The Equine Heart
how it works &
what can go wrong
By Todd C. Holbrook, DVM
The horse's heart and blood vessel
(cardiovascular) system is one of the key components required for
optimal athletic performance. The modern-day horse has evolved over
thousands of years with selective pressures aimed at improving
athletic performance. This has resulted in an animal with a large
heart which has high blood pumping capacity. In this article I will
review some of the basic functions of the heart, the cardiovascular
response to training and exercise, as well as review some causes of
poor performance related to heart function.
The heart as a pump -- why is
this important?
The horse's heart, like ours, has
four chambers with associated valves that open and close as the heart
muscle relaxes and contracts to insure blood flows in the right
direction. Specialized cells within the heart conduct electrical
activity that coordinates the muscles of the heart to contract in an
appropriate manner to optimize blood pumping. The right and left atria
are smaller volume chambers at the top of the heart that receive blood
returned from either the lungs (left atrium) or the body (right
atrium).
When the atria contract, each
delivers blood to the larger volume ventricle that lies beneath. The
right side of the heart receives unoxygenated blood from the body and
pumps it to the lungs to allow the red blood cells to uptake oxygen.
Oxygenated blood returns to the left side of the heart, and the left
ventricle pumps it out the aorta to the rest of the body.
Simply stated, the larger the heart
is, the more volume of blood it can pump. Thus, heart size is a major
determining factor of stroke volume (SV) or the volume of blood pumped
per heart beat. Stroke volume in a horse at rest is around 1000 ml,
and increases to around 1700 ml at maximal exercise. The cardiac
output (CO), or volume of blood pumped per minute, is equal to the
heart rate (HR) multiplied by the stroke volume.
With exercise onset, the horse's
heart rate increases from a resting rate of 30 to 40 beats per minute
to around 110 beats per minute. Stroke volume increases also, due to
increased volumes of blood returned to the heart. The heart muscle
responds to this increased volume and pressure with stronger
contractions.
Thus with the initial demands of
exercise, cardiac output rapidly increases. This increase is required
to supply working muscles with oxygenated blood. The maximal oxygen
uptake or VO2 max is also known as aerobic capacity. Among animals of
comparable size, the horse is superior in its ability to consume
oxygen (>200mlO2/min/kg).
It is generally accepted that the
ability of the horse's muscle mass to consume oxygen far exceeds the
ability of the heart and lungs to provide oxygenated blood. Thus,
conditions that result in improved cardiac output positively impact
performance, whereas conditions resulting in reduced cardiac output
can directly negatively impact performance.
Do athletes with bigger hearts
perform better?
As explained above, the larger the
heart, the higher the cardiac output. The impact of heart size on
performance is supported by relative heart sizes in famous human and
equine athletes. One multiple Olympic champion distance runner had a
heart mass that was almost three times larger than predicted for his
body size. Triple Crown winner Secretariat's heart was estimated to be
22 pounds, nearly 10 pounds heavier than the heart of an unexceptional
stallion. The impact of cardiac size on endurance performance in the
horse is currently unknown; however, a number of famous endurance
horses anecdotally had large hearts.
In racing thoroughbreds, maximal
oxygen consumption is an excellent determinant of performance, and
heart size is related to both VO2 max and racing performance. Whereas
elite thoroughbred racehorses perform near VO2 max at top speeds,
endurance horses typically compete at 30% to 55% VO2 max.
What's different about the
cardiovascular requirements of endurance exercise?
Exercising muscles produce excessive
heat during work that must be balanced with efficient body cooling.
Increases in cardiac output are required not only for working muscle,
but also to supply blood flow to the skin to allow for optimal
sweating and heat dissipation.
In the endurance horse, the
thermoregulatory system is of utmost importance. Comparatively, much
more cardiac output is required for thermoregulation with endurance
work than with short distance racing. During endurance exercise, fluid
loss from sweating can reduce circulating blood volume. The resulting
competition for cardiac output between working muscles and skin can
become critical. Because the central cardiovascular needs take
precedence, skin blood flow is reduced to maintain circulating blood
volume and heat stress often results.
The maximal heart rate in the horse
varies from just over 200 to around 240 beats per minute. During
racing, thoroughbreds typically achieve heart rates that are close to
maximum, while endurance racing horses typically maintain a lower
working heart rate for long periods (120 to 160 beats per minute).
However, with sprints at race finishes, elite endurance horses also
likely achieve heart rates approaching maximum.
How does the cardiovascular
system respond to training?
As horses become more fit with
training, a number of adaptations occur in the cardiovascular system.
In both humans and dogs, endurance training causes a reduction in
resting heart rate. Although I am not aware of a study that addresses
this question in endurance horses, my impression is that resting heart
rate also decreases with training in endurance horses. Most literature
of cardiovascular exercise physiology addresses the thoroughbred race
horse, wherein there is not a reduction in resting heart rate with
fitness.
Heart rate during submaximal
exercise does decrease with training. In other words, the heart rate
required to trot or canter at a set speed from point A to point B will
be reduced as the horse becomes more fit. The heart rate monitor is a
valuable tool when utilized to assess fitness in this manner.
Although there is a linear
relationship between running speed and heart rate, maximal heart rate
is not a good measure of athletic fitness. Furthermore, maximal heart
rate is not increased with training. Speed or velocity at a specific
submaximal heart rate is related to stroke volume, fitness and
performance.
The reduced heart rate during
submaximal exercise is due to a number of factors that are altered
with training. Blood volume increases in horses due to training by
about 10%. This increased blood volume is due to both increased plasma
volume and red blood cell volume. These changes optimize
oxygen-carrying capacity, and provide greater plasma volume for
improved thermoregulation. The increased blood volume is one factor
that leads to increased cardiac stroke volume secondary to training.
Mathematically, because CO = HR x SV, the increased stroke volume
induced by training provides the maintenance of cardiac output for
submaximal exercise, while allowing a reduction of heart rate.
The heart itself remodels in humans
and dogs as a result of endurance training. Recent studies in
racehorses indicate remodeling also occurs in the horse in response to
training. Heart mass and the left ventricular internal diameter
increases with training. The size of the left ventricle has also
recently been correlated with performance in both Standardbred and
Thoroughbred racehorses. The impact of endurance training on cardiac
remodeling in the horse, and the importance of heart size on endurance
performance, have not been studied.
How is heart rate recovery
affected by training and exercise?
As with improvement in fitness, the
heart rate recovery time post-exercise is shortened. In other words,
the horse's heart rate drops more rapidly toward normal after exercise
as fitness improves. Checking the heart rate monitor at intervals
after completing a defined distance is a valuable method to assess
fitness during training. During competition the rider and veterinarian
often utilize this information on the ride card to monitor the horse's
condition. With dehydration, metabolic dysfunction, fatigue and even
lameness, heart rate recovery is adversely affected. This is the basis
of the cardiac recovery index (CRI).
How can heart problems cause poor
performance?
Adequate cardiac output must be
maintained to sustain muscular work; therefore, any condition that
results in reduced cardiac output below that necessary for a given
workload can reduce performance. In the horse, reduced cardiac output
can result from arrhythmias (abnormal heart rhythm) that cause
uncoordinated contraction of the heart.
Heart murmurs can also be associated
with poor performance. If one or more of the heart valves does not
close properly, this can disrupt normal blood flow in the heart and
cause a heart murmur. Depending on the location, severity, and
duration of the valve leakage, changes in cardiac function can result
which may negatively impact athletic performance.
The most common arrhythmia that
causes poor performance in horses is atrial fibrillation. During this
condition, the electrical activity of the top chambers of the heart
(atria) is uncoordinated. This results in reduced pumping of blood to
the ventricles, as well as an irregular ventricular rhythm. The
resulting reduced cardiac output can cause poor performance.
I have diagnosed paroxysmal atrial
fibrillation that has developed during competition in a number of
endurance horses. In this situation it may be related to electrolyte
disturbances. Indeed in one recent case it was associated with
inappropriate calcium supplementation during competition. Horses that
develop atrial fibrillation during competition should receive
veterinary attention. Evaluation will usually include assessment of
the horse's serum electrolyte status and electrocardiogram recording.
The condition may resolve over time with correction of the electrolyte
abnormality. Because there can be other causes of atrial fibrillation,
and this arrhythmia can indicate underlying heart disease, a thorough
cardiac workup is recommended.
Other exercise-induced arrhythmias
may also result in poor performance. The diagnosis of these problems
often requires evaluation of the horse's heart rhythm during exercise
on a treadmill.
Heart murmurs are quite common in
horses, especially athletes. Rapid blood flow out the major vessels at
the heart base during contraction of the heart commonly results in
innocent flow murmurs that are not caused by valve leakage. These are
normal murmurs caused by turbulent blood flow, and do not impact on
the horse's performance. It can be difficult to determine the
seriousness of a murmur by the stethoscope alone, and sometimes even
suspected innocent flow murmurs should be confirmed with a more
in-depth cardiac workup. Because specialty equipment and advanced
veterinary training is often required for extensive cardiac workup,
this often necessitates transport of the horse to a university
teaching hospital or referral clinic.
Murmurs associated with the valves
between the atria and the ventricles can result in poor performance,
especially on the left side of the heart (mitral valve). Mitral valve
insufficiency can result in heart enlargement, atrial fibrillation,
and ventricular arrhythmias. A thorough cardiac examination including
ultrasound of the heart (echocardiography) is warranted in any horse
that has a significant heart murmur.
One of the more common murmurs I
recognize in older endurance horses is called aortic insufficiency. In
this disease, there is a progressive degeneration of the aortic valve,
and as the valve function worsens it results in reduced performance.
After the left ventricle contracts to pump oxygenated blood out the
aorta, the insufficient valve allows blood to flow backwards into the
left ventricle during the resting phase of the heart cycle. This
results in a reduced effective cardiac output. This murmur is more
common in horses over 15 years of age. I have heard a number of rather
loud aortic insufficiency murmurs in older endurance horses that have
competed for years with the condition at lower levels. Because it is a
progressive disease, horses with aortic insufficiency should be
monitored over time with echocardiography.
In this article I have briefly
reviewed some basic concepts of how the horse's heart functions during
exercise and how a few conditions can affect performance.
Todd C. Holbrook, DVM, Diplomate
ACVIM, Assistant Professor of Equine Medicine at Oklahoma State
University in Stillwater, Oklahoma
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