Page 5 - Equine Matters - Winter 2014

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HEART D I SEASE
WINTER 2014 ISSUE
EQUINE MATTERS
4
These are very common due to turbulence
in large blood vessels associated with
high flow rates of blood filling, or being
pumped out of, the heart. They can also
sometimes be heard transiently during
episodes of fever, colic or anaemia.
These murmurs are usually quieter than
pathological murmurs and are heard only
over a short period of the heart cycle.
l
Holes in the heart are the most
common birth heart defect (figure 2).
Horses with small defects may be able
to exercise normally (and even race
successfully) and have a normal
life expectancy.
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Other heart birth defects are rare.
Figure 2:
Ultrasound scan of the heart
showing abnormal blood flow through a
hole in the heart.
Causes of equine cardiac
murmurs
A.
Physiological / functional
flow murmurs
B.
Murmurs associated with
heart disease
1. Birth defects
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These are most commonly caused
by degeneration of the valve and
surrounding tissues. Infection of the
heart valve (endocarditis) is rare in
horses. Horses with significant valve
murmurs may have exercise intolerance
e.g. become unusually tired during
faster work. Mild leakiness of some
heart valves can be seen in fit, healthy
horses but valve disease can be very
serious and may lead to heart failure.
Severely leaky valves cause secondary
enlargement of heart chambers which
makes horses unsafe to ride. It is
therefore important to fully evaluate
these murmurs with heart scans.
2. Heart valve murmurs
It is important to characterise the abnormal
heart rhythm by the heart rate (abnormally
fast or slow), the presence of abnormal
pauses or premature sounds in the heart
cycle and whether such abnormalities are
heard regularly or irregularly. Other signs
of heart disease will also be evaluated as
described for murmurs.
The heart rhythm should then be evaluated
by performing an ECG (figures 3 and 4).
This will be performed at rest and then
possibly also at exercise if required.
Modern ECG units can be fitted under
the saddle and the heart trace recorded
digitally. Sometimes it is necessary for
a horse to wear an ECG for 24 hours
to be able to record and identify
intermittent arrhythmias.
Occasionally further diagnostics such as
blood tests may be recommended. An
ultrasound scan of the heart may also be
advised to assess heart muscle function
and investigate concurrent murmurs.
Cardiac Arrhythmias:
Evaluation of an Arrhythmia
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These arrhythmias are heard at rest
and should disappear when the
heart rate elevates during excitement
or exercise.The most common
physiological arrhythmia is
2nd degree
atrioventricular block
(AV block) and
is very common in fit,healthy horses.
A regularly, irregular rhythm with
occasional pauses (dropped beats) is
heard e.g. four normal beats and then
a dropped beat every fifth beat.
Physiological Arrhythmias
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Atrial fibrillation:
the most common
arrhythmia associated with poor
performance in horses. With this
arrhythmia, the top chambers of the
heart (atria) are working ineffectively.
Output from the horse is normal at rest
but submaximal and inadequate during
exercise such that the horse suffers
exercise intolerance. This arrhythmia
can occur constantly or intermittently
in short episodes, usually at exercise.
It may be associated with whole
electrolyte imbalances or underlying
heart disease. Compared with AV
block, the heartbeat is irregularly
irregular and there is no set pattern to
the rhythm. This arrhythmia may not
even need to be treated in horses used
for light work only but can often be
treated successfully in performance
horses if diagnosed shortly after onset.
Common Pathological Arrhythmias
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Ventricular tachycardia:
the most
common life-threatening arrhythmia in
the horse characterised by disordered
and irregular beating of the large
pumping chambers of the heart
(ventricles). This usually occurs as a
consequence of other systemic disease
(especially gastrointestinal disease),
electrolyte disturbances, heart infection,
immune-mediated heart disease or
accidental consumption of ionophores,
which are used as growth promoters in
farm animals. Ventricular tachycardia
may result in respiratory distress,
fainting, and heart failure. Treatment
can be effective in the early stages.
Common Cardiac Arrhythmias
Conclusion
Murmurs and arrhythmias are relatively
common in horses, many of which do not
cause the horse any problem. The most
common clinical sign of heart disease an
owner will pick up is exercise intolerance.
If you are concerned about your horse's
health and possible heart disease, contact
your veterinary surgeon for further advice.
Figure 3
Figure 4
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Premature heartbeats:
occasional
early beats are heard. Occasional
early beats are unlikely to be significant
but if frequent or at exercise they
should be investigated. Electrolyte
imbalance and underlying systemic or
heart disease may cause premature
beats. Premature beats can progress
into more serious arrhythmias and
therefore should be investigated.