Cardiovascular effects of amphetamine in the horse.

The study investigates the effect of amphetamine, atropine, and their combination on the heart rate and rhythm of horses in resting, exercising, and immediately after exercise situations. The research found that amphetamine caused irregular heartbeats and a specific type of heart block in the horses, but these effects were lessened when combined with atropine.

Research Objectives and Methodology

• The study aimed to understand the cardiovascular effects of amphetamine, atropine, and their combination on horses in different physical states.
• The researchers used nine horses for this experiment and studied their cardiac activity through electrocardiography in conditions of rest, exercise, and immediately after exercise.

Findings of the Research

• All treatments led to an increased heart rate in horses when at rest.
• However, a significant heart rate increase during exercise was only observed when amphetamine and atropine were combined.
• Amphetamine treatment caused the most rapid decrease in heart rate immediately after exercise.
• Amphetamine induced a second-degree atrioventricular (AV) block in all horses during the post-exercise period. This is a heart condition where the electrical signals between the upper and lower chambers of the heart don’t transmit properly.
• Amphetamine also caused ectopic beats or irregular heartbeat either during or right after exercise in six out of nine horses.
• Atropine, when used alone or in combination with amphetamine, did not cause the second-degree AV block.
• Atropine also lessened the irregular heartbeats when used with amphetamine.

Interpretations and Conclusions

• The response of heart rate to the treatments indicates that amphetamine’s direct effect was more potent than the reflex effect both at rest and during exercise. However, the opposite was found in the post-exercise phase.
• The second-degree AV block observed during the post-exercise period was attributed to excessive activity of the <a href="/equine-cranial-nerve-anatomy/" title="Cranial Nerves in Horses: Role in Sight, Hearing, Chewing & Balance”>vagus nerve (or vagotonia) due to stimulation of the baroreceptor, a bodily sensor that detects changes in blood pressure.
• The release of norepinephrine hormone by amphetamine likely caused many of the irregular heartbeats by triggering subordinate pacemakers in the heart.