Understanding Biomechanical Changes in Heart Valves Following Pregnancy
The human body changes drastically during pregnancy. It changes in ways that are visible and can be felt, and it changes in imperceptible ways that, nonetheless, can have large impacts on the body. Some of these changes occur in the heart.
The specific changes in a pregnant woman’s heart—specifically in the heart’s valves—are the subject of research being conducted by Associate Professor Rouzbeh Amini, mechanical engineering, jointly appointed in bioengineering.
“Women’s bodies go through this amazing period during pregnancy,” Amini explains. “At some point in a pregnancy, there is a 40% increase in cardiac output—the heart has to deliver blood to a body that is almost 40% above what it used to be before pregnancy.”
This increased load produces a mechanical engineering problem that the body has to solve: keep blood pressure constant, while pumping a lot more blood.
“For someone like me, with a mechanical engineering background and training, the cardio-vascular system is somewhat like a plumbing system,” Amini describes. “The heart is the pump to get blood throughout the body. If you increase the amount of blood you need to deliver, that puts extra load on the heart. This leads to very interesting changes in a woman’s heart.”
“In a normal pregnancy, blood pressure remains constant,” he continues. “So the size of the openings of heart valves change—they actually become slightly bigger.”
The metamorphosis that allows heart valve openings to expand happens at a microscopic level. To understand what exactly these changes are, and how they happen, Amini was awarded a National Science Foundation CAREER Award in 2018 while at the University of Akron.
“What happens to the heart valve structure—from the point of the proteins that are in the underlying layer of the valve—that allows the body to maintain normal function while mechanical loads increase?”
Amini proposes that the valves somehow remodel themselves—changing their structure and protein structure—to bring back the microenvironment for the cells that they had before pregnancy.
“When you look at these tissues under a microscope, they look like these beautiful, interwoven layers of protein, it looks almost like a bowl of angel hair pasta,” he explains. “A cell in this network of proteins has to be anchored. Then once you start stretching this network, it’s like if there are more ropes, the load can be carried by the extra ropes. When you apply excessive mechanical loading on a tissue, they start developing these sort of fibrous proteins around them.”
Professor Amini uses the analogy of supporting ropes helping to hold up a growing tree. If you add more ropes, you can support more total weight to help keep the tree straight while it grows. But it’s not just the amount of proteins, it’s how they work. If a tree is constantly being blown by a North wind, adding extra support in an east-west direction isn’t going to help much. The same is true in heart valves.
“During pregnancy, the amount of structural proteins that exist in the heart valves increases,” he says. “But it’s not just the amount of proteins that increase, it’s the architecture: the fibers start organizing towards the direction of the stress to help carry the load.”
The long-term potential for this research includes improving maternal health outcomes, even years after pregnancy. For example, the stress pregnancy puts on the heart valves of certain groups of people, especially those who suffer from connective tissue disorders, can lead to the need for valvular surgeries. By understanding the stresses and changes valves undergo, better treatments may become possible in the future.
“There are a lot of diseases that affect women more than men, and some of them have a relationship to pregnancy or the hormonal changes that occur during pregnancy,” Amini concludes. “If we learn about how the human body responds and adapts during pregnancy, maybe there are other things we can learn about how biomechanical changes can increase the risk factors for certain diseases for women that may not even be as obvious as heart disease.”
Abstract Source: NSF
The four one-way valves of the heart open and close to prevent the backflow of blood, and they do this more than three billion times during an average person’s lifetime. During pregnancy, the workload of the heart increases, and the total volume of blood in the body rises by 40 percent to accommodate circulation in the placenta. Such an immense increase in cardiac workload has been shown to cause changes in the shape and structure of the heart valves. Currently, it is not clearly understood how changes in the mechanical loads on the heart valve during pregnancy lead to such modifications in shape and structure. It is known that the cells in heart valves (and in other soft tissues) try to maintain a stable equilibrium in their mechanical environment. Any deviations from the normal environment can cause cells to respond in an attempt to restore this balance. For example, cells may produce more proteins to build a stiffer and stronger surrounding structure in order to lower the amount of mechanical strain they experience. In this Faculty Early Career Development Program (CAREER) project, the PI combines experimental measurements and computer simulations to determine how changes in the heart’s output and mechanical loading affect the tricuspid valve (the least understood valve in the heart) at the cellular level. The project is expected to widely impact the field of biomechanics, society in general, and engineering education. Specifically, this research project is expected to add knowledge of how mechanical loading affects cells and their surrounding structure, which is useful for future studies of the heart valves and other soft tissue. It will also provide useful information about how valve dysfunctions are brought on by pulmonary hypertension, which is a major cause of tricuspid valve disease in the broader public as well as in pregnant women.