Each year in the United States, 500,000 coronary artery bypass surgery procedures are performed after coronary arteries become clogged with fatty tissue. Although the surgery may be successful in reestablishing blood flow to the heart, some patients have neurological complications after surgery including mental confusion, memory loss and stroke.
But Hawaiian researchers have found a 30-fold reduction in the micro-embolic burden – tiny debris that break off in surgery which and can travel to the brain and result in neurological complications – if the coronary bypass surgery is performed without placing the patient on a heart-lung bypass machine.
"We have shown that off-pump coronary bypass surgery can dramatically reduce the number of micro-emboli that travel to the brain, says Jeffrey D. Lee, M.D., associate professor of surgery, University of Hawaii. This is a very promising finding that can help us make heart bypass surgery safer."
In the past, most patients undergoing bypass surgery had their hearts stopped, and their body's circulation temporarily supported with a heart-lung bypass machine. While the pump is still used for most patients undergoing open-heart surgery, during the past several years an increasing number of surgeons have started performing off-pump bypass. In off-pump bypass, the heart is kept beating and just the portion of the heart with the affected artery is held still, while the bypass graft is sewn in place.
The study included 60 patients. Half were prospectively randomized to undergo bypass without the pump (Off-Pump Coronary Artery Bypass or OPCAB) and half were assigned to undergo bypass surgery with the pump.
During surgery, transcranial Doppler ultrasound of the middle cerebral arteries was performed to detect micro-emboli. Probes were placed on both sides of the temple region of the patient's head directly over these arteries, which feed the brain. Passing micro-emboli emitted a characteristic high frequency chirp and a sonographer listened and manually counted all micro-emboli in these arteries. Timing of each major surgical event was noted and any emboli occurring during the next minute were attributed to that surgical maneuver. The surgical staff was not told of the findings.
The researchers found that there were significantly more micro-emboli in patients who received coronary artery bypass surgery using the pump than those who had the off pump bypass procedure. Those placed on the pump had an average of 771 micro-emboli versus only 26 in patients undergoing off pump bypass surgery.
The researchers also found that major surgical events (aortic cannulation or termination of the bypass, for example) did not play a major role in micro-emboli production. Major surgical events only accounted for 15.5 percent of the micro-emboli, whereas almost 74 percent of micro-emboli were associated solely with the cardiopulmonary bypass pump. In the patients receiving off pump coronary bypass surgery, only 20 percent of the micro-emboli were associated with major surgical events. Almost 80 percent were random in occurrence and not associated with any known surgical maneuver.
With both procedures, the vast majority of micro-emboli were not associated with "surgical maneuvers, such as clamping and unclamping of the aorta," Lee says.
"Clinical stroke occurs in about 3 percent of patients undergoing coronary bypass surgery," says Lee. "But other neurological complications such as confusion, memory loss, and difficulty in thinking and cognition are more common."
Earlier studies have shown that half of all bypass patients have some cognitive decline at discharge, and at six-months a fourth still have some decline. At five years, 42 percent have significant cognitive decline.
Part of the problem is that with the advent of better coronary stenting and angioplasty, fewer patients are undergoing coronary bypass surgery in mid-life, Lee says. Now patients undergo bypass surgery only after they have exhausted all other treatments, he says. "An older population is now undergoing coronary bypass surgery and older individuals clearly are at a higher risk for serious neurological complications."
What Lee suspects is happening is that debris caused by years of atherosclerosis breaks off the aorta and goes to the brain. When a patient is placed on cardiopulmonary bypass the heart is temporarily stopped to allow the bypass grafts to be sewn on a stilled heart. To support the body's circulation, the blood is removed from the body, oxygen added, and then the blood is pumped back into the body through a tube placed into the aorta.
"When the blood is pumped back in, it can stir up debris within the aorta which can then travel to the brain." Presently, off pump bypass surgery is utilized in about 30 percent of patients nationwide, but that number is increasing steadily, he says.
"This is a small study and I can't recommend off pump bypass for everybody," he says. "It has to be based on the surgeon's experience. However, if the surgeon feels comfortable with off pump bypass surgery, then I believe it should be seriously considered in almost all patients undergoing heart bypass surgery."
Lee says the study may also provide hope for improved outcomes in other types of surgery within the heart such as valve replacement. In valve replacement, the heart must be stopped and the pump used, he says. However, knowledge gained from studies such as this may aid researchers in developing new designs for heart-lung pump and aortic cannulas that may help reduce the amount of debris going to the brain.
Co-authors are Collin R. Dang, M.D. and Hideko Yamauchi, M.D.
NR03-1073 (AsiaPacific/Lee)
Abstract 58