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Every year in the United States, some 35,000 babies are born with congenital heart defects: holes or narrowings in the wrong places, misshapen chambers and valves too stiff or too weak to open and close properly. One in three cases requires surgery, and most of these operations necessitate stopping the heart and using
cardiopulmonary bypass, allowing
surgeons to work on a still, bloodless heart. It's the best way to see and reach structures inside the heart, the surest way to avoid accidentally jabbing delicate spots and the only way to work on fast-moving parts, such as valves.
But it's still risky enough to worry Pedro del Nido, MD, chief of Cardiac Surgery at Children's Hospital Boston. The child's immune system may react to the cardiopulmonary bypass machine, causing rare but dangerous complications such as blood clotting and inflammation in
different organs of the body. Although bypass techniques have become safer over the years, micro air bubbles may still enter through the open heart and travel to the brain, causing damage. And because the heart doesn't receive circulation
during surgery, sometimes for lengthy periods during complex repairs, it may
suffer damage and fail to beat properly once it's restarted.
For about a decade, Dr. del Nido,
who repairs about 250 children's hearts a year, has been exploring ways to avoid stopping the heart during surgery. Currently, beating-heart repairs are limited to atrial septal defects (ASD) and a small number of the simplest ventricular septal defects (VSD). (The majority of septal defects in children can't be repaired on beating hearts because existing repair devices are too bulky for newborns and don't fit many types of holes.) During the procedure, cardiologists, guided by an X-ray and 2-D ultrasound imaging, pass a catheter from the groin blood vessels into the heart and place a metal plug in the hole. Rare complications, like bleeding, clotting, infection and tears of the heart muscle, still occur, and in some cases, the plug can get dislodged. In addition to these risks, surgeons lack many of the catheter-compatible tools and clear 3-D real-time imaging of the heart's inner structures to perform more complex repairs.
Taking matters into their own hands, Dr. del Nido and Nikolay Vasilyev, MD, research associate in Cardiovascular Surgery, are inventing the tools and imaging techniques themselves, thanks to two, five-year, $5 million NIH grants and a partnership with engineers at MIT, Boston University, Harvard University, Philips Medical Systems and Microfabrica, Inc.
Their first step is
getting a better look inside the beating heart. The best imaging available is
real-time 3-D ultrasound, which shows all the
dimensions of the interior of the beating heart (as shown in image 1 at left, an ASD). While the images look realistic, in practice, they may actually disorient surgeons, who are used to looking inside the heart either on 2-D ultrasound or directly, when the heart is stopped and opened. When put to the test, surgeons patching septal defects in animals under this type of guidance anchored their patches askew (image 2 below, left panel). This is because traditional ultrasound machines render depth using grayscale shading, whereas humans see depth in stereo by looking at a different angle with each eye.
Tough repairs of the rapidly moving inner structures of the heart call for better depth perception, and to get it, Dr. del Nido is exploring the world of video games. Today's most realistic 3-D video games abandon grayscale shading, instead rendering depth by splitting images into right- and left-eye versions, cocked at slightly different angles. Dr. del Nido is applying the technique to ultrasound images (image 3 above right, right panel), using a computer to churn out new left- and right-eye images fast enough to keep up with the beating heart. When viewed through ’Äúblinking" glasses that allow only one eye to see parts of the heart at a time (Image 3, left panel), the two images merge, allowing surgeons to see the beating heart as a hologram. ’ÄúAs far as I know, we're the only team who can do that," says Dr. Vasilyev. ’ÄúBy applying the ultrasound probe on the patient's heart and putting on the
shutter glasses, you're entering the inner universe of the contracting heart muscle."
But surgeons still need special instruments that are sophisticated enough to precisely remove a piece of tissue or sew a suture, but small enough to fit through a child-sized catheter. Dr. del Nido and his collaborators have developed millimeter-sized, catheter-based tools: hole-patchers (image 4 below, top panel), tissue-nibblers (middle panel) and suture-clippers (bottom panel) that surgeons will be able to use to operate through catheters or small incisions, with the assistance of surgical robots. Dr. Vasilyev has successfully tried the patching device in animals, but the other tools remain to be tested.
Using a hole-patcher guided by stereo 3-D
ultrasound, Dr. Vasilyev has repaired an ASD in an animal's beating heart, securing the patch with evenly spaced anchors (image 2, right panel). These results were
as good as those from
open-heart surgery. Dr. del Nido's imaging system's first clinical trial will begin for beating-heart repair of ASDs in children next year.
The next frontier is repairing a moving mitral valve. While a faulty mitral valve is a common
congenital heart defect, no one has ever attempted a beating-heart repair in
children because the valves are delicate and flutter
faster than can be
perceived by the human eye. Dr. del Nido is already dreaming up instruments to do the job’Äîfor instance, a valve leaflet catcher that can, according to Del Nido, ’Äúspear the mitral valve like a harpoon and then fix the abnormal leaflet."
Dr. del Nido discusses the past and future of beating heart
surgery in adults and children: childrenshospital.org/beatingheart.
Dr. del Nido talks about how robots might be used to improve
beating-heart
surgery: childrenshospital.org/robots.
Dr. del Nido performs open-heart surgery on a patient with borderline left
ventricle: childrenshospital.org/heart.
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