New Orleans—Surgery patients who experience prolonged episodes of decreased oxygen to the brain are markedly more likely to suffer serious postoperative complications, including stroke, heart attack and death, a new study has found.

The observational study, by researchers from Mount Sinai School of Medicine, in New York City, was conducted in patients undergoing aortic surgery during which the circulation to their brains was arrested for as long as 30 minutes under hypothermic conditions. However, the investigators said the findings could apply to most surgical procedures for which monitoring and maintaining cerebral oxygenation is critical.

Indeed, an unrelated study by a team in Canada found an apparent correlation between episodes of cerebral desaturation and postoperative cognitive dysfunction among patients undergoing thoracic surgery. Both studies were presented at the 2009 annual meeting of the American Society of Anesthesiologists.

Gregory W. Fischer, MD, director of adult cardiothoracic anesthesia at Mount Sinai Medical Center, called his group’s findings “extremely interesting” and said they have the potential to change practice.

“The only small beauty spot to this study is that it’s not an interventional study,” Dr. Fischer said. “The next level that we have to go to is if intervening in patients who have low oxygen values improves outcomes.” He and his colleagues are now recruiting patients for such a trial.

For their study (abstract A897), which was funded by CAS Medical Systems through a grant from the National Institutes of Health, Dr. Fischer’s group collected data on 30 patients—22 men, eight women—undergoing aortic surgery with deep hypothermic circulatory arrest. During the procedure, the investigators tracked cerebral oxygenation using the company’s Fore-sight monitor, which provides absolute measurements of oxygen saturation in brain tissue (SctO₂). They set SctO₂ thresholds of 55%, 60% and 65%, and calculated the risk for postoperative complications associated with each.

Patients who endured longer periods of reduced cerebral oxygenation were at least 50% more likely—and often much more so—to experience a serious postoperative complication; and the deeper and longer the trough, the greater the risk (Figure 1). For example, patients whose SctO₂ fell below 55% for 30 minutes had about 3.5 times the risk for developing a postoperative complication as those whose SctO₂ remained above the 65% threshold. Even a 10-minute spell at an SctO₂ below 55% was associated with a 50% increased risk for adverse events after surgery, the researchers said.

“It appears for every SctO₂ threshold decrease of 5%, the increment exposure time for a given odds ratio decreases by approximately a factor of two” for SctO₂ thresholds between 55% and 65%, the investigators wrote.

The second study (abstract A1415), by David Bracco, MD, and colleagues at McGill University in Montreal, included data on 22 patients (10 men, 12 women) undergoing thoracic surgery with single-lung ventilation at the Quebec institution (the study so far has enrolled 100 subjects). The procedures were lobectomy (n=14), wedge resection (n=5) and pneumonectomy (n=3) lasting an average of 172 minutes, of which about 83% was spent under single-lung ventilation.

As with the Mount Sinai study, Dr. Bracco’s group used the Fore-sight monitor to assess cerebral oxygenation. Cognitive dysfunction was assessed before surgery, and two and 24 hours after the procedure, using the Mini-Mental State Examination (MMSE).

Cognitive function appeared to dip immediately after surgery compared with the baseline MMSE score, but rebounded by the 24-hour mark, the researchers said. Episodes of cerebral desaturation during single-lung ventilation correlated significantly with postoperative cognitive dysfunction (Figure 2; P<0.001). However, no other clinical variables, including peripheral oxygen saturation and PaO₂ readings, appeared to predict changes in cognitive function after surgery.

Competition Driving Studies

Although cerebral oximeters have been available for nearly two decades, with the arrival of the Invos System from Somanetics, only in recent years have researchers begun to seriously investigate whether the data these devices generate are clinically meaningful, Dr. Fischer said. That might reflect mounting competition in the market for such monitors, which now includes a field of three: the Invos and Fore-sight monitors, and the Model 7600 “Equanox” cerebral oximeter from Nonin, which won FDA approval in July. “You’ll see a lot more publications” of research involving cerebral oximetry, he predicted.

The findings arrive at a time when anesthesiologists are increasingly coming to understand that long-standing clinical practice may not always lead to good results. Examples abound: the emerging link between exposure to general anesthesia in early childhood and subsequent learning difficulties; evidence that certain anesthetic techniques may hasten the spread of breast cancer; and neurologic injuries resulting from poor cerebral perfusion during “beach chair” surgeries (see Anesthesiology News, September 2009, page 1). “There are multiple reports in the literature where we have caused a lot of harm by not respecting cerebral oxygenation,” Dr. Fischer said.

John Murkin, MD, professor of anesthesiology and director of cardiac anesthesia research at the University of Western Ontario, in London, Canada, has led studies showing that clinicians can reduce the morbidity of patients undergoing cardiac surgery by monitoring cerebral perfusion during the procedure and taking steps to increase blood flow when it falls.

In one such study, published in Anesthesia & Analgesia (2007;104:51-58), Dr. Murkin’s group showed that using an algorithm to increase cerebral perfusion during cardiac surgery may improve postoperative outcomes.

“We showed a significant reduction in perioperative morbidity in these patients versus a control group,” Dr. Murkin said. They also found that correcting desaturation episodes could prevent major organ morbidity and mortality—including death and stroke, the need for more than 48 hours of ventilation and the need for kidney dialysis.

Along with researchers from the Montreal Heart Institute, Dr. Murkin published an intervention algorithm to treat cerebral desaturations (Semin Cardiothorac Vasc Anesth 2007;11:274-281). “It’s not really a laundry list we’re providing as much as a guide to the physiologic rationale for these interventions,” Dr. Murkin said.

Monitoring is not foolproof, however. “It’s important to acknowledge the limitations,” Dr. Murkin said. “It’s a relatively small area of the cerebral cortex that’s being interrogated, and there are confounders that can significantly influence the measured values.” Still, he said, “if you pay attention, you can reduce the amount of cerebral ischemia, and that has a huge impact on patient outcomes.”

Oximetry also may prove useful for procedures below the brain. Dr. Murkin and his colleagues have been using the technology to monitor perfusion in the spinal cords of patients undergoing endovascular surgery. Others have applied it to neonates at risk for splanchnic ischemia.

But while researchers have long suspected that low levels of oxygen in the brain might be to blame for poor outcomes, particularly cognitive decline, after cardiac bypass surgery, that perception may be shifting somewhat.

New data suggest that a patient’s overall health at the time of bypass surgery—the heart failure, diabetes and high blood pressure—may be a stronger predictor of negative outcomes than what happens during the procedure. “Is it surgery or the comorbidity load?” Dr. Fischer asked. “That’s really hard to say.”