Heart’s hydraulics proven for the first time

The question of what role – if any – hydraulics played in filling the heart’s ventricle chambers has long fascinated lead author Elira Maksuti, an engineer who recently earned a joint PhD from KTH Royal Institute of Technology and Karolinska Institute in Stockholm.

She says the study, which was published by an international research team offers a better understanding of diastolic function, and a step forward for research into diagnosing and treating heart problems, as well as development of new medical devices.

The central feature of this overlooked function is the mitral valve plane, a formation that acts as a piston-like barrier between the atrium, which receives the blood, and the ventricle, which sends it back out of the heart. The team measured the hydraulic force that helps move blood through this valve and concluded that it accounts for one-third of the peak diastolic force in the left ventricle of a healthy heart.

“This is the first time this force has been measured in vivo,” says Maksuti, who works in the Department of Medical Imaging at KTH. The team includes research physicians at Karolinska Institute and Lund University in Sweden, as well as at Washington University in St. Louis, USA.

Maksuti says a spring-like cardiac muscle protein, known as titin, has been widely thought to be largely – if not totally – responsible for diastole. “The spring-mechanism does not do all the work,” she says. “It initiates the process and the hydraulic force completes it.”

The mitral valve plane, and the two unequal-sized chambers it connects, functions according to something called Pascal’s principle – the law behind all hydraulics. The principle explains why if you squeeze a water-filled balloon near one end, the liquid will rush into the larger part.

The water pressure is equal throughout, but the chambers have different sizes. Squeeze it in the middle and nothing happens because the area within each section is equal, and hydraulic force is defined as pressure multiplied by area. The researchers built a model to demonstrate this simple principle in the infinitely more complex system of the heart.

“The model is filled with water and has a small and a large chamber, like the heart,” she says. “These chambers are connected by a piston with a small tunnel in it, so that water can easily flow between them, and the pressure is the same in each part.

“When we move the piston toward the large chamber and then release, it comes back towards the small chamber and the water flows into the large one,” she says. If the two chambers were the same size, the piston would not move because the forces would be in balance.

Maksuti says her inquiry began when she was doing her master’s thesis in cardiac mechanics. “Everyone can see that the heart has this shape and a lot of people know the physics of hydraulics. Some had also suggested that this mechanism could be present in the heart; but no one has measured the force and asked whether its magnitude is important.”

With researchers Martin Ugander at Karolinska Institute and Michael Broomé, who is employed at both KTH and KI, she used magnetic resonance imaging (MRI) to make in vivo measurements of the moving heart in a group of healthy people. “Before this study, we developed a simulator in the computer and we wanted to see if the mechanism was present also in the real heart,” she says.

The study focused solely on the left atrium and ventricle, which takes oxygenated blood from the lungs and redistributes it to the rest of the body. The hydraulic force is likely to work on the right side of the heart, although the pressure level is different.

The right ventricle doesn’t need to generate as much power as the left does, since it only sends blood a short distance to the lungs and this circulation has a low resistance, she says. As a hydraulic system, the sizes of the atrium and ventricle then have an optimum ratio for perfect diastolic function, Maksuti says.

“In patients with heart failure, the atrium might be the same size, or even larger than the ventricle,” she says. “When diastolic dysfunction is diagnosed, clinicians are only looking at the size of the atrium now. But this mechanism suggests that what is important is the size of the atrium in comparison to the size of the ventricle.”