August 16, 2013
News Review From Harvard Medical School -- Brief Pre-Surgery Blood Cutoff May Aid Heart
Cutting off blood supply to the arm, briefly, before heart surgery may improve survival by reducing injury to the heart. That's the conclusion of the first study to look at results of the technique after a year. The study included more than 300 people who were scheduled to have heart bypass surgery. They were randomly divided into 2 groups. People in the first group received remote ischemic preconditioning. Right after they received received anesthesia for surgery, a blood pressure cuff was applied to one upper arm. The cuff was inflated and kept in place for 5 minutes to restrict blood supply to the arm. Then it was removed for 5 minutes. This was done 3 times in a row before surgery. The other group did not have preconditioning. Three days after surgery, researchers measured troponin, a protein that indicates heart muscle damage. Blood levels of troponin were 17% lower in the group that had preconditioning. During the next year, people who had preconditioning were 73% less likely to die of any cause than people in the other group. They were 86% less likely to die of a heart attack or stroke. The journal Lancet published the study August 15.
By Mary Pickett, M.D.
Harvard Medical School
What Is the Doctor's Reaction?
"Squeeze me!" It's not something you'd naturally think of saying to your heart surgeon. But consider a fascinating new strategy that is being used to improve results after bypass operations.
The strategy is not one you would guess. Just before heart surgery, doctors inflate a blood pressure cuff on a patient's arm to a high pressure. They cut off circulation to the arm and hand for five minutes. Then they deflate the cuff and give the arm a few minutes to recover its feeling. They repeat the process for a total of three sessions. Researchers call this "remote ischemic preconditioning."
A study "preconditioned" 162 heart surgery patients in this manner. They were compared with patients who did not have the process before the surgery.
Something about the arm preparation caused the heart to be more resilient during the operation. After surgery, patients in the "conditioned" group had a 17% lower level of a protein called troponin in the blood. Doctors use the levels of troponin to measure heart muscle injury.
Remarkably, the patients in the preconditioned group had a 73% lower chance of dying within 4 years of the operation. They were 86% less likely to die of a heart attack or stroke in this time than patients without preconditioning.
This result delighted researchers. But it did not totally surprise them, because the results match earlier studies that were done in pigs.
How could this possibly work, you might ask? Squeeze the arm a few times, and make the heart more resilient? Researchers suspect that chemicals released from tissues that are oxygen starved must circulate through the heart. This must somehow put the heart tissue on alert, triggering the muscle cells to brace for some serious stress.
Doctors first began experimenting with the idea of preconditioning in the 1980s. Early studies didn't cut off blood flow to the arm. Instead, the cutoff was directly to the heart. In animals, briefly cutting off blood flow to the heart seemed to make the heart more resistant to later injury.
Cardiologists have adopted the idea. They frequently use conditioning before or after putting in a stent. For these procedures, the conditioning is done by inflating a miniature balloon several times in the artery where the cardiologist is working. Surgeons, too, have used preconditioning in some bypass operations. They have put a clamp on the largest blood vessel, the aorta, to briefly cut off blood flow to the heart. Doing this seems to help the heart withstand the stress of the operation.
Here are other examples of preconditioning. You should stop exercising if you get chest pain (angina). However, people who don't follow this advice sometimes report that their most bothersome pain happens when they first begin exercise. Then it becomes milder despite extra exercise effort. This is known as "warm-up angina." It is probably an example of heart preconditioning.
What Changes Can I Make Now?
Do not try this at home.
It is possible to create irreversible damage to an arm if you cut off its blood supply for long stretches.
The preconditioning protocol seems to provide its strongest -- perhaps only -- protection within the first hour after the series of blood pressure inflations. It is helpful only if you can anticipate it will be followed right away by a major stress on the heart, such as surgery or an artery procedure.
What Can I Expect Looking to the Future?
Thanks to this study, there will be great interest in preconditioning among surgeons. More studies will certainly be done. Cardiologists already use preconditioning. So far, though, they apply it at the site of the balloon used to open a blockage inside a coronary artery. It is likely that cardiologists also will decide to try remote (arm or leg) preconditioning as a lead-in to balloon angioplasty and stent placement.