Mending broken hearts

In experiments with dogs,

researchers successfully used a 3D map of the heart and sensor-guided catheter to perform cardiac ablation, a mainstay treatment that stops abnormally fast and potentially fatal heartbeats, or arrhythmias.

The Johns Hopkins findings were presented at the American Heart Association's Scientific Sessions 2005 on November 13 in Dallas, Texas.

The Johns Hopkins team created their own 3D models of each heart from images obtained by integrating and superimposing CT and MRI scans. Using an upgraded computer software program known as electro-anatomic mapping, the scientists are able to colour-code the heart models' structures.

The scientists safely ablated, or destroyed, tiny areas of diseased heart tissue that facilitate rhythm disturbances, guided only by these anatomically precise, reconstructed 3D map.

During the procedure, a catheter containing a magnetic sensor in its tip is inserted through a vein in the dog's leg, then guided to the heart, where it is used to burn off the small part of heart muscle that gives rise to the errant signalling responsible for the arrhythmia.

A magnetic location pad was placed under the operating table and situated directly beneath the animal's body to detect the position of the catheter and compare it to the computer-generated image of the heart displayed on a screen.

"This is a significant improvement for patient safety and the performance of minimally invasive procedures in the heart," says study senior investigator and cardiologist Timm Dickfeld, M.D., Ph.D., an adjunct assistant professor at The Johns Hopkins University School of Medicine and its Heart Institute. Dickfeld points out that current methods for visualising the heart, using X-rays and fluoroscopy, are potentially hazardous because they involve radiation and produce only two-dimensional images, which are not as accurate as 3D images.

"Our study proves the effectiveness of 3D mapping in guiding and pinpointing the catheter's tip during a procedure, giving the electrophysiologist a more concrete awareness on the placement of lesions, and it eliminates the need for an X-ray, reducing the amount of radiation exposure for the patient," he adds.

As part of the study, the Johns Hopkins scientists performed catheter ablation in nine dogs who had nearly 50 CT markers, small wafer-like targets that could be seen by a scanner, surgically implanted on the outside of their hearts' muscle wall. The placement of the wafers covered all regions of the heart, especially those where arrhythmia are known to occur. Three-dimensional maps of the hearts, made shortly before the procedure, were then used to guide the catheter to their targets.

After the procedure, analysis of heart tissue showed that the 3D maps worked very well and were extremely precise and safe, Dickfeld says. The range of errors in pinpointing therapy to a lesion was between 1.9 millimetres and 4.9 millimetres, and well within lesion boundaries that average 6 millimetres in diameter.

"A physician really needs precise anatomical information during ablation because one move in the wrong direction could damage healthy heart tissue or puncture the organ," commented study lead investigator Jun Dong, M.D., a postdoctoral research fellow at Johns Hopkins.