1.00, odds ratios for death during the hospitalization were 1.32, 1.89, and 1.81 for borderline, mild, and severe hyponatremia, respectively, and 1.48, 2.32, and 3.62 for borderline, mild, and severe hypernatremia, respectively. Hospital mortality was 14.6% for normonatremic patients, 32.9% for mildly and 33.6% for severely hyponatremic patients, and 45.3% and 57.8% for mildly and severely hypernatremic patients, respectively. Using the initial serum sodium value as an indicator, without attempts to account for potential confounders, and compared to a mortality of roughly 15% for normonatremic patients, hospital mortality was about 33% for patients with Na < 130 mmol/L and more than 50% for patients with Na > 150 mmol/L. Multiple regression analysis suggested that hyponatremia and hypernatremia were independent risk factors for mortality.

Commentary

This is the largest study reported to date that associates initial derangements in serum sodium values with outcomes in critically ill patients. It does not prove that an abnormal sodium level increases mortality in and of itself, but the relationship between sodium levels and mortality persisted with all of the adjustments the authors made for patient demographics and severity of illness. It also does not say anything about the treatment of hyponatremia and hypernatremia, and specifically that optimal correction of these electrolyte abnormalities would improve the results observed here. However, in a large cohort of mixed adult ICU patients with the full spectrum of medical and surgical diagnoses, it confirms previous associations between the presence of substantially abnormal serum sodium values on ICU admission and the likelihood of in-hospital mortality.