In one of the first studies of its kind, researchers at the University of Ottawa Heart Institute have found that hypotension during cardiopulmonary bypass is associated with postoperative cardiovascular accidents, such as stroke. The researchers also found evidence of a dose–response relationship with increasing severity and duration of hypotension.

“Although the etiology of stroke is multifactorial, hypotension is thought to play a key role,” began Amy M. Chung, MSc, a medical student at the University of Ottawa Faculty of Medicine, in Ontario. “However, no model to date has investigated the role of hypotension during distinct phases of cardiac surgery. Accordingly, our work sought to examine whether hypotension before, during and after bypass was associated with postoperative strokes.” Cerebrovascular accidents, she added, can occur in up to 10% of patients undergoing major cardiac surgery.

A Retrospective Study

To investigate the potential association between varying magnitudes and durations of intraoperative hypotension and postoperative cardiovascular accidents, Ms. Chung and her colleagues conducted a retrospective cohort study of 7,779 patients, all of whom underwent major cardiac surgery requiring cardiopulmonary bypass between November 2009 and June 2014. Patients undergoing off-pump procedures were excluded.

The study’s primary exposure was longest duration of mean arterial pressure (MAP) less than 55, less than 65, and less than 75 mm Hg before, during and after bypass. The primary outcome was a postoperative ischemic cardiovascular event, defined as new focal or global neurologic deficit of cerebrovascular origin lasting at least 24 hours and nonhemorrhagic in nature.

The diagnosis of a cardiovascular event was verified by reviewing reported postoperative brain CT or MRI studies. Intraoperative invasive blood pressure measurements were recorded every 15 seconds in an electronic patient record; artifacts were removed using an automated algorithm.

As Ms. Chung reported at the 2016 annual meeting of the Canadian Anesthesiologists’ Society (abstract 151847), cardiovascular accidents occurred in 148 patients (1.9%) and were associated with any duration of MAP less than 75 mm Hg during cardiopulmonary bypass. What’s more, each additional 10 minutes of intraoperative hypotension with MAP less than 55 mm Hg was associated with a 17% increased odds of cardiovascular accidents (propensity-adjusted odds ratio [OR], 1.17; 95% CI, 1.07-1.28).

Similar results were found for each additional 10 minutes of MAP less than 65 and less than 75 mm Hg, which were respectively associated with 9% (OR, 1.09; 95% CI, 1.03-1.16) and 5% (OR, 1.05; 95% CI, 1.01-1.10) increased odds of cardiovascular accidents. Pre– and post–cardiopulmonary bypass intraoperative hypotension were not associated with cardiovascular accidents.

Independent Risk Factors

“We also found that there was a dose-dependent relationship for greater deviations of mean arterial pressure from baseline,” Ms. Chung reported. “For a MAP decrease of 30%, there was a 10% increase in cardiovascular events for the during-bypass period. And for a MAP decrease of 50%, there was a 21% increase in events.”

The researchers also identified a variety of other independent risk factors for cardiovascular accidents. According to Ms. Chung, these included:

• older age;
• combined valve and bypass surgery;
• emergent surgery;
• surgery on the thoracic aorta;
• preoperative shock;
• cooling while on bypass;
• hemodynamic instability;
• new-onset atrial fibrillation; and
• reopening following surgery

“To conclude, we found that hypotension was associated with postoperative cardiovascular accidents,” Ms. Chung said. “Future work aims to establish a multicenter study as well as overcome the limitations of our study. We also hope to establish a prospective stroke registry, which may show us ways that we can intervene to help prevent this complication as well as define personalized, goal-directed therapy.” Indeed, the ability to define critical thresholds and durations of hypotension associated with ischemic brain injury may lead clinicians to prompt preventive interventions.

Hilary P. Grocott, MD, professor of anesthesia and surgery at the University of Manitoba, in Winnipeg, questioned whether the study’s definition of stroke might have left some patients behind. “One of the difficulties I see is that your patients had to present with a clinically overt stroke that was confirmed with a radiographic image, such as a CT or MRI scan,” Dr. Grocott said.

“But there are many studies that now show that if you rigorously evaluate everybody before and after surgery, particularly with MRI, the stroke rates are substantially higher than if you just wait for them to present clinically. So you have shown a signal, but the strength of the signal was likely weakened because you may have missed at least half or more of all the strokes.”

“It’s possible that we’re missing some patients that weren’t identified,” Ms. Chung acknowledged.