ROCHESTER, Minn. -- By using sound waves Mayo Clinic researchers have described subtle changes in the motion of the heart that are measurable by ultrasound and may improve understanding of heart function, and possibly be a noninvasive aid in predicting impending heart damage including heart attacks. The study could also contribute to optimal adjustment of cardiac pacemakers or perhaps better design of artificial hearts. The findings, published in the current Journal of Applied Physiology [JApplPhysiol.000191.2005], are based on "snapshots" of the mechanical transitions that occur between the main relaxation and contraction phases of the heartbeat. During these split-second transitions, the heart muscle "shifts gears" or prepares for the upcoming phase.
"This is only a start and much work is needed, but we are optimistic that our research will ultimately lead to development of noninvasive, broadly clinically available methods in diagnostic ultrasonography," says Marek Belohlavek, M.D., Ph.D., Mayo Clinic ultrasound imaging specialist and senior researcher of the study. "These methods could improve our chances in predicting cardiac events, so that preventive measures could be taken. And in patients with an existing heart condition, a detailed analysis of cardiac function could contribute to therapeutic optimization of heart performance." A patent application has been filed based on this research.
Researchers at the Mayo Clinic Translational Ultrasound Research Unit study the mechanical, biochemical and electrical aspects of these transitions which occur between phases of relaxation -- when the heart ventricles fill with a volume of blood -- and contraction -- when the heart ejects most of the blood volume into body circulation. Recently advanced, high-resolution ultrasound tissue Doppler imaging allowed them to experimentally measure these transitional tissue deformations, which last only milliseconds and are unnoticeable to the human eye. The technology allows slow-motion comparisons of these events separately between the inner and outer layers of the cardiac left ventricle. The researchers' published measurements demonstrate how a rapid succession of motions occurring within tissue of the ventricular wall can appear chaotic if not observed closely and with high temporal resolution. The data also show how these transitions "reorganize" the ventricle to best perform its cycles of filling and ejection.
Significance of the Findings
Alterations in the cardiac mechanical transitions detected by ultrasound imaging can be used as early indicators to predict heart problems, without the risk of an invasive procedure. Such an early warning system could allow physicians to intervene with appropriate therapies and thus prevent problems that could lead to heart attack or heart failure. The knowledge may also help researchers to develop new and targeted treatments in some heart diseases or further improve cardiac pacemakers or artificial hearts.
Animal Model of Heart Functioning
Until recently, it was thought to be sufficient to study the function of the heart muscle during the relaxation and ejection phases of the heartbeat. Now, technological improvements in imaging have allowed studies of the heart muscle condition during the transitional phases. These short-lived mechanical transitions are successfully accomplished and prepare the heart for the next beat optimally only if the mechanical, biochemical and electrical events in the cardiac muscle work in concert and delivery of nutrients and oxygen are uninterrupted. Understanding these rapid transitional events not only improves fundamental understanding of heart functioning, but their dependence on various conditions makes these events vulnerable. This vulnerability translates into early changes in the transitional events detected by the state-of-the-art diagnostic imaging methods.
Using pigs as a very close model to human heart function, researchers established benchmarks for measuring normal and abnormal transitions in heart muscle layers. Accurate analyses of motion, deformation (strain), electrical impulses and other parameters characterize the transitional events between the phases of cardiac filling and ejection.
Collaborations and Support
Besides Dr. Belohlavek, the research team included Partho Sengupta, M.D., first author, designer of the study and recipient of the Young Investigator Award from the American Society of Echocardiography for this work; Bijoy Khandheria, M.D.; Josef Korinek, M.D.; and Jianwen Wang, M.D. The work presented in this paper was supported by the National Institutes of Health, GE Healthcare and Mayo Clinic.
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