Factors determining delayed peak flow in canine myocardial reactive hyperaemia

Area of Science:

• Cardiovascular Physiology
• Myocardial Blood Flow Dynamics

Background:

• Reactive hyperemia (RH) is characterized by a gradual increase in coronary blood flow to peak rate after a period of occlusion.
• A leading hypothesis suggests delayed flow restoration to the subendocardium drives this gradual rise.

Purpose of the Study:

• To investigate the spatial distribution of coronary blood flow during reactive hyperemia.
• To determine if subendocardial perfusion dynamics fully explain the time course of RH.

Main Methods:

• Open-chest dog model with controlled left circumflex artery occlusion and subsequent reperfusion.
• Radiomicrosphere injection to measure myocardial perfusion distribution.
• Comparison of endocardial to epicardial (end/epi) perfusion ratios in hyperemic and control regions.

Main Results:

• During occlusion and early reperfusion, the end/epi ratio in the hyperemic zone was 50% of control.
• The end/epi ratio progressively increased during RH, reaching control levels at peak flow.
• Control regions (left anterior descending perfused) showed a stable end/epi ratio of 1.14.

Conclusions:

• While subendocardial ischemia may play a role, it does not solely account for the gradual increase in flow rate observed during reactive hyperemia.
• The progressive recovery of subendocardial perfusion during RH is a key factor, but other mechanisms likely contribute to the overall response time.