6 March 2000
Heart Ops Create Brain Blocks
by Kate Melville
The longer heart surgery takes, the more tiny particles of fat enter the bloodstream and circulate to the brain, blocking tiny blood vessels, Wake Forest University researchers reported in the March issue of Stroke, an American Heart Association journal.
The number of fat particles, called microemboli, increase dramatically the longer a patient remains on the heart-lung machine, which temporarily does the work of the heart and lungs during surgery. By blocking the tiny blood vessels, the microemboli can cause brain deficits that may be permanent in ten to thirty percent of affected patients.
For each hour spent on the heart-lung machine, the number of emboli increased by 90.5 percent, reported William R. Brown, Ph.D., Dixon M. Moody, M.D. and other researchers.
The heart-lung machine -known as cardiopulmonary bypass -- is most commonly used in coronary artery bypass grafting, done in 598,000 patients in 1996, according to the Heart Association. Other forms of heart surgery -- such as heart valve replacement -- also use cardiopulmonary bypass.
"Current estimates indicate that more than 50 percent of patients who undergo cardiopulmonary bypass have neurologic or neuropsychological deficits during the first week after surgery," Brown, Moody and their colleagues said. "Ten to thirty percent have long-term or permanent deficits and 1 to 5 percent suffer severe disability or death."
They said microemboli "have long been a suspected cause" of memory loss and other brain problems following cardiopulmonary bypass.
The emboli are generated during the operations, the Wake Forest team reported. After surgery has ended, the number of microemboli plugging blood vessels in the brain begins to decline. "Lipid [fat] emboli are known to pump slowly through the brain," they said. Over time, the larger emboli break into even smaller fat globules.
The research team has worked on the effects of heart surgery on the brain since 1990.
In the current study, the team studied brain specimens from 36 patients who died within three weeks of cardiopulmonary bypass, counting the microemboli that were present. Of the 36, 24 had only coronary artery bypass grafts and 12 had cardiac valve repair (some of whom also had coronary artery bypass grafts.)
"Microemboli were found in 'all 36 subjects," they reported. The microemboli often were clustered where these tiny arteries fork.
While they found increases in microemboli tied to duration of the operation in the 36 patients, the number went up even faster in the subgroup of patients undergoing heart valve surgery, by 145 percent per hour. And these operations tended to last longer.
"That longer cardiopulmonary bypass might cause more emboli has long been suspected, but until now the number and volume of cerebral microemboli have not been systematically quantified," they said.
The team also suggests that one possible cause for the microemboli might be the practice of suctioning up -- scavenging -- blood from the opened chest and returning it to the body through the bypass apparatus. "Eliminating the use of scavenged blood or removing lipid emboli from scavenged blood may be important for reducing post-cardiopulmonary bypass cerebral dysfunction," they said.