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Role of capillaries in determining CBF reserve: new insights using myocardial contrast echocardiography
A R Jayaweera1, K Wei, M Coggins
1Cardiovascular Division, University of Virginia School of Medicine, Charlottesville, Virginia 22908, USA.
Insights
Capillaries significantly regulate coronary blood flow (CBF) reserve, especially during hyperemia and stenosis. Their resistance increases substantially, contrary to prior beliefs, playing a key role in CBF control.
Area of Science:
- Cardiovascular Physiology
- Hemodynamics
Background:
- The role of capillaries in regulating coronary blood flow (CBF) reserve is not fully understood.
- Previous notions suggested limited capillary contribution to myocardial vascular resistance.
Purpose of the Study:
- To define the role of capillaries in the control of coronary blood flow (CBF) reserve.
- To evaluate experimental data within a developed coronary circulation model.
Main Methods:
- Developed a three-compartment (arterial, capillary, venous) model of coronary circulation.
- Derived vascular resistance from hemodynamic data in dogs during rest, hyperemia, and stenosis.
- Assumed constant capillary hydrostatic pressure.
Main Results:
- Capillary resistance increased from 25% at rest to 75% during hyperemia in healthy conditions.
- With noncritical stenosis, capillary resistance became predominant (84%) during hyperemia.
- Myocardial video intensity (MCE) decreased distal to stenosis, indicating increased capillary resistance.
Conclusions:
- Capillaries play a vital and often predominant role in regulating coronary blood flow (CBF) reserve.
- Capillary resistance is a key factor in CBF regulation, particularly under conditions of stenosis and hyperemia.
- Findings offer an alternative explanation for critical coronary closing pressure.
Abstract:
To define the role of capillaries in the control of coronary blood flow (CBF) reserve, we developed a model of the coronary circulation and evaluated experimental data in its context. Our model comprised three compartments connected in series (arterial, capillary, and venous), each with its own resistance. The resistance in each vascular compartment was derived from the model based on hemodynamic data obtained in nine dogs during baseline and stenosis, both at rest and during hyperemia. The capillary hydrostatic pressure was assumed to be constant in all stages. Although in the absence of stenosis, the contribution of capillaries to total myocardial vascular resistance was only 25 +/- 5% at rest, it increased to 75 +/- 14% during hyperemia, despite the total myocardial vascular resistance decreasing by 51 +/- 13%. In the presence of a noncritical stenosis, total myocardial vascular resistance decreased by 22 +/- 10% at rest, with no change in capillary resistance. During hyperemia, total myocardial vascular resistance increased by 58 +/- 50% in the presence of the noncritical stenosis. In this situation, because arteriolar and venular resistances were already minimal, the increase in myocardial vascular resistance was due to increased capillary resistance, making it the predominant source (84 +/- 8%) of total myocardial vascular resistance. Myocardial video intensity (VI) on myocardial contrast echocardiography (MCE), which reflects capillary blood volume, decreased distal to the stenosis during hyperemia. In the presence of a flow-limiting stenosis at rest, myocardial VI also decreased, indicating that decrease in CBF was associated with an increase in capillary resistance. Our findings also provide an alternative explanation for the critical coronary closing pressure. Thus, contrary to previously held notions, capillaries play a vital role in the regulation of CBF.