North Carolina neuropsychologists believe they have gathered reliable evidence linking cardiopulmonary bypass surgery to impaired memory and attention. Claims about this relationship have been made before, but the current team used statistical methods that they consider to be more sound, less biased and less likely to over-estimate occurrence than those used in previous research. The researchers also, for the first time, documented that the cognitive declines persist beyond the first couple of weeks. The study appears in the July issue of Neuropsychology, published by the American Psychological Association (APA).
Cardiopulmonary bypass (CPB) surgery, notes Julian Keith, Ph.D., of the University of North Carolina at Wilmington, and his co-authors, is performed in the United States on more than 500,000 people annually. The procedure — like many major surgeries — exposes patients' brains to a variety of abnormal physiological conditions, including inflammation, lack of oxygen, elevated blood sugar, lowered body temperature, showers of microscopic blood clots, and the presence of a lot of amnesia-causing drugs.
The Neuropsychology article is both a reminder of the brain's vulnerability during surgery, and an open discussion of how scientists look for reliable ways to understand that vulnerability — in hope of finding ways to preserve brain function.
"There is no question that post-operative performances by bypass patients on some measures tend to be worse than one would expect, compared to control participants," says Keith. "the question is, what does this finding mean?"
Keith and his colleagues studied cognition in 39 patients, one to two days before and three to four weeks after elective bypass surgery performed in Wilmington, N.C., by a single surgeon between May 1996 and May 1999. The researchers also analyzed test data from 49 control participants recruited through a local senior wellness program.
The bypass patients and control participants showed significantly different neuropsychological performance, both before and after the surgery. The authors attribute the patients' lower pre-surgery memory scores to anxiety, medically based declines or both.
The medical aspect might be key, says Keith, "especially because it now seems as if some of the disease processes involved in coronary artery disease may be the same as those involved in dementias, including Alzheimer's Disease." If so, the disease necessitating the surgery, not so much the surgery itself, may be hurting memory.
Further, in the "after" testing (for controls, a second session), control participants significantly outperformed bypass patients on two important tests of attention and memory. The data reinforce the supposition that brain systems that support attention may be particularly vulnerable. Because these are complex systems, says Keith, "the more machinery required to do the task, the more likely it is that a brain insult will disrupt the process."
Although post-bypass patients have shown a harder time concentrating since the advent of the surgery some 40 years ago, it's been hard to confirm the problem and define its cause. First, there are the complexities of studying the phenomenon in actual patients, outside the controlled conditions of the laboratory. Second, human cognition is itself a slippery subject, comprising many aspects such as memory and decision-making, some of which are difficult to quantify. Third, in people who take the same tests twice — as patients do, before and after surgery — a "practice effect" can boost performance. Fourth, the influence of practice can mix with the influence of surgery, making it hard to tease out the effects of different factors.
Keith and his team raise the point that these kinds of post-operative cognitive declines may not be unique to bypass surgery per se, but rather associated with major surgery itself. An expanded, rigorous research program might shed light on this medical mystery — especially once researchers settle on valid statistical approaches. Researchers in the field are fairly certain that persistent, long-lasting cognitive effects not caused by anesthesia.
In addition, the UNC Wilmington researchers urge investigators to focus on the post-surgery cognitive problems most important to daily life. Because a small change in one cognitive domain (for example, attention) may actually hurt everyday functioning more than a big change in another (for example, fine motor skills), Keith and his co-authors draw a distinction between statistically significant and functionally meaningful problems.
"What are you willing to trade," says Keith, "for 10 percent of your short-term memory ability or a 10 percent loss of your ability to take in and process information about your world?"