Page 727 - The Veterinary Care of the Horse

• Longer than normal recovery time after exercise.

• In some cases the first sign is a sudden onset of ataxia, i.e. the horse becomes
VetBooks.ir • uncoordinated and slightly wobbly during exercise.

Some affected horses have exercise-induced pulmonary haemorrhage and bleed from the

nostrils following moderate work.
• On auscultation the heartbeat is irregular and often faster than normal. If it is more than

50 beats per minute, there is likely to be underlying heart disease.

• During fast work the heart rate may increase to around 300 beats per minute in an
attempt to pump sufficient blood around the body to meet the requirements of the horse.

Diagnosis

This is made on the results of the clinical examination and confirmed by taking an ECG. The
horse will have completely irregular heartbeats of different intensity and variations in the rate

and quality of the pulse. Only three of the four normal heart sounds are present. The fourth
sound, which is heard when the atria contract, is absent in horses with atrial fibrillation.

Some horses develop atrial fibrillation for a short period of time during the last stages of
a race. This may cause the horse to pull up or slow down. The fibrillation may persist for up

to 48 hours, but some horses spontaneously convert back to sinus rhythm immediately,
making it very difficult to diagnose. When this occurs, it is known as paroxysmal atrial
fibrillation.

Treatment

When atrial fibrillation is diagnosed, treatment should be delayed for 48 hours as a number of

horses spontaneously convert back to normal. The exception to this is treating electrolyte
imbalances in exhausted endurance horses; these should be treated with appropriate fluid
therapy straight away.

It is also important to establish whether there is any sign of co-existing heart failure

before treatment begins. Ideally, an ECG and echocardiography should be performed to
determine this. In the absence of heart failure, atrial fibrillation is most commonly treated
using a drug called quinidine sulphate. This is given by stomach tube every 2 hours until the

heart returns to its normal rhythm. During treatment continuous ECG monitoring is
performed and the horse is closely observed for any signs of toxicity. Symptoms such as

anorexia, depression and swelling of the nasal mucosa are quite common during the
treatment period. Other side effects are subcutaneous oedema, diarrhoea, colic, laminitis,
increased heart rate, reduced blood pressure, weakness, difficulty breathing and even death.