Ultrasound Imaging of Blood Flow Provides New Clues into Cardiac Abnormalities, Dysfunction

By concentrating on fluid dynamics--specifically, the researchers believe they can detect heart disease even when conventional diagnostics reveal no sign of abnormality.

In addition to improving diagnoses, this move in focus from muscle mechanics to fluid mechanics could lead to more effective therapeutic interventions. The study’s findings were published by two Mount Sinai School of Medicine (New York, NY, USA) cardiologists and a team of international collaborators in a recent issue of JACC Cardiovascular Imaging, a journal of the American College of Cardiology.

The ultrasound strategies cardiologists employ today frequently cannot identify changes in the heart until there is overt dysfunction. Blood flow imaging, however, may provide better clues in diagnosing heart failure. Sinai investigators reason that flow should be immediately affected by changes in cardiac function--such as those revealed in image analysis by the chaotic behavior of tiny whirlpools.

The computer-aided visual study of these abnormalities could dramatically improve the assessment of patients with heart failure and lead to a fresh understanding of normal and abnormal pumping and circulatory function. Visual blood-flow analysis could also yield improved therapies for arrhythmias and other disorders requiring cardiac synchronization. Researchers are actively exploring applications in aortic atherosclerosis, before and after valve replacement, and congenital abnormalities.

“With visualization, we are looking at the ultimate measure of the efficiency of the heart - how the blood is brought in and how it is sent out,” said Jagat Narula, MD, PhD, director of cardiovascular imaging at Mount Sinai and the senior author of the paper. “Today, cardiologists place great weight on a gauge called the squeeze fraction, or ejection fraction--the portion of blood pumped from the ventricle with each heartbeat. What we are doing is a complete departure from the view of the heart as a squeezing, pressure-generating chamber.”

Mount Sinai researchers and their collaborators have experimented with a range of imaging techniques to grasp the characteristics of normal and abnormal blood flow. The approaches include phase-encoded MRI, cardiac magnetic resonance (CMR) and several forms of ultrasound-based imaging known as echocardiographic particle imaging velocimetry.

“The most effective technique involves injecting a stream of bubbles that behave exactly like red blood cells and using echocardiography to track their path through the left ventricle,” said Partho Sengupta, MD, director of cardiac ultrasound research at Mount Sinai, and the first author, with Narula, of the JACC article. In these studies, the computer-enhanced video output portrays normal and turbulent flow in vivid detail, with arrows plotting the direction as the bubbles swirl through the heart chamber.

“Not only are you following the path of the blood, but you can actually identify the amount of energy that is being distributed,” said Dr. Sengupta. “Like other forms of ultrasound, that means low-cost heart tests using this technology could be performed on a simple outpatient basis.”

The echocardiography technology pioneered by Drs. Sengupta and Narula sheds light on diagnostic discrepancies that have puzzled cardiologists relying on pressure measurements. “After sustaining significant damage, a patient's heart may not have the greatest squeeze, but there could be good trafficking of blood through the heart and the patient could remain asymptomatic,” Dr. Sengupta explained. “The normal ejection fraction is around 60%, but we sometimes see a patient with 20% walking around and playing golf. Other people who are at 50% may be short of breath. Flow visualization is one way to capture the essence of why the patient is or is not symptomatic.”

Diagnosing cardiac disease by searching for structural defects in the heart is similar to analyzing highway traffic by examining the road, Dr. Narula said. “The structure may not be great, but how does that affect the cars that are actually traveling on the road? It’s the same thing if you fail to look at the blood.”

Similarly, a plumber’s search into pipes only matters or makes sense in relation to how the water flows, noted Dr. Sengupta. A new study these investigators hones in on specific correlations between blood flow and cardiac pathology. “We will be able to demonstrate that efficiency may be lost even though the structure is maintained,” said Dr. Senguptra. “In other words, the façade is good, but inside, you have lost it.”

Dr. Sengupta stressed that the combined visualization and computation techniques in the study are still new and require additional study, including development of appropriate flow-based indexes for applications in various cardiac disorders. Forces acting on flow are extremely complicated and dynamic, according to the researchers. Pumped by the heart at a rate of 3,785-7,571 cm³ per minute, blood interacts with the contours of the valves, myocardium, vessels, and other areas, which are also in motion. The flow is multidirectional--curling, spinning, and forming eddies that are affected in innumerable ways by structural changes in heart tissue. As with any new observational techniques, data from novel cardiac visualizations in complex environments are subject to interpretation.

“We have started using these imaging techniques in clinical trials,” Dr. Narula said. “They will require careful evaluation.”

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