Table 7.3 Summary: clinical features of atrial fibrillation

Most common arrhythmia affecting athletic performance

Horses are predisposed to AF because they have high vagal tone and large atria

Horses often develop AF without underlying heart disease

May be paroxysmal or sustained

Paroxysmal AF may be difficult to identify

AF may develop in some animals with congestive heart failure but underlying heart disease is the cause of the CHF

Presenting signs


Poor athletic performance

Incidental finding




collapse (usually at exercise)


Weight loss

Clinical recognition

Irregularly irregular rhythm

Absence of atrial contraction sound ('A' sound)

Pulse quality may be variable

Heart rate normal or raised


No P waves

f waves

Irregular R-R interval

QRS complex normal (although may vary fractionally)

If CHF is present, do not treat with quinidine. The prognosis is poor.