Changes in left ventricular dynamics during graded exercise.

The study investigated how the dynamics of the left side of a horse’s heart changed as the horse’s exercise intensity increased. The key objective of the study was to understand whether the Frank-Starling mechanism of the heart, which enables the heart to change its force of contraction based on the degree of stretch of the heart muscle, is evident in horses during high levels of exertion.

Methodology

• To gather necessary data, three Thoroughbred horses were surgically prepared with ultrasonic sonomicrometer crystals attached to their ventricular pericardia, the sac surrounding the heart.
• The signals from the crystals recorded the dimensions across the left side of the horse’s heart and these dimensions were used to estimate the volume of the chamber.
• The volume estimates were compared with measurements of stroke volume (the volume of blood pumped from the left ventricle per beat) made using the Fick principle, a common method to measure cardiac output and oxygen consumption in the heart.
• Measurements were taken when the horses were at rest and during bouts of varying intensities of exercise, with the highest intensity being equivalent to the horse’s maximum rate of oxygen consumption (VO2max).

Findings

• Although there was no noticeable difference in stroke volume at different exercise intensities according to the Fick principle measurements, stroke volumes determined using sonomicrometer were significantly lower at rest and increased at VO2max.
• These variations were on par with changes noticed in the end-diastolic volume (the volume of blood in the left ventricle at the end of filling), while the volume of blood remaining in the left ventricle at the end of contraction (end-systolic volume) remained consistent.
• At all exercise intensities, stroke volume was the most variable measurement, whereas end-diastolic volume hardly varied.
• Consistent with the Frank-Starling mechanism, the researchers concluded that during high-intensity exercise, the cells of the ventricle generate high pressure and experience increased stress due to the increased dimensions of the chamber.

This research provides insights into how the cardiac muscle of horses acts under varying levels of exercise, demonstrating a likely Frank-Starling response during intense exercise.