Transmural coronary vasodilator reserve and flow distribution during severe exercise in ponies.

This research investigates the effect of severe exercise on the heart’s blood flow and coronary vasodilator reserve in healthy adult ponies. It shows that during intense exercise, there is a notable increase in different aspects related to blood flow and pressure in different parts of the heart, and despite these changes, there is still a significant coronary vasodilator reserve in the heart across different layers.

Study Design and Observations

• Researchers studied the transmural distribution of myocardial blood flow and coronary vasodilator reserve in 11 healthy adult ponies at rest, during moderate exercise, and severe exercise, with the ponies put on a treadmill.
• Heart rate benchmarks for rest, moderate, and severe exercise were established.
• During intense exercise, the study observed significant increases in various aspects of heart function – cardiac output, mean aortic pressure, right ventricular systolic and end-diastolic pressure, left ventricular end-diastolic pressure, and the maximum rate of rise of left ventricular pressure. Alongside these changes, there was a significant increase in transmural myocardial perfusion.
• The subendocardial/subepicardial perfusion ratio for the left ventricle decreased significantly from its control value. However, it was not quite different from 1.00, which might be indicative of normal function.

Adenosine Infusion Observations

• With an additional infusion of adenosine (a naturally occurring substance that widens blood vessels) during severe exercise, the myocardial blood flow throughout the cardiac ventricles increased further, even though heart rate, aortic pressure, and right and left ventricular end-diastolic pressures remained unchanged.
• During this severe exercise with adenosine, endo-to-epi perfusion ratios for both ventricles increased.
• The coronary vasodilator reserve was least in the papillary muscles, and largest in the middle layers of the left ventricular myocardium.
• Also, with adenosine infusion during severe exercise, coronary vascular resistance in both ventricles significantly decreased to levels observed during maximal coronary vasodilation induced by resting adenosine infusion.

Conclusions

• The findings of this research demonstrated that despite the pressure put on the heart during severe exercise, there was still a significant coronary vasodilator reserve in the pony myocardium. This showcases the heart’s ability to not only cope with significant cardiovascular stress but also its mechanisms to cope with such situations.