Pulse pressure and fluid viscosity act in a synergistic fashion on the forces disrupting arteriosclerotic plaque in an experimental model – new insights into mechanisms underlying plaque disruption

The human heart is a relentless pump, sending blood surging through a vast network of arteries with every beat. For decades, doctors have known that the most dangerous events in heart disease—heart attacks and strokes—usually begin when a fatty deposit, known as a plaque, builds up inside an artery and then suddenly breaks open. When this happens, the body's clotting system rushes to the site, often blocking the vessel and cutting off blood flow to vital organs. While the presence of plaque is a well-known danger, the precise moment of rupture has remained a complex puzzle. It depends on a tug-of-war between the forces trying to hold the plaque in place and the forces trying to tear it loose. Two of the most significant forces at play are the pressure of the blood pushing against the artery wall and the thickness, or viscosity, of the blood itself. Just as thick honey flows differently than thin water, blood with higher viscosity creates different physical stresses on the vessel walls. Understanding how these factors interact is crucial, because if we can identify exactly when and why a plaque gives way, we might be able to prevent the catastrophic events that follow.

In a recent study, researchers set out to investigate how the thickness of the fluid flowing through an artery combines with the pressure of that flow to threaten the stability of a plaque. To do this, they built a controlled experiment using segments of aorta taken from pigs. These vessels were cleaned and prepared to mimic the human circulatory system. Inside each vessel, the scientists placed a small, artificial plaque made of epoxy resin. This fake plaque was designed to block about thirty percent of the tube's opening, similar to a moderate blockage found in a human patient. The setup allowed the team to apply a magnetic force to hold the plaque against the inner wall of the artery, simulating the natural glue that keeps real plaques attached. By carefully adjusting the strength of this magnetic hold, they could measure the exact amount of force required to make the plaque detach. This measurement, which the researchers call the critical detachment force, represents the point where the blood's push overcomes the plaque's grip.

The team then perfused these vessels with a series of liquid solutions. They started with fluids that had the same thickness as normal blood and gradually increased the viscosity, making the liquid thicker and thicker, up to a point where it was more than seven times as thick as normal. They tested these fluids under two different pressure conditions: one representing a normal blood pressure pulse and another representing a much higher, more stressful pulse. As they ran the fluids through the arteries, they observed a clear pattern. When the fluid was only slightly thicker than normal, the force needed to rip the plaque away did not change much. However, once the fluid thickness crossed a specific threshold, the situation changed dramatically. Above this point, the force required to detach the plaque increased significantly, indicating that the stress acting on the plaque had risen to a level where it was much harder for the plaque to remain attached.

The most striking discovery was that this effect did not happen in isolation. The researchers found that high fluid thickness and high blood pressure worked together in a synergistic fashion. When the fluid was thick and the pressure was high at the same time, the stress on the plaque was amplified, acting in a way that combined the effects of both factors to increase the load on the plaque. The study showed that once the fluid viscosity passed a certain limit, the stress on the plaque rose sharply, and this stress was amplified if the patient also had high pulse pressure. The researchers were careful to rule out other possibilities; they tested whether the increased weight of the fluid alone was the cause, but found that simply making the fluid denser without making it thicker did not change the results. This confirmed that the thickness of the fluid, not its weight, was the key driver of the increased stress.

These findings offer a theoretical explanation for observations made in large population studies, where people with thicker blood have been shown to have a higher risk of heart attacks. The study suggests that there may be a tipping point for blood thickness, roughly between 1.17 and 1.36 millipascal-seconds, beyond which the stress on the plaque increases rapidly. This threshold is significant because it is a level that can be reached in everyday life. A person might temporarily exceed this limit due to dehydration, an acute infection, or a heavy fatty meal. For a patient who already has high blood pressure or stiff arteries, crossing this threshold could be the final push that causes a stable plaque to rupture. However, the author cautions that due to the limitations of the experimental model, care should be taken before drawing firm clinical management conclusions. The research implies that managing cardiovascular risk requires looking beyond just cholesterol and blood pressure numbers to include the physical properties of the blood itself, but further investigation is needed to confirm these findings in patients.

Beyond the immediate physical forces, the paper proposes a deeper connection between these mechanical stresses and the body's biological response. The author suggests that the repeated strain of high pressure and thick blood acting on a plaque is similar to a form of repetitive injury. Just as a tendon can become inflamed and weakened from constant overuse, the interface between the plaque and the artery wall may suffer from a similar "repetition strain injury." This constant mechanical stress appears to trigger a local inflammatory response, where immune cells gather at the site of the strain and release enzymes that weaken the plaque's attachment. This biological process is linked to markers of inflammation found in the blood, such as C-reactive protein. The study posits that the physical forces do not just tear the plaque apart directly; they also set off a chain reaction that weakens the plaque from the inside, making it more likely to fail.

While the experiment used artificial materials and pig vessels, the results align with what is known about human heart disease. The study does not claim to have solved the problem of heart attacks, but it provides theoretical support for the idea that fluid viscosity is a potential independent risk factor that warrants further study. It suggests that for patients with vulnerable plaques, keeping blood pressure under control is not enough; they must also be aware of factors that might suddenly thicken their blood. The research opens the door to new ways of thinking about prevention, suggesting that measures to lower blood viscosity, such as hydration or specific medications, could be valuable tools for reducing risk in high-pressure situations, though whether reducing plasma viscosity in at-risk patients would actually reduce hs-CRP and cardiac risk remains to be examined. Ultimately, the work highlights that the heart is a system where physics and biology are inextricably linked, and that the simple act of blood flowing through a vessel carries a complex mechanical story that can determine life or death.