SAEVA Congress 2018 Proceedings | 12-15 February 2018 | ATKV Goudini Spa
predisposition to AF in the Standardbred, particularly in male Standardbreds and in
pacers. Presenting complaints include poor performance, tachypnea, dyspnea,
exercise-induced pulmonary hemorrhage, myopathy, colic, collapse (rare),
vocalization and congestive heart failure (CHF). Sudden cardiac death, while
extremely rare in horses with AF, has been reported in horses. Atrial fibrillation can
also be an incidental finding during a routine examination. Most horses with AF have
little or no underlying cardiac disease. AF is also a common finding in horses with
advanced valvular heart disease, particularly mitral (most common) and tricuspid
regurgitation, as well as in those with CHF.
Horses are predisposed to the development of AF due to their high resting vagal
tone, autonomic imbalance associated with exercise and large atrial mass. Transient
potassium depletion secondary to the administration of furosemide or excessive
sweating is also a known predisposing factor. Excess thyroid hormone has also been
associated with AF in horses on Thyro-L or supplements containing kelp. Shortening
of the effective refractory period, atrial inhomogeneity and atrial premature
depolarizations also set the stage for the development of AF.
Horses with AF usually have normal resting heart rates (28 - 44 beats/minute),
although the rhythm is irregularly irregular and no fourth heart sound (S4) is
produced. The intensity of the peripheral arterial pulses is also irregularly irregular.
Pulse deficits may be present, particularly in horses with two conducted beats
occurring in rapid succession. The heart should be carefully ausculted for murmurs
of mitral, tricuspid, and less frequently aortic regurgitation may be present in horses
with AF and influence their prognosis.
The ECG is diagnostic for AF and reveals irregularly irregular R - R intervals, no P
waves and normal appearing QRS complexes. Rapid baseline fibrillation "f" waves
are usually present which may be small (fine) or large (coarse). In approximately
10% of horses with AF, QRS complexes of different morphology that may be
aberrantly conducted or originating from the ventricle are present.
AF Base-apex lead, 25 mm/sec, 10 mm=1 mV
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