Operating inside a Beating Heart
Fixing the heart is hard. Certain procedures have to be performed on a stationary organ, so the heart is stopped and the patient put on a cardiopulmonary bypass machine. But stopping the heart increases the risk of brain damage. Now researchers at Harvard University and Children’s Hospital Boston are testing a robotic system that could help surgeons perform a common valve repair while the heart beats on. The system uses 3-D ultrasound images to predict and compensate for the motion of the heart so that the surgeon can work on a patient’s mitral valve as it moves.
“Some 50,000 people a year, in the U.S. alone, get mitral-valve surgery,” says Robert Howe, a professor of engineering at Harvard and a researcher on the project. “It is a pressing clinical concern.”
The goal of the procedure is to decrease the size of the valve. Traditionally, this is done by placing a stiff ring around the valve and suturing it in place by hand.
“We know how to repair valves. But what patients and doctors want is a more rapid recovery,” says Marc Gillinov, a cardiac surgeon at the Cleveland Clinic who was not involved in the research. It can take two or three months for a patient to recover from an open-heart procedure; if the heart didn’t have to be stopped, the recovery time could drop significantly. Performing the surgery on a beating heart would also give the surgeon instant feedback on the effectiveness of the procedure. “You’d know just as you do it whether the valve is working well,” Gillinov says.
Howe says that, moreover, a number of studies show that stopping the heart can result in long-term cognitive deficits, and that older or frail people in particular don’t always respond well to bypass machines. He hopes that his system will make heart surgery safer.
Unlike traditional mitral-valve repair, Howe’s procedure does not involve opening up the heart itself. Instead, a hollow needle is inserted into the organ. The needle is used to introduce small anchors into the heart and affix them to the tissue around the mitral valve. The anchors can then be pulled together by a suture wire to decrease the size of the valve opening. “The challenge here is that [to affix the anchors] we need to keep track of where the heart tissue is, as the heart moves continuously,” Howe says. Howe’s team opted to use 3-D ultrasound to visualize heart movement because with other imaging techniques, such as video, the internal structures of the organ would have been concealed by circulating blood.
Data from the 3-D ultrasound images is analyzed using special software written by the researchers. The software can predict where heart tissue will be approximately 70 to 100 milliseconds in the future, so the position of the tip of the handheld surgical tool can be adjusted accordingly. Sensors in the tool also detect whether it comes in contact with the tissue. “We can detect very quickly if things deviate greatly from what’s predicted and then pull back the [instrument] to get it out of the way,” Howe says.
After studying the motion of real hearts, the researchers developed a foam model to test whether their device increased the dexterity of a small group of surgeons asked to affix anchors to the foam in particular positions. Howe says that the surgeons’ performance was notably improved when they used the motion-compensation system. “Without it, there was a far higher failure rate, and the forces they applied were much higher as well,” he says. In a clinical setting, applying too much force to the valve could damage heart tissue. Howe says that the system allows surgeons to affix the anchors within one to two millimeters of their intended position, which is fine, given the pliancy of heart tissue.
“It is very promising research,” says Cenk Cavusoglu, an associate professor of electrical engineering and computer science at Case Western Reserve University. Cavusoglu is working on a similar system to allow surgeons to perform coronary-artery bypass surgery. While the procedure itself is quite different, the need for motion compensation is the same. Cavusoglu says that he is impressed by the simple design of the valve-repair tool and by the researchers’ results so far.
Shelten Yuen, a Harvard PhD student who worked on the motion-compensation system, says that preliminary animal trials have already begun. But there is still much work to be done to perfect the tool. “There’s a lot of interest on my part in terms of incorporating additional sensors, such as electrocardiograms and force sensors,” Yuen says.
Romuald Ginhoux, a medical-software systems analyst at Median Technologies, in France, agrees that additional sensors could make the system more accurate. Ginhoux was also impressed by the small size of the device, which is about as big as a soldering iron. Ginhoux says that back in 2003, he worked on similar robots for heart surgeries, but that they were “the size of a real arm.”
Yuen says that he hopes to make the device even smaller and lighter so that it will respond better to slight pressures, giving surgeons a better feel for the heart’s tissue.
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