Electromechanical Drivers of Diastolic Dysfunction During Hypertrophic Cardiomyopathy Progression

The human heart is a tireless pump, but its job involves more than just squeezing blood out; it must also relax completely to let fresh blood flow back in. This relaxation phase, known as diastole, is just as critical as the squeeze. When the heart muscle becomes stiff or fails to relax properly, the chamber cannot fill with enough blood, even if the squeeze itself remains strong. This condition, called diastolic dysfunction, is a hallmark of hypertrophic cardiomyopathy, a common inherited heart disease where the heart muscle thickens. For decades, doctors have relied on a single number, the ejection fraction, to judge how well the heart is working. This number measures the percentage of blood pumped out with each beat. However, a patient can have a perfectly normal ejection fraction while their heart is struggling to fill, a situation that often leaves the true severity of the disease hidden from standard tests. Understanding exactly why the heart fails to fill, and how different biological changes contribute to this failure, remains a major challenge for researchers trying to improve care for these patients.

To untangle these complex causes, a team of researchers built a detailed digital twin of the human heart. Instead of studying patients directly, they created a sophisticated computer simulation that mimics the electrical signals, the chemical movements inside cells, and the physical squeezing of the heart muscle. They focused specifically on a form of the disease driven by a mutation in a gene called MYH7, which is responsible for producing a key protein in the heart's muscle fibers. The researchers did not just look at the final, sick heart; they constructed the disease in stages, adding one problem at a time to see how each piece changed the whole. They began with a healthy heart model, then introduced the genetic mutation, then added the thickening of the heart wall, and finally increased the stiffness of the muscle tissue. At each step, they also tested what happened when the cells' electrical systems changed, a common secondary effect of the disease.

The simulations revealed a surprising sequence of events that challenges the traditional view of how this disease progresses. The very first sign of trouble appeared even before the heart muscle began to thicken. The genetic mutation alone caused the heart to become hyperactive, squeezing harder than normal but filling with less blood. This created a small, tight chamber that pumped efficiently but held very little volume. As the disease progressed and the heart wall thickened, this constraint worsened, further limiting the space available for blood to enter. When the researchers added increased stiffness to the muscle, the filling problem became even more severe, and the heart's ability to relax slowed down significantly. Crucially, throughout all these stages, the ejection fraction remained high and looked normal. The heart was still pumping out a healthy percentage of the blood it had, but because it started with so little blood to begin with, the total amount of blood moving through the body was actually dropping.

The study also showed that changes in the heart's electrical system played a major role in the filling problem. When the researchers simulated the secondary electrical changes that often occur in this disease, the heart's relaxation slowed down even more. The muscle held onto its tension for too long, making it difficult for the chamber to open up and accept new blood. This electrical issue reduced the volume of blood the heart could hold by a measurable amount, comparable to the reduction caused by the physical thickening of the muscle. The simulations demonstrated that these electrical changes prolonged the time it took for the heart to reset between beats, effectively shortening the window available for filling.

Perhaps the most significant finding was that the standard measure of heart health, the ejection fraction, failed to capture the growing damage. Even as the heart's ability to fill deteriorated and the total volume of blood it moved decreased, the percentage of blood pumped out remained in the normal range. This suggests that relying solely on this number could lead doctors to underestimate how sick a patient is. The researchers found that looking at the heart's shape and how different parts of the muscle stretch and shorten provided a much clearer picture. By examining specific regions, particularly the thickened wall, they could see that the muscle was struggling to relax and was becoming increasingly uneven in its movement. These regional signs of distress appeared long before the overall pumping power looked abnormal.

The work suggests that the disease is not caused by a single factor but is the result of several distinct problems working together. The genetic mutation starts the process by making the muscle contract too tightly. The physical thickening of the wall then squeezes the chamber smaller. The increased stiffness of the tissue makes it harder to stretch open. Finally, the electrical changes slow down the relaxation process. Each of these factors adds to the difficulty of filling the heart, yet they can all happen while the heart still appears to be pumping strongly. The researchers propose that to truly understand the burden of this disease, doctors need to look beyond the simple pumping percentage. They suggest that combining electrical readings, pressure measurements, and detailed maps of how the muscle stretches could reveal the hidden severity of the condition. This approach could help identify patients who need treatment earlier, before their heart function visibly declines, by detecting the specific mechanical and electrical signs that precede the obvious failure of the pump.