Biomechanical strain keeps heart valves strong for decades

Collagen plays a critical role in keeping heart valves strong, but the heart is surrounded by enzymes designed to break down collagen. So how can heart valves function efficiently decade after decade? A new study shows that the biomechanical tension applied to these valves every time your heart beats plays a key role in protecting the collagen from breaking down.

"We know that collagen is critical for keeping heart valves strong, and that the leaflets in a heart valve open and close millions of times every year," says Hsiao-Ying Shadow Huang, first author of a paper on the work and an associate professor of mechanical and aerospace engineering at North Carolina State University. "We also know that the fluid surrounding the heart contains collagenases that are designed to break that collagen down. Yet those leaflets remain sturdy for decades."

"We also know that when people develop heart disease, the structural integrity of those leaflets can degrade very quickly. We wanted to explore why these things happen, and what role - if any - biomechanics might play in these processes."

Leaflets are the flaps of tissue in a heart valve that open to allow blood to be pumped in one direction, then shut to prevent that blood from flowing back.

For this study, the researchers conducted experiments using pig hearts. The researchers tested different levels of strain on heart valve leaflets, pulled the tissue in different directions, and varied the levels of collagenase in the fluid surrounding the hearts.

Hsiao-Ying Shadow Huang, associate professor of mechanical and aerospace engineering, North Carolina State UniversityWhen a healthy heart pumps, and the aorta valve leaflet is in the open position, the leaflet is stretched to 70% of its maximum capacity lengthwise, but to only 20% of its maximum capacity laterally. We tested the leaflets under those conditions, but also looked at other levels of strain that retained that same 70% to 20% ratio, such as 35% to 10%. We also tried changing the ratio so that the leaflets experienced the same amount of strain in both directions. And we conducted these experiments with both normal levels of collagenase and elevated levels of collagenase."

After each experiment, the researchers evaluated the microstructure of the leaflet tissue to see if the tissue was intact or had been damaged.

"We found that increasing the strain did not damage the collagen, even with higher levels of collagenases - as long as the ratio of the lengthwise strain and lateral strain stayed the same," says Huang. "When we changed the strain ratio, the collagen broke down - regardless of the collagenase levels."

In other words, stretching the leaflet in the same direction as the collagen-rich fibers serves as a biomechanical protective mechanism, as long as the naturally-occurring strain ratio is preserved.

"Previous studies have identified this protective mechanism in collagen-rich tissues in tendons, but that research only pulled the tissue in one direction," says Huang. "Heart valve leaflets experience strain in more than direction every time they open and close, so this finding is a step forward.

"Our findings also offer insights into why leaflets degrade so quickly when heart disease alters the amount of strain the leaflets are subjected to - they've lost that biomechanical defense mechanism," says Huang.

"These findings advance our understanding of how the heart works, how it fails, and how we can engineer new biomaterials for use in heart valve replacements."

The paper, "Strain-Stabilization of Collagen in Heart Valve Tissues: A Biomechanical Account of Proteolysis," is published in the Journal of Biomechanical Engineering. The paper was co-authored by Siyao Huang, a former Ph.D. student at NC State; and Kaitlyn Barbour, a former master's student at NC State.

Source:

North Carolina State University

Journal reference: