Inspiratory and expiratory muscle perfusion in maximally exercised ponies.

This research article investigates the allocation of blood flow in the respiratory muscles of ponies during periods of rest and maximum exercise. The study quantifies the proportion of cardiac output that goes towards perfusing these muscles under various conditions.

Research Methodology

• The study was conducted on seven healthy ponies.
• Tissue blood flow was analyzed using radionuclide-labeled microspheres with a diameter of 15 microns, introduced into the left ventricle.
• Changes in cardiac output and blood flow to the respiratory muscles were measured both at rest and during maximum exercise.
• The variety of respiratory muscles examined included both inspiratory muscles (those used for inhaling) and expiratory muscles (used for exhaling).

Findings – Resting Period

• At rest, the inspiratory and expiratory muscles comprised 2.41% and 3.05% of the pony’s body weight respectively. They received 6.17% and 3.75% of the cardiac output.
• The renal proportion (amount diverted to the kidneys) of cardiac output at rest was confirmed as 21%.

Findings – Maximum Exercise

• During periods of maximum exertion, significant changes were observed in heart rate, mean aortic pressure, and cardiac output. Heart rate increased from an average of 55 to 218 beats per minute, aortic pressure rose from 125 mmHg to 170 mmHg, and cardiac output increased markedly from 96 ml.min-1.kg-1 to 730 ml.min-1.kg-1.
• Blood flow to all respiratory muscles also increased significantly during exercise as vascular resistance decreased drastically.
• There was noticeable variation in perfusion across different inspiratory and expiratory muscles during exercise. The most perfused inspiratory muscle was the diaphragm, followed by the serratus ventralis. Among the expiratory muscles, the internal oblique abdominis and the transverse thoracis (triangularis sterni) had the highest blood flow.
• Combined, blood flow to the inspiratory and expiratory muscles comprised about 14.8% of the cardiac output during maximum exercise—a substantial increase compared to the resting state.
• Conversely, during maximum exercise, the renal proportion of cardiac output decreased significantly from 21% (at rest) to 0.72%.

Implications

• The study provides valuable insight into how exercise affects blood flow allocation in ponies, particularly towards the respiratory muscles. It shows that the horses’ bodies prioritize perfusion of these muscles above others during strenuous activity, even at the expense of reduced perfusion to the kidneys.
• These findings may contribute to understanding the physiological adaptations during exercise in other species too, including humans.
• Beyond this, the research could have practical applications in equine athletic training and healthcare by helping to design programs that consider the body’s perfusion priorities during exercise.