Regional myocardial blood flow and coronary vascular reserve in unanesthetized ponies during pacing-induced ventricular tachycardia.

The study investigates the impact of rapid heart rates on blood flow in the hearts of six healthy adult ponies. The results suggest that the coronary arteries maintain the flow of blood in the heart at a rate of 200 beats per minute, but are unable to increase blood flow further at a rate of 250 beats per minute due to exhaustion in deeper layers.

Objective of the Research

• The study was designed to examine the effects of tachycardia (abnormally rapid heart rate) on the coronary circulation, which refers to the movement of blood through the vessels in the heart.
• The researchers particularly focused on understanding the transmural distribution of myocardial blood flow (MBF) – that is, how blood flow changes across the wall of the heart.

Methodology

• The subjects of the study were six healthy adult ponies. They were studied at rest, during ventricular pacing at different rates (150, 200, and 250 beats per minute), and also during maximum dilation of the coronary vessels stimulated by the infusion of adenosine.
• The researchers used radionuclide-labeled microspheres of 15-micron diameter to study the MBF. These are tiny particles that emit radiation, allowing the blood flow to be mapped accurately.

Results

• The study found that when the heart rate was increased to 150 and 200 beats per minute, there was a proportional uniform increase in MBF, with the ratio of blood flow between the inner (endocardial) and outer (epicardial) layers of the heart wall well maintained.
• However, when the heart rate was further increased to 250 beats per minute, there was no corresponding increase in MBF. Furthermore, the ratio of blood flow between the endocardial and epicardial layers decreased significantly, indicating lesser blood flow to the inner layers of the heart.
• Further, when vasodilation was induced using adenosine at this heart rate, there was a substantial decrease in aortic pressure and further reduction in blood flow to the inner layers, while MBF increased dramatically in the outer layers.
• By contrast, for the right ventricle, MBF increased significantly and the perfusion ratio was well maintained.

Conclusion

• These findings illustrate that the coronary arteries are able to maintain blood flow in the hearts of ponies at a rate of 200 beats per minute.
• However, at a rate of 250 beats per minute, the coronary arteries exhaust their reserve in the inner layers, thus restricting any further increase in MBF.