Cardiac Parameters During Exercise in Horses:
Horses possess extraordinary cardiovascular capacities due to their size, breed, and selective breeding for athletic performance.
Key metrics:
Resting heart rate: 30-40 bpm; increases to 220-240 bpm during intense exercise.
Cardiac output increases dramatically from ~5 L/min at rest to up to 450 L/min during intense exertion.
VO2max (maximum oxygen uptake) in horses exceeds 200 ml/kg/min, far surpassing elite human athletes (e.g., 97.5 ml/kg/min for the human record holder). ventilatory response increases to meet the demands for gas exchange.
"Blood Doping": Splenic contraction releases splenic reserve (80% hematocrit blood), increases hematocrit (the proportion of red blood cells) from 32% at rest to 65% during exercise, which helps boost oxygen delivery.
Fitness comparison: a less fit horse's HR will increase along a steep slope with increased velocity while running when compared to a fit horse because the fit horse is able to go faster for the same HR.
ECG and Arrhythmias During Exercise:
Heart rate increases linearly with speed during exercise, but ECG analysis is necessary for more detailed cardiac function monitoring. Arrhythmias (irregular heartbeats) are common in horses, especially after exercise. These include:
Atrioventricular block: only observed at rest, P waves are not followed by QRS waves.
Supraventricular premature contractions (SVPC): Early beats from the sinus node with normal P and QRS complexes. ECG waves appear very close together.
Ventricular premature contractions (VPC): Early beats originating from the ventricles, often with wide/abnormal QRS complexes very close to one another.
Atrial fibrillation: poses a risk for sudden cardiac issues during competitions.
Studies have examined the frequency and effects of arrhythmias in chuckwagon racing and horse metabolism. hscTnT in the blood is an indicator of heart tissue damage and can be used as a biomarker for monitoring cardiac health or identifying horses at risk for bad cardiac arrhythmia.
Respiratory Parameters During Exercise:
Horses are obligate nasal breathers, meaning they can only breathe through their nose (as opposed to other mammals that can mouth breathe). Catecholamines will dilate nares and head extension opens up airways between nares and lungs. These are critical features during exercise, where airflow is generated by visceral piston movements, lumbosacral flexion, and limb impact.
Exercise induced hypercapnia (increased CO2 levels) and hypoxemia (reduced oxygen levels) in horses due to the coupling of ventilation with stride frequency, and a large lung capacity (18 ribs, massive diaphragm).
Limiting factor is not the heart but the lungs - don’t have enough oxygen coming in - why? Better to spend energy on speed (running) than on bringing in oxygen so they have developed a buffering system to deal with the disruption in CO2 levels.
Upper and Lower Airway Issues:
The upper airways (nasal passages, pharynx, and larynx) offer significant resistance to airflow, with the nares being the greatest point of resistance.
The lower airways include the trachea, bronchi, and alveoli, which are responsible for gas exchange.
Pulmonary issues such as Exercise-Induced Pulmonary Hemorrhage (EIPH) can occur, where high-pressure blood flow ruptures capillaries, leading to bleeding in the lungs.
Guttural pouches play a role in cooling the blood going to the brain to prevent hyperthermia during exercise.
Exercise Physiology Takeaways:
Training can improve VO2max by 20%, but ventilatory parameters worsen post-training, indicating a trade-off between oxygen delivery and gas exchange efficiency.
Dynamic respiratory endoscopy (DRS) allows for real-time assessment of airway function during exercise.