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Compression of intramyocardial arterioles during cardiac contraction is attenuated by accompanying venules
M A Vis1, P Sipkema, N Westerhof
1Laboratory for Physiology, Vrije Universiteit, Amsterdam, The Netherlands.
Insights
The unique arrangement of coronary vessels, forming a triad, protects arterioles from changes in area during cardiac contraction. This structure also reduces the increase in coronary resistance compared to unaccompanied vessels.
Area of Science:
- Cardiovascular Physiology
- Microcirculation Dynamics
- Biomechanical Modeling
Background:
- Cardiac contraction significantly impacts coronary blood flow.
- Understanding the mechanical interactions within the coronary vasculature is crucial for explaining blood flow regulation.
- Previous models often simplify the complex microvascular architecture.
Purpose of the Study:
- To investigate the influence of cardiac contraction on coronary arteriole and venule cross-sectional areas within a "triad" arrangement.
- To compare the mechanical effects on accompanied versus unaccompanied coronary vessels.
- To analyze the impact of vessel arrangement on overall coronary resistance during systole.
Main Methods:
- Computational modeling was used to simulate the effects of cardiac muscle shortening on vascular geometry.
- Calculations were performed for a triad of one arteriole and two venules, and for unaccompanied vessels.
- Vascular resistance was determined based on simulated changes in luminal area and pressure.
Main Results:
- During 14% cardiac contraction, arteriole area in a triad increased by 4%, while venule area decreased by 55%.
- Unaccompanied arterioles and venules experienced area reductions of 45% and 36%, respectively.
- Coronary resistance increased 1.4-fold in a triad but 2.8-fold for unaccompanied vessels.
Conclusions:
- The triad arrangement provides a protective effect, maintaining arteriole area and attenuating resistance increases during cardiac contraction.
- Extravascular pressure is dependent on vascular loading conditions, not an independent variable.
- This arrangement highlights a significant functional advantage in regulating coronary blood flow during the cardiac cycle.
Abstract:
It was calculated how cardiac contraction influences the luminal cross-sectional area of a maximally dilated coronary arteriole (37-micron inner diameter at a pressure of 35 mmHg) that is accompanied by two equal venules (45-micron inner diameter at a pressure of 17 mmHg), forming a so-called "triad." It was found that, during a contraction with 14% cardiac muscle shortening, arteriolar area is virtually unaffected (increase of 4%) at the expense of a large (55%) decrease in venular area. For comparison, the areas of an unaccompanied arteriole and an unaccompanied venule were calculated to be reduced by 45 and 36%, respectively, demonstrating the "protective effect" on accompanied arterioles in a triad. During contraction, the overall resistance of a system consisting of one arteriole in series with two parallel venules of equal length was calculated to increase about twice as much for nonaccompanied vessels (resistance increases by a factor of 2.8) than for vessels in a triad arrangement (resistance increased by a factor of 1.4). The calculations show that the extravascular (intramyocardial) pressure, which determines vascular area, is not an independent variable as in the intramyocardial pump and waterfall models but depends on the vascular "loading" conditions. Thus the small venular pressure together with the large venular compliance causes the extravascular pressure to remain low during contraction, thereby protecting the stiff arteriole at high pressure. We conclude that the triad arrangement of intramyocardial coronary vessels attenuates the increase in coronary resistance during cardiac contraction and thus has an important functional advantage.