For years, it has been thought that coronary artery bypass grafting surgery (CABG) often leads to long-term cognitive decline. Now evidence from an ongoing Hopkins study of heart patients suggests this may not be the case.
During CABG, the heart is stopped and the patient kept alive on a mechanical pump so that surgeons can carefully sew grafts onto the spaghetti-sized arteries that provide blood to the heart muscle. Previous studies had shown that a significant number of CABG patients experience increased depression and cognitive decline.
“It was previously thought that up to 40 percent of CABG patients suffered long-term cognitive decline,” says Guy McKhann, MD, founding director of the Hopkins Mind-Brain Institute and professor of Neurology and Neuroscience at the School of Medicine. “Clearly, this would be a huge problem.”
But McKhann and Ola A. Selnes, a Neurology professor, weren’t convinced the numbers suggested by previous studies were correct. Working closely with Biostatistics chair Scott Zeger, PhD, and Biostatistics research associate Sarah Barry, McKhann and colleagues found that previous studies lacked adequate control groups. The team therefore set out to look at four distinct populations over time: those who had CABG; those who had off-pump surgery (in which the surgeon sews grafts to the beating heart); those who had stents implanted in the occluded arteries instead of the surgery; and finally, healthy people who had no heart disease or risk factors.
In journal articles, the team has reported that cognitive functions in all three heart patient groups remain about the same, casting into doubt the widely held assumption that stopping the heart during the surgery in some way leads to later neurological impairment. With its use of different statistical models, the study is a startling departure from previous published reports. “We all think we know what cognition is, but the difficult part is, how do you measure it?” explains Zeger. “The problem with these measurements is they have a lot of noise in them.”
Eliminating “noise”—erratic and non-significant variations in data—is a challenge in situations where researchers can only hope to measure degrees of difference rather than absolute change. Cognitive ability is typically measured by repeatedly administering a battery of short tests—such as asking the subject to listen to 10 spoken words and then repeat them back—over a period of months or years. The trouble with this method is that most subjects will tend to vary in their responses, even if they are perfectly healthy. “You have good days and you have bad days,” is how Zeger puts it, leaving his team of biostatisticians with the difficult task of separating significant changes from normal fluctuations.
There is also the issue that statisticians call multiplicity: “The more tests you look at, the more likely you are to find evidence of decline—if that’s what you’re looking for,” says Zeger. “But it is significant that in our study we found just as many people got better as got worse.”
Still, overall there was a difference between the three kinds of heart patients and the healthy group, with the heart patients showing some cognitive decline.
“What this suggests is that the folks who have heart disease in fact have severe vascular disease of both the heart and the brain,” says McKhann. “That in itself could prove a very significant finding for post-operative care, and we are starting to see changes in how physicians look at these patients.” —Mike Field