Heart and lungs: the horse's power train

The power train

The Royal Veterinary College describes a horse's athletic engine as a power train of lungs, heart and muscle. The lungs take in fuel (oxygen) and pass it into the blood. The heart pumps that blood to the engine (the muscles), where the oxygen is burnt to create the power that drives the skeleton. Few animals can match the horse for the efficiency of this system: the RVC says even the most elite human athletes have only half the power-train efficiency of the average horse.

The heart

The RVC calls the horse's heart one of the most efficient pumps found in nature, and a major reason the horse is such an excellent athlete.

• A huge range of rates. A horse's heart beats from a very slow 30 beats a minute at rest to 240 when galloping, a much wider range than ours. How quickly it returns to its resting rate after exercise is an indicator of fitness.
• A huge volume. At rest, a Thoroughbred's heart pumps about 35 litres of blood a minute, compared with 5 litres for a human heart. Combined with the heart's large size and its ability to keep beating fast, that lets it deliver large amounts of oxygen to the muscles for long periods of hard exercise.
• Big hearts, big athletes. Horses with larger hearts generally have more athletic ability. Some of the world's most famous racehorses, including Eclipse, Phar Lap and Secretariat, had much larger hearts than average. A big heart isn't everything, though: elite horses also need good skeletal function and a desire to win.

Murmurs and rhythms

Fit horses often have heart sounds that would worry a doctor in a person but are perfectly normal in a horse:

• Murmurs are caused by turbulent blood flow. The RVC says 30–40% of fit Thoroughbreds may have one. At rest, the relatively small volume of blood flowing through the large vessels can become turbulent, and training enlarges and strengthens the heart until its pressures can make the valves leak slightly. But some murmurs are caused by valve disease or heart defects, and these can get worse over time.
• Irregular rhythms are also common. Most fit horses have a rhythm with occasional pauses, which lets the heart keep a slow rate at rest.
• Atrial fibrillation is the most common and important abnormal rhythm in horses, especially in large and fit horses. Because the equine heart has so much capacity and beats so slowly, a leisure horse with it may show no obvious signs. In athletes it is a fairly common cause of poor performance.

Your vet can tell normal from abnormal by listening to the heart and, if needed, with an ultrasound scan, an ECG at rest and during exercise, and blood tests.

The lungs and airways

Horses evolved on open pasture with unlimited fresh air. The British Horse Society notes that keeping them in more confined spaces can compromise their breathing over time, affecting their wellbeing and performance.

Respiratory disease is very common in horses. The RVC divides it into two main kinds:

• Infectious diseases, such as strangles, equine influenza and the equine herpesviruses, which spread most easily among young horses and in large groups or yards. Mandatory vaccination of horses that travel to compete, show or breed has greatly reduced influenza since it was introduced in the early 1980s.
• Non-infectious diseases, mostly allergic reactions to moulds, fungi and other dust in stable air. Inflammatory airway disease affects younger horses, mainly up to about six years old. Equine asthma, previously called recurrent airway obstruction (RAO), COPD or "heaves", affects older horses. Equine asthma tends to persist, so it is managed by permanently changing the horse's environment: turning it out, keeping stables clean and well ventilated, and using bedding such as wood chips or paper.

A cough always means the airways are affected, but many horses with respiratory disease cough only occasionally or not at all. Other signs include nasal discharge, reluctance to work, faster or more effortful breathing, and taking too long to recover after exercise.

Know your horse's normal

Counting your horse's breaths and heartbeats at rest gives you a baseline to spot illness and to track fitness. World Horse Welfare gives normal resting ranges for a healthy adult horse of 36–42 beats a minute and 8–12 breaths a minute. The British Horse Society's riding-award guidance gives a normal breathing rate of 8–16 breaths a minute. Horses vary, so learn your own horse's normal values; the daily care guide explains how.

After exercise, a fit horse should get back to its normal breathing rate within about five minutes. A horse that breathes hard for the work it is doing, or takes a long time to recover, may not be fit enough for it.