NEW ORLEANS – Preoperative hyponatremia was common in patients undergoing cardiac surgery and associated with a dramatic increased the risk of death and surgical complications in a large retrospective analysis.
Of the 4,370 patients who underwent cardiac surgery between 2002 and 2009 at the Ohio State Medical Center in Columbus, 931 (21%) had hyponatremia, defined as serum sodium less than 135 mEq/L on at least one of three sodium tests obtained before surgery.
Dr. Juan Crestanello
In unadjusted analysis, hyponatremic patients had significantly higher rates than did those without hyponatremia of early mortality (9% vs. 4%), late mortality (24% vs. 15.6%), longer mean hospital length of stay (12.6 days vs. 8.2 days), and operative complications (13% vs. 7%), Dr. Juan Crestanello and his colleagues reported in a poster at the annual meeting of the American College of Cardiology.
This isn’t all that surprising since hyponatremic patients were sicker at baseline, but the poor outcomes remained, even after adjustment for a slew of baseline clinical and operative variables.
In multivariable logistic regression analysis, preoperative hyponatremia was independently associated with a 31% increase in overall mortality, a 52% increase in early mortality, and a 56% increase in late mortality, said Dr. Crestanello, a thoracic surgeon at the medical center.
Hyponatremia also significantly increased the risk of operative complications by 30%, pulmonary complications by 73%, renal failure requiring dialysis by 64%, and hospital length of stay by 26%. Infectious and neurologic complications were not independently associated with preoperative hyponatremia.
"Hyponatremia identifies a group of patients at higher risk for cardiac surgery," Dr. Crestanello said in an interview. "It constitutes another tool to risk-stratify these patients and will help surgeons advice patients of their risk associated with surgery."
At baseline, patients with hyponatremia had a significantly lower mean ejection fraction than did patients without hyponatremia (39% vs. 46%), higher mean pulmonary artery pressure (32 mmHg vs. 28 mmHg), and higher incidence of comorbidities, including diabetes (47% vs. 34%), chronic obstructive pulmonary disease (24.5% vs. 18%), and history of a previous MI (44.5% vs. 35.6%).
Hyponatremic patients also had significantly higher New York Heart Association functional class, higher surgical risk as predicted by the European System for Cardiac Operative Risk Evaluation (19% vs. 9%) and underwent more complex surgical procedures including coronary artery bypass surgery with valve replacement and ventricular-assist device placement.
Their mean age was 62 years, 66.5% were male, and the mean sodium level was 134 mEq/L.
Sodium levels are one of the most common laboratory values measured and are routinely obtained preoperatively.
"In spite of being widely available, they are often overlooked," Dr. Crestanello said.
Data on other factors that may have influenced serum sodium like medications, glucose levels, and fluid use were not accounted for in the study because of its retrospective nature, he noted.
When asked to speculate on the mechanism behind the association between hyponatremia and poor outcomes, Dr. Crestanello said they are not well understood. "Obviously, hyponatremia is a marker for high-risk patients, but at the same time it is likely that it has pathophysiological effects on its own ... hyponatremia is also associated with changes in the neurohormonal milieu that, by itself, can have deleterious effects."
The analysis was based on data from electronic medical records, the Society of Thoracic Surgery database, and Social Security Death Index. Mean follow-up was 2.2 years.
The authors plan to analyze the effects that correcting preoperative hyponatremia has on outcomes of cardiac surgery. The current study was supported by a grant from Biogen Inc.