In the past, physicians would typically write off such thinned heart tissue as unsalvageable and would not pursue revascularization therapy with coronary artery bypass surgery or angioplasty. However, the Duke researchers said that the cardiac MRI allows them to visualize the beating heart with a precision and specificity that conventional methods cannot match.
Although their study was based on a small sample of heart patients seen at Duke University and Northwestern University, Chicago, the researchers believe that as cardiac MRI is used more routinely in the imaging of the heart, it could become the new "gold standard" for determining heart muscle that while damaged, is still viable given appropriate treatment.
The results of the Duke study were presented today (Nov. 12, 2003) at the 76th annual scientific session of the American Heart Association, by cardiologist Dipan Shah, M.D., consulting assistant professor of medicine at Duke.
"Most cardiologists are beginning to recognize that MRI is becoming the gold standard for viability testing, because with its extremely high spatial resolution, it can detect details not seen before," Shah said. "The ability to differentiate between living and dead cells makes MRI a more direct measure of tissue viability than any other method."
During a cardiac MRI examination, which is non-invasive and radiation-free, a patient is guided through the cavity of a large doughnut-shaped magnet. The magnet causes hydrogen nuclei in cells to align, and when perturbed by radio waves, they give off characteristic signals, which are then converted by computers into three-dimensional images of the heart and its structures. While MRI technology itself is 20 years old, only in the past few years has technology improved to the point where accurate images of moving tissues can be taken.
"Not only have we found thinned areas that could be saved by revascularization, we have learned some new concepts that will allow us to predict which of the thinned areas could be saved," said cardiologist Raymond Kim, M.D., co-director of the Duke Cardiovascular Magnetic Resonance Center and member of the research team. "It appears to depend on the ratio of viable cells to scar tissue in the thinned area and not just the absolute amount of viable cells.
"This is important since most other viability tests measure only the amount of viable tissue and not the amount of scar," Kim continued. "Thus, thinned areas will generally be interpreted as dead because the amount of viable tissue is small by definition."
When a portion of heart muscle is deprived of blood flow, as in a heart attack, those muscle cells are deprived of needed oxygen and nutrients. As the muscle cells in the walls of the heart die, they are replaced by collagen, which makes the walls thinner and less effective in pumping.
For their study, the Duke team evaluated 30 patients with chronic coronary artery disease who had large areas of thinned myocardium, or heart muscle. Each patient received an initial MRI scan to determine myocardial viability, and then another after having a revascularization procedure.
Patients were then divided into two groups -- 19 who had less than 50 percent viability before revascularization, and 11 who had greater than 50 percent.
"After revascularization, the patients in the first group demonstrated minimal functional improvement in the thinned region, while the second group had significant improvement not only in contractile function but also wall thickness," Shah said.
Specifically, the group with greater than 50 percent alive-to-dead cells experienced an increase in diastolic wall thickness from 4.4 millimeters to 7.4 millimeters, while the systolic wall thickening increased from 4 percent to 32 percent. Systole is that part of the heart's pumping cycle when it contracts and pushes blood out to the body, while the diastole is when the heart relaxes and fills with blood in preparation for the next heart beat.
"Prior studies have suggested that myocardium which is thinned to less than 5.5 millimeters is not viable and cannot improve in contractile functioning even after revascularization," Shah said. "However, our study has shown that there are some patients with severely thinned myocardium who can be helped by revascularization."
Kim said the larger studies need to be conducted to see how often patients with thinned muscle actually have tissue that can be saved, and what impact revascularization can have on that tissue.
"But we have already proven that we can detect alive but injured tissue and bring it back with revascularization," Kim added.
Other members of the team were from Duke Robert Judd, Ph.D., Michael Elliott, M.D., Igor Klem, M.D., Louise Thomson, M.D., and Michele Parker. From Northwestern, team members were Robert Bonow, M.D., and Edwin Wu, M.D. Ham Kim, M.D., Cornell University Medical Center, also participated in the study.