A simple blood test done immediately after cardiac surgery may provide an effective indication of a patient’s postoperative stroke risk. An acutely elevated blood urea nitrogen (BUN) level after an operation was “by far” the most powerful predictor of postcardiac surgical stroke in a case-control study.

The researchers, led by Martinson K. Arnan, MD, of Wake Forest Baptist Medical Center in Winston-Salem, N.C., analyzed 5,498 adults ages 18 to 90 who had undergone heart surgery from 2005 to 2010 at the Johns Hopkins Medical Institution in Baltimore. Within 10 days of surgery, 180 patients had suffered a stroke.

Postoperative BUN values greater than 25 mg/dL predicted stroke with a sensitivity of 80 percent and a specificity of 75 percent. That greatly exceeded the predictive power of other, more commonly recognized risk factors such as smoking, preoperative hypertension and emergency surgical procedures.

“We do not believe that the relationship between elevated BUN and stroke risk is causal. Still, this novel finding has important implications,” Arnan et al wrote. Elevated BUN is generally considered to reflect intravascular volume depletion or inadequate renal perfusion. It’s possible patients with elevated postoperative BUN suffered parallel embolic hits to the brain and kidneys.

The results suggest patients identified postoperatively with strokes had suffered from relatively inadequate renal perfusion during the surgical procedures, even though the same cardiopulmonary bypass perfusion protocol was used for all patients. But the analysis revealed postoperative BUN is only weakly correlated with procedure type and cardiopulmonary bypass time and not correlated with lowest intraoperative mean arterial pressure. This led the researchers to posit that the postoperative BUN level cannot be fully explained by intraoperative factors.

“This lends some support to our conclusion that the rise in BUN among patients with postoperative stroke could reflect renal vascular vulnerability, which may be associated with stroke susceptibility,” Arnan et al wrote.

The findings are specific to one institution and a fixed set of surgeons with a certain level of experience and may not be generalizable to other clinical settings, Arnan wrote. But the researchers said the thrust of their findings is that evidence of vascular disease is associated with postoperative strokes and likely has general applicability.

The study was published online Feb. 12 in the Annals of Thoracic Surgery.