Page 997 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

972 CHAPTER 8

VetBooks.ir ARRHYTHMIAS

A wide range of arrhythmias has been documented
an increased number of re-entrant wavelets (electri-
in the horse. These arrhythmias are for the most it and simultaneous depolarisation. This may lead to
part benign in nature, although there are some cal signals conducting around a self-perpetuating
notable exceptions. Differentiation of arrhythmias circuit), increasing the propensity for AF mainte-
based on auscultation alone can lead to confusion. In nance. It is considered a re-entrant arrhythmia if the
particular, second-degree heart block and AF may arrhythmia is self-sustaining.
be similar on auscultation in the resting horse, but
the clinical consequences are drastically different. Clinical presentation
Electrocardiography is indicated to determine the AF is not usually associated with any underlying
clinical significance of any arrhythmia detected on cardiovascular or systemic disease in the horse (lone
clinical examination. AF). However, it is also commonly encountered
The clinical consequences of relevant arrhyth- in horses with extensive heart disease and in asso-
mias are, for the most part, a result of their haemody- ciation with heart failure. The heart rate at rest is
namic effects. Suboptimal ventricular performance usually normal, except in animals with advanced
and cardiac output are often encountered. Syncope heart failure. The clinical consequences of AF are
is uncommon but may occur during episodes of also not commonly encountered at rest. The heart
profound bradycardia or tachycardia. Syncope is of rhythm, however, is absolutely irregular and the
particular concern in horses, as opposed to other peripheral pulse is variable, both in rhythm and in
species, because of the potential for injury to riders quality. Unlike second-degree heart block, there
or people handling affected horses. is no predictability to the cardiac rhythm and the
A number of commonly encountered benign rhythm remains irregular during excitement or
arrhythmias, as well as some clinically significant exercise. AF has significant clinical consequences
arrhythmias, are discussed in this section. AF, at elevated heart rates and it profoundly limits per-
because of its clinical implications and common formance. The classical presentation for a horse in
occurrence in horses, is described in greater detail. AF is one of poor performance. A tendency to tire
during exercise is common. Sudden fading during a
ATRIAL FIBRILLATION race is a common finding associated with the onset
of AF in racehorses. Syncopal episodes are uncom-
Definition/overview mon. Exercise-induced pulmonary haemorrhage has
AF is the most common clinically relevant dysrhyth- been associated with AF by some authors; however,
mia in the horse, with an estimated incidence rang- the relationship is not proven. Elevated heart rates
ing from 0.3% to 2.5% of the equine population. at rest, peripheral oedema and venous distension
are not encountered in lone AF. These findings in
Aetiology/pathophysiology horses with AF are related to the underlying cardiac
The initiating event is not known in the horse. Factors disease rather than to the AF, and the arrhythmia
such as electrolyte abnormalities and premature should be considered a consequence of the underly-
atrial contractions may be contributory. A genetic ing heart failure.
factor has been suggested in the Standardbred. In most instances, AF is persistent once initi-
Horses may be predisposed to AF because of high ated. However, in some horses, short episodes of AF
vagal tone and large atrial dimension. AF results may occur with spontaneous onset and resolution.
from a loss of coordinated electrical and mechanical This is known as paroxysmal AF, and episodes are
function within the atria. Vagal tone leads to vari- often associated with exercise. In humans, parox-
ability in action potential duration in the atrial tissue ysms gradually increase in duration and persis-
and the large dimensions of the atria in the horse tent AF often develops. Such an association has
allows conduction through multiple pathways within not been documented in horses. The differences