when cardiac surgeons stop the heart during an operation, they sometimes hurt the organ they’re trying to help.
A new chemical regimen may help stop the damage caused by surgery-induced cardiac arrest. “Much of the success [of cardiac surgery today] is derived from the fact that we can stop the heart and make it bloodless, so during this time intricate operations can be performed,” John E. Foker of the University of Minnesota in Minneapolis said this week at the American Heart Association Science Writers Forum in Monterey, Calif. “But there’s a price to be paid.
” When the heart is cooled and stopped during an operation, its metabolism slows but does not come to a complete half. During this time, the concentration of its main energy source, the molecule adenosine triphosphate (ATP), is depleted. What Foker and his colleagues have done is find a quick way to restore ATP levels. In the process of providing energy, ATP loses one of its phosphates and becomes ADP, which in turn becomes AMP. The heart cells usually recycle AMP into ATP, but during heart-stopping surgery, enzymes destroy the AMP. The goal, then, is to resupply the heart with ATP. But injected ATP is broken down in the blood and doesn’t cross the cell membrane.
“So the problem,” says Foker, “is to regenerate the precursor.” He and his colleagues first tried adenosine, the backbone of the ATP molecule, but found that it cut blood flow to the kidney. They wne back one step farther, to adenine and ribose, the precursors of adenosine. In experiments on dogs, they found that administering these two chemicals as the heart was being restarted cut the time it took ATP levels to return to normal following surgery from 10 days to only a couple of days. The hearths physical recuperation paralleled the biochemical change.
The organ doesn’t immediately recover its ability to pump blood when it is turned back on; Foker and his colleagues found that it ordinarily takes 10 days to relax fully during each beat. Without fully relaxing, the heart doesn’t take in as much blood, and so it doesn’t pump as much blood. When adenine and ibose are given, the recovery of full heart function takes one to two days. After further refinement of the procedure, they expect to try it in humans. Says Foker, “I would hope that we could be under way with this in about six months’ time.”