Investigators simulated blood flow in a sophisticated computational model of the internal carotid artery (ICA) at three stages of stenosis ― without blockage, with mild (30%) blockage, and with moderate (50%) blockage. They compared the effect of exercise-induced elevated heart rate (140 bpm) to that of two resting heart rates (67 and 100 bpm).
As expected, intense exercise was beneficial for the patients with healthy arteries and for those with mild stenosis, but it showed potential for adversely affecting arteries among patients with moderate or higher levels of stenosis because it increased the arterial wall shear stress (WSS) at the stenosis zone.
"The behavior of arterial WSS is substantially influenced by the local hemodynamics, which is directly related to the development of atherosclerosis," the authors write.
"Disturbed flow in the post-stenotic region further promotes arterial dysfunction by activating various pro-atherosclerotic and pro-inflammatory genes in the endothelial cells," they add.
Arterial locations that are subjected to low or oscillatory WSS "behave as atherogenic sites." Two hemodynamic parameters — time-average WSS and oscillatory shear index (OSI) — are used extensively to "quantify the low and fluctuating behavior of WSS in an arterial flow.
"Stenosis in the internal carotid arteries can disrupt the blood supply to the brain and may lead to ischemic stroke," Roy said.
Heart rate increases during exercise, which brings stress to the arteries. Previous research shows that in healthy arteries without atherosclerosis or in arteries that are only mildly affected, the change in arterial stress levels produced by exercise is beneficial, he explained. However, many clinicians are confused about prescribing exercise regimens for patients who have a history of stroke or arterial blockage, said Roy.
"We have noted reports of sudden vascular strokes in individuals undergoing heavy exercise. We have noted that the flow behavior in arteries with stenotic blockage at high heart rate is not yet well explored."
For the study, the researchers observed the flow features and wall stress levels in a human carotid model at heart rates that correspond to those when at rest and under exercise conditions.
They used a computational model that enabled them to "solve the mass and momentum equation of blood flow in the virtual arterial domain," said Roy. This produced a "physics-based prediction of what might happen in human artery under such conditions."
Prescribe Exercise Carefully
The researchers found that strong recirculation, secondary velocity, and OSI zone develop inside a severely stenosed carotid artery at normal heart rate.
They observed that higher heart rates may improve arterial health by reducing OSI, but only in the absence of stenosis or in cases of mild stenosis.
By contrast, the increased heart rates (>120 bpm) worsened the arterial health of the severely stenosed arteries (≥50% blockage) by causing the onset of flow instabilities, which enhance the spread and severity of the recirculation zone and the magnitude of secondary velocity.
This leads to a drop in pressure across the stenosis and to the spread of the high OSI (≥0.2) zone downstream. Moreover, in cases of severe stenosis, the WSS at the stenosis throat rises significantly, which can contribute to plaque rupture and thrombus development.
Although the plaque rupture phenomenon itself was not studied, "the stress field that came through our prediction for stenosed artery at 120 bpm suggests that the risk of plaque rupture is high," Roy noted.
This ruptured plaque may then go directly to the brain through the ICA and block the blood supply, causing ischemic stroke. Apart from that, intense exercise also increases the risk of another stenosis formation in the poststenotic zone by "spreading the region exposed to high oscillating WSS," he said.
Roy hopes that the "detailed understanding of the flow features and stress distribution in stenosed carotid arteries at different heart rates may help clinicians to prescribe better exercise schedules for patients."
He and his colleagues "recommend checking arterial health regularly for the people who are doing intense workouts in gyms, or any other intense exercise."
Exercise regimens should be "carefully prescribed" for people with moderate or severe stenosis or who have a history of strokes "in light of the risk factors that emerged in the study," he advised.
Commenting for Medscape Medical News, Seemant Chaturvedi, MD, Stewart Greenebaum Endowed Professor of Neurology and director of the Stroke Program at the University of Maryland School of Medicine, noted that it is known that WSS increases in narrowed (stenotic) carotid arteries with increased heart rate. However, the observation regarding "variance with differing heart rates are novel," he added.
At this stage, "there may not be a clear actionable item for health care providers. Sometimes, increased heart rate is beneficial," continued Chaturvedi, who was not involved with the study.
"For example, exercise boosts the heart rate and is recommended for preventing vascular disease. It is possible that patients with extreme narrowing may want to avoid very fast heart rates, but this should be an individual decision in consultation with a physician."