Cardiac contraction affects deep myocardial vessels predominantly

Area of Science:

• Cardiovascular Physiology
• Myocardial Perfusion Dynamics
• Vascular Biology

Background:

• Myocardial blood flow is crucial for cardiac function.
• Understanding regional differences in flow is vital for diagnosing and treating heart conditions.
• Intramyocardial forces and ventricular pressure are potential regulators of coronary blood flow.

Purpose of the Study:

• To investigate the distinct roles of intramyocardial forces and systolic ventricular pressure on myocardial blood flow in different cardiac layers.
• To analyze the impact of systolic arrest versus diastolic arrest on subendocardial and subepicardial flow.
• To examine the structural characteristics of intramyocardial vessels in relation to myocardial layers and cardiac states.

Main Methods:

• Rabbit hearts were excised and perfused.
• Myocardial flow was measured during induced systolic arrest (low Ca2+, Ba2+ perfusion) and diastolic arrest (pentobarbital, cardioplegia).
• Left ventricular pressures of 60 and 0 mmHg were maintained during systolic arrest.
• Intramyocardial vessel number and size were assessed post-perfusion fixation.

Main Results:

• Subendocardial flow was lower than subepicardial flow during systolic arrest, irrespective of left ventricular pressure.
• During diastolic arrest, subendocardial flow exceeded subepicardial flow.
• Terminal arteriole and capillary diameters were significantly smaller in the subendocardium compared to the subepicardium during systolic arrest.
• Vessel densities did not differ between layers, but diameters were altered by cardiac state.

Conclusions:

• Cardiac contraction, driven by intramyocardial forces, predominantly constricts subendocardial vessels.
• This constriction leads to reduced subendocardial blood flow relative to subepicardial flow during systole.
• Left ventricular pressure has a lesser impact on this flow disparity compared to the mechanical effects of contraction.