More than five million Americans are diagnosed with heart valve disease each year, and for some of these patients, a heart valve replacement may be an inevitable option. Mechanical valves have proven to be lifesaving for many. However, due to their design, they fall short in their ability to replicate physiological blood flow, directly impacting patients' health.
To further improve the health outcomes of patients with artificial heart valves, Dr. Iman Borazjani, associate professor in the J. Mike Walker '66 Department of Mechanical Engineering at Texas A&M University, uses his understanding of fluid-solid surface interactions to simulate blood flow through mechanical valves.
As their name suggests, heart valves are flaps of tissue that are attached to ventricles and allow unidirectional flow of blood from one side of the heart to the other. If a valve becomes impaired due to disease or aging, it can be replaced with a mechanical valve. However, research shows mechanical valves currently cannot exactly replicate a natural heart valve's movement. In particular, they create unnatural blood flows, thereby increasing the patient’s susceptibility to clotting.
“Mechanical valves can replace damaged valves and can restore unidirectional blood flow, but we are now finding that the flow patterns are not entirely physiological,” said Borazjani. “This nonphysiological flow triggers an immune reaction, causing clotting. And so, these patients remain on blood thinners for the duration of their lives.”
To determine the causes of unnatural blood flow, Borazjani focused on the design of the mechanical valves that are made of polycarbonate, a material known for its rigidity and toughness. Although durable, polycarbonate is not flexible. Unlike a natural valve that can bend or bulge out of the way by the pressure coming from blood flow, the leaflets of mechanical valves cannot distort their shape and stay in the middle of the flow. The presence of the leaflets in the middle of the orifice, when the valve is open, causes the blood to form swirls, called eddies.
“Picture a rock in a river. As the water flows past the rock, ripples form around the rock,” Borazjani said. “The eddies created by the leaflets of mechanical valves are conceptually similar, they hinder the streamline motion of blood.”
Borazjani’s research reveals that the eddies are a result of the design. Mechanical valves that are currently on the market have two leaflets, but bioprosthetic (tissue) valves have three leaflets. The presence of the rigid leaflets in the middle of the flow creates eddies even when they are fully open. However, the leaflets of bioprosthetic valves bend out of the way when fully open and do not produce significant eddies.
Looking ahead, Borazjani hopes their findings lead to new mechanical valve designs that provide not just symptomatic relief to patients suffering from heart valve disease but cuts their dependence on blood thinners over time. He is working with a heart valve company, Novostia, SA, whose new valve design might achieve this.
“We are still in the early stages of research and development, and there is still a lot of work to be done before we can introduce new valve designs to patients,” said Borazjani. “Having said that, my team has leveraged its knowledge in fluid-structure interaction and computational fluid dynamics to come a step closer to elucidating the mechanics that guide cardiovascular flow in our body.”