Is it possible to accurately measure the intrinsic filling function of the heart? School of Medicine scientists have found the answer to that 50-year-old question.
Sound esoteric? Consider that about half of people with heart failure have problems related to how well the heart fills with blood during the relaxation phase — referred to as diastole.
Furthermore, these problems often develop earlier than problems with the contraction phase of the heartbeat — called systole. And consider that a person can have normal systole and yet have abnormal diastole. That fact, coupled with the lack of a reliable way to measure intrinsic filling function, has caused abnormalities of the filling process to be incompletely recognized.
"Only in the last decade have physicians really become aware of the importance of the diastolic process and have come to recognize the syndrome of diastolic heart failure," said senior author Sándor J. Kovács, M.D., Ph.D., associate professor of medicine, of cell biology and physiology and of biomedical engineering and adjunct associate professor of physics.
"When heart muscle loses its normal ability to simultaneously relax and spring back after contracting, it fails to move properly during filling. This causes blood to start backing up into the lungs with the patient developing life-threatening pulmonary edema (fluid in the lungs) and related symptoms," he continued.
Until this discovery of a method for reliably measuring intrinsic diastolic (filling) function, cardiologists couldn't get a truly accurate read of the heart's ability to fill because filling is affected by factors such as blood pressure, blood volume, body movements and posture. For about 50 years, researchers tried and failed to find a method that was independent of these factors. That failure meant that diastolic dysfunction — particularly in its early stages — could be overlooked, even with a thorough physical examination.
Kovács and Leo Shmuylovich, an M.D./Ph.D. student in physics in Arts & Sciences, both members of the Cardiovascular Biophysics Laboratory, developed their method for measuring intrinsic diastolic function by mathematically analyzing echocardiograms of the heart. Their method is described in the July issue of the Journal of Applied Physiology.
Echocardiograph machines obtain images of the heart using echoes from ultrasonic pulses. The machines also measure the velocity of the blood flow into and out of the heart's chambers as the heart relaxes and contracts. This velocity measurement appears on the instrument's screen as an image that takes on a wavelike shape (the velocity wave) with each heartbeat. The trough of the wave corresponds to the slowing of the blood flow, and the peak of the wave corresponds to speeding up of the blood flow.
"The key to our method is that it's a mathematical approach for analyzing the echocardiogram's velocity wave," Shmuylovich said. "Cardiologists are used to looking at these waves and recognizing certain shapes that tend to be associated with normal or abnormal filling function. But our method eliminates the need for these subjective assessments of echocardiograms."
Kovács and Shmuylovich used a combination of physics and physiology to develop mathematical parameters that describe velocity wave during diastole. They showed that by obtaining the heart's velocity wave with the patient in two different positions, such as lying down and sitting up, they could use their mathematical toolbox to derive a number that was independent of the patient's position.
This number is "load independent" because it isn't affected by the amount of blood reaching the heart. Termed "load" by cardiologists, the amount of blood reaching the heart previously has interfered with the accurate measurement of intrinsic diastolic function. Load is affected by blood pressure and volume and by bodily position.
"We've shown what no one could before — that a load-independent index of diastolic function exists and that it has certain values in normal hearts and different values in abnormal hearts," Kovács said. "Our contribution will allow physicians to measure the filling process in a way that reflects the intrinsic capability of the heart to fill itself with blood — and this is a reflection of the relaxation and recoil properties of the heart muscle."
The technique is particularly useful when two measurements are taken of the same patient at different times. Comparison of the two measurements can assess the presence of diastolic dysfunction and monitor the effect of medications on the filling process, the authors said.
Next the researchers will publish an article that describes how to approximate their mathematical formulation using simpler measurements of the velocity waves. But by using the descriptions in this current report, mathematically savvy physicians already can obtain these numbers from their patients' echocardiograms.
"Additionally, this is an analytical method that can be incorporated into software built into echocardiograph machines," Shmuylovich said. "That way the analysis of the data can be streamlined, and the physician just needs to read the output value of the load-independent index of diastolic function."