For as long as horses have been bred, raised, and used by humans, their
owners and caretakers have praised their "heart." This reputation for
quality or quantity of "heart" is based on a horse's psychological
rather than anatomical characteristics: stoicism, bravery, dedication, and
nobility of character. But in fact, these attributes of character also reflect
the typically robust nature of the horse's anatomical heart and cardiovascular
system as well.
A horse’s heart supplies blood to all parts of one of the largest
domesticated land mammals (often 1,000 pounds or more), not only at rest and
during routine activities, but also during periods of extreme physical stress
such as that encountered during performing, racing and the training required
for those activities. It is a power plant unequaled in any other creature. That
it is able to do so attests to the system’s remarkable efficiency and
adaptability.
The cardiovascular system of a horse consists of a pump (the heart), a
distribution system (arteries), exchange areas (capillary beds), and a collection
and return system (veins). The heart of a 1,000-pound adult horse is about the
size of a large melon and weighs about 10 pounds. As in all mammals, it
consists of the left and right atrial receiving chambers, left and right
atrioventricular inflow valves, left and right pumping ventricles, and aortic
and pulmonic semilunar outflow valves, plus attached inflow veins and outflow
arteries (see photos above).
The left heart collects blood returning from the lungs via the pulmonary
veins and pumps it to the body via the aorta. The right heart collects blood
returning from the body via the large veins and directs it to the lungs via the
pulmonary artery. The output of each ventricle in an adult horse at rest is
about 25 to 40 liters/minute (approximately 7 to 10 gallons), compared with
four to five liters/minute in an average adult human (1 liter equals about 1.05
quart).
This
Horse Report describes some conditions most often encountered
by equine cardiologists, including some common congenital defects, some causes
of heart problems, and the diagnostic methods that are currently used. In
general, however, horses have excellent cardiac health.
Cardiac Disorders
Horses generally do not have the same types of cardiac problems experienced
by humans, such as coronary heart disease (atherosclerosis) and heart attacks.
But because most horses are not kept as companion pets and are expected to
perform work or athletic feats with a rider, the consequence of any kind of
cardiovascular disease in a horse could be greater than it might be in a dog or
a cat. In addition to the potential effect of a cardiac problem on performance,
the safety of the rider must also be considered in determining the future of
the horse.
The main observable signs of a heart problem in horses include:
- Loss of condition;
- Increased fatigue during
exertion;
- Shortness of breath;
- Increased rate or effort of
breathing;
- Weakness occasionally
resulting in collapse or fainting; and
- Signs of fluid accumulation
in the abdomen or beneath the skin of the lower thorax.
Depending on the severity of the problem, these signs might initially appear
only when the horse is subjected to moderate or strenuous exercise. With a
severe cardiac disorder, these signs could appear during normal non-strenuous
activities or even at rest.
A physical examination in a horse with cardiac disease will usually reveal
an abnormality in the audible heart sounds or in the heart rate and rhythm.
When an abnormality is suspected, further evaluations by electrocardiogram
(EKG) and echocardiogram (ultrasound imaging) might be performed to more
precisely identify the nature and severity of the abnormality. Chest
radiographs could also be used to help determine heart size and abnormalities
in the lungs and chest cavity, but these are often difficult to obtain with
conventional equipment in adult horses because of their large body size.
Heart Murmurs
During an examination of a horse’s chest and heart with a stethoscope, a veterinarian
might detect an abnormality by the sound of a heart murmur, which is the sound
of turbulent blood flow usually caused by an abrupt increase in the velocity of
blood flow. When blood moves smoothly through the heart and blood vessels, very
little sound is produced (like water flowing smoothly through a hose).
Most heart murmurs are caused by blood flow that becomes turbulent because
of increased velocity due to a leak or obstruction in one of the heart valves
or because of abnormal communication between different parts of the heart (like
the increased velocity and spraying sound you get when you put your thumb
across the end of the hose). However, there are some soft, short, variable
heart murmurs that can be heard with no other detectable evidence of heart
disease. Such murmurs are referred to as normal or “innocent.”
If there is uncertainty about the origin or significance of any heart
murmur, further evaluation is usually performed by echocardiography, which
provides detailed images of the inside of the heart and can detect abnormal
blood flow patterns.
Heart murmurs are graded on a scale of Grades 1 to 6, as follows:
- Grade 1—Very soft murmur
that requires extended auscultation to detect.
- Grade 2—Readily audible
murmur that is softer than S1 (the first of the two beats in a heartbeat)
or S2 (the second of the two beats in a heartbeat).
- Grade 3—Readily audible
murmur that is moderately loud and similar in volume to S1 and S2.
- Grade 4—Readily audible
murmur that radiates widely and is louder than S1 or S2.
- Grade 5—Very loud murmur
with a palpable thrill (vibration) that is detectable with fingertip
pressure over the heart.
- Grade 6—Very loud murmur
associated with a palpable thrill that is audible with the stethoscope
held just off the chest.
Heart murmurs are further classified by when in the cardiac cycle they
occur—during ventricular filling (diastole, after S2 and before S1) or during
ventricular contraction (systole, between S1 and S2). Finally, they are
described by their length and musical qualities.
The majority of heart murmurs heard in the horse are physiologic or benign.
These murmurs can increase in intensity with submaximal exercise and can also
be heard with high vagal tone or when the horse is in an excited state. One
classic example of this is colic. Often during a painful colic episode a murmur
can be heard with a stethoscope that has not been heard before and that will
resolve when the colic resolves.
It is important to know which heart murmurs merit further diagnostics and
which heart murmurs can be considered incidental. In general, heart murmurs
should be assessed by a specialist, especially when accompanied by other signs
of cardiac dysfunction or illness. In addition to listening to the murmur with
a stethoscope, a cardiologist will be able to assess the heart via
echocardiography (cardiac ultrasound). Blood flow through the heart can be
analyzed through color flow echocardiography, pinpointing the cause or causes
of the murmur. Additionally, the whole heart can be scanned to assess chamber
size and contractility.
Congenital Heart Defects
Congenital heart defects are abnormalities that are present at birth. They
occur much more common in humans and dogs than in horses. These abnormalities
are often discovered within the first few weeks to months of life when a heart
murmur is heard during stethoscopic examination of the chest and heart.
Although rare, congenital heart defects can prevent the development of an
athletic career and in some cases can be life-threatening. For this reason, every
newborn foal should receive a thorough cardiac examination.
A wide variety of simple or complex congenital heart defects can occur, but
only a few have been recognized often enough to be reported in more than a few
individual horses. The most accurate technique for identifying specific defects
and evaluating their severity is two-dimensional echocardiography (ultrasound
imaging) supplemented by Doppler echocardiography (imaging of blood flow within
the heart and associated blood vessels).
The most commonly reported congenital heart defect in horses is ventricular
septal defect, described below. Another defect, patent ductus arteriosus, which
is common in humans and dogs, is relatively rare in horses beyond one to two
weeks of age.
The ductus arteriosus—a large blood vessel in the fetus that connects the
pulmonary artery with the aorta, bypassing the lungs—closes more slowly after
birth in horses than in humans and dogs, so that a soft, continuous murmur can
be heard in newborn foals up to about a week of age. Horse owners should not
panic if their veterinarian pronounces that their newborn foal has a heart
murmur, although a follow-up exam should be conducted to make sure the murmur
disappears.
Ventricular Septal Defect
Ventricular septal defects consist of a hole in the muscular wall between
the two ventricles and are the most commonly recognized congenital heart defect
in horses. It also occurs as one part of more complex defects.
In simple cases, the hole results in the passage of oxygen-rich blood from
the higher pressure left ventricle to the lower pressure right ventricle and
pulmonary artery, primarily during ventricular systole. Because some of this
blood bypasses the lungs, it is not fully oxygenated. A systolic heart murmur
is usually heard on the right side of the chest over the cranial part of the
heart.
Depending on the size of the hole and the amount of blood passing through
it, the pulmonary arteries and veins and the left atrium and ventricle are
subjected to an increased workload because of this extra volume of blood. If the
hole and the resulting shunt are small, the adverse effect on cardiac function
might be minimal, and the horse might be fully capable of engaging safely in
moderate physical activities without evidence of fatigue or shortness of
breath. If the hole is larger and the shunt is greater, there might be signs of
cardiac insufficiency with minimal exertion, and the horse could be very
limited in its athletic ability. The nature of the defect can usually be
confirmed using two-dimensional and Doppler echocardiography.
Patent Ductus Arteriosus
The ductus arteriosus is a large blood vessel connecting the fetal pulmonary
artery to the descending aorta, allowing blood from the right ventricle to
bypass the non-functioning lungs and be directed toward the abdomen and
placenta. In all mammals, the ductus constricts at or shortly after birth,
eliminating this fetal connection and allowing for the normal development of
the blood vessels in the lungs.
Unlike most other domestic animal species, persistent slight opening of the
ductus arteriosus is quite common in newborn foals. Because the pressure in the
aorta is higher than that in the pulmonary artery throughout the cardiac cycle,
blood flows through the ductus from the aorta to the pulmonary artery, and a
“continuous” murmur can be heard over the pulmonary artery on the left side of
the chest.
Closure of the ductus usually occurs within the first week of life and the
murmur disappears. If the ductus remains open beyond the first week, it is
called a persistent or patent ductus arteriosus. The resulting shunt might
cause blood volume overload in the pulmonary arteries and veins and the left
atrium and ventricle. Although slight patency (an open state) in the first week
is very common in foals, patency beyond the first week is rare.
Complex Defects
Congenital heart defects are uncommon in horses, but when they occur,
multiple or complex defects appear to be more common than in other species such
as dogs and cats. These can occur as combinations of embryologically unrelated defects,
or as recognized combinations such as tetralogy of Fallot (consisting of a
ventricularseptal defect, pulmonic stenosis, rightward malpositioning of the
origin of the aorta, and right ventricular hypertrophy/thickening) or truncus
arteriosus (consisting of a ventricular septal defect and a single large
arterial trunk exiting both ventricles).
In the most severe cases, there could be shunting of oxygen-poor, darker
venous blood from the right heart chambers to the left heart, bypassing the
lungs and causing cyanosis (a bluish color to the membranes of the mouth and
eyes) at rest or during exertion. These defects can be diagnosed accurately
only by X ray angiography or, more recently, two-dimensional and Doppler
echocardiography.
Heart surgery is rarely performed in horses. Congenital heart defects can
now be accurately diagnosed using sophisticated ultrasound imaging, but
treatment options are very limited for the types of defects horses tend to get.
Acquired Heart Disease
Generally speaking, acquired heart disease is relatively uncommon in horses,
although it is encountered slightly more frequently than congenital heart
defects. It occurs most often in horses older than five years and only
occasionally in younger horses. Degenerative changes affecting the heart
valves, myocardium (heart muscle), and lungs are associated with aging,
increasing in frequency with age.
The most commonly diagnosed conditions are heart rhythm irregularities and
leaks in one or more heart valves. The most common signs associated with heart
disease include a reduction in exercise capacity (exertional fatigue),
shortness of breath especially following exertion, or the detection of a heart
murmur, irregular heartbeat, or other audible abnormality in a horse without
other signs of illness. Identification of the electrical rhythm of the heart
requires recording of an electrocardiogram.
The normal resting heart rhythm of horses is usually slow (28 to 48 beats
per minute) and regular (called sinus rhythm). Many horses also have short pauses
in their resting heart rhythm caused by “dropped beats” (called second degree
AV block). These are considered to be normal if they disappear during exercise.
Atrial Fibrillation
Atrial fibrillation is an electrical disorder of the heart rhythm—also known
as an arrhythmia. There are different kinds of arrhythmias, but the most
commonly recognized one associated with diminished athletic performance or more
serious signs of cardiac insufficiency is atrial fibrillation. With this
arrhythmia, the normally regular organized atrial electrical waves become
irregular, disorganized, and chaotic, and the atria fail to contract normally.
This results in a very unpredictable irregular heartbeat.
Although atrial fibrillation often develops in horses with advanced structural
heart disease and atrial dilation, horses most often develop this arrhythmia
with minimal or no detectable additional signs of heart disease. Draft breeds
are more commonly affected. In such cases, signs of cardiac insufficiency are
usually not recognized at rest or with mild to moderate exertion, but become
apparent at more strenuous levels of exercise.
Accurate diagnosis of arrhythmias requires evaluation of an
electrocardiogram, where the lack of normal atrial waves and the very irregular
ventricular waves can be readily identified. Further evaluation of the
structure and mechanical function of the heart by echocardiography is also
recommended because the prognosis for treatment, recovery, and return to
previous activity levels is directly related to the presence or absence of
underlying mechanical cardiac dysfunction.
If there is little or no evidence of underlying cardiac dysfunction,
administration of oral or injectable drugs, especially quinidine, is often
successful at converting the arrhythmia to a normal rhythm. Most of these
horses are able to return to their previous levels of activity and performance,
although some experience one or more recurrences of the arrhythmia,
necessitating retreatment or retirement from strenuous activity. Quinidine can
become toxic to horses at higher dosages and occasionally treatment must be
suspended before conversion of the rhythm can occur. A newer catheter option
for cardioconversion exists for cases that do not respond to quinidine
treatment. This option requires general anesthesia and specialized equipment
and is still in the developmental stages.
If serious cardiac disease with atrial dilation is present, the prognosis
for functional recovery is poor and conversion of the arrhythmia is usually
unsuccessful or temporary. Treatment of the signs of cardiac insufficiency with
drugs such as digitalis and diuretics can be considered in selected cases where
little physical activity is expected.
Valvular Heart Disease
The most commonly recognized acquired structural heart disorders in horses
are degenerative valvular deformities. The process causes thickening and
deformity of valve leaflets. These defects result in incompetence and
insufficiency of one or more heart valves, associated heart murmurs, and
dilation of the chambers that must handle the extra regurgitated blood on
either side of the incompetent valve. If the valve leak is severe enough,
pressure in the veins leading to the affected side of the heart increases to
the point where fluid accumulation (edema) occurs.
Valvular disease is initially diagnosed by the detection of a heart murmur
during a physical examination. It is very important, however, to understand
that “innocent” murmurs are often heard in normal foals and adult horses. In
order to advise an owner or rider about the significance of any heart murmur,
it is critical to distinguish between a normal murmur and a pathologic murmur
of valvular regurgitation, and to assess the severity of any suspected valve
leaks. Two-dimensional and Doppler echocardiography are the most accurate and
least invasive methods to help make such determinations.
In general, mild to moderate valvular insufficiency in a horse without
reported signs of illness is compatible with continued use for mild to moderate
physical activity. More severe valvular disease, especially when it is
accompanied by obvious signs of cardiac insufficiency, atrial fibrillation, or
severe enlargement of the heart, is cause for a poor prognosis and a strong
recommendation against any riding or forced physical activity.
Myocardial Disease
Myocarditis is occasionally suspected in a horse that develops an arrhythmia
or other electrical disorder following an infectious disease such as strangles,
influenza, or an internal abscess. Toxic damage to the heart muscle can also
rarely occur as a result of severe dietary deficiency of vitamin E and
selenium, or as a result of ingesting the chemical monensin (usually from
cattle feed).
Vascular Disease
Horses are known to develop several types of disorders that affect primarily
their blood vessels. However, atherosclerosis—vascular disease associated with
high blood pressure, high cholesterol, and fats in humans—is exceptionally rare
in domestic animals, including horses.
Therefore, the consequences of this condition in humans—including heart
attack, stroke and other peripheral arterial disease—are also rare in horses.
The only common condition affecting the veins of horses is thrombophlebitis of
the jugular vein(s) caused by repeated jugular vein puncture, injection of
material outside the vein, or use of a jugular vein catheter. The resulting
chemical or physical irritation or infection in or around the vein causes
inflammation, swelling, and tenderness, followed by formation of a firm clot in
a small portion or long segment of the vein. Treatment involves removing the
cause and applying symptomatic treatment for discomfort and any associated
infection. Thrombophlebitis can occur in other large veins in the horse.
Several conditions can affect the systemic arteries in horses. The most
common condition is parasitic arteritis (inflammation of the walls of the
arteries) due to the vascular migration of the larval forms of the intestinal
parasite
Strongylus vulgaris. The resulting dilation and thrombosis
(and potential obstruction) usually occur at the origin of the large arteries
to the intestines, although other arteries might be affected. Fortunately, this
condition can usually be treated or prevented by an appropriate antiparasitic
drug treatment program. The incidence of this problem declined significantly
with the introduction of ivermectin dewormer in the 1980s.
The other most commonly recognized arterial disorder is called aorto-iliac
thrombosis. In this condition, a clot develops at the point where the abdominal
aorta branches toward the hind legs. The resulting restriction of blood flow to
the hind limbs can cause signs of lameness, stiffness, weakness, and abnormal
gait that develops during exercise and usually disappears at rest. This
condition is often progressive and rarely reversible, markedly limiting the
athletic uses of an affected horse.
Finally, degenerative changes in the wall of large arteries can weaken a
vessel and predispose it to rupture and bleeding. The most commonly reported
sites of such rupture are the root of the aorta in stallions and the uterine
artery in mares.
Summary
The causes of heart disease in horses are often multiple and difficult to
determine in individual cases. There is increasing evidence that genetic
background might play a major role in a horse’s susceptibility to developing
disease. Some heart muscle disorders have clearly been shown to have a major
genetic component, and familial or breed tendencies in some conditions strongly
suggest that genetics play a role in these conditions.
Unlike in humans, diet and exercise have not been shown to be factors in
heart disease in horses, since horses almost never develop atherosclerotic
vascular disease leading to stroke or heart attack. Almost all heart diseases,
except the congenital defects, tend to increase with age in horses, just as
they do in humans, dogs, and cats.
There are no known preventive strategies to reduce the likelihood of heart
disease in horses. However, owners of all animals should avoid inbreeding,
which might increase the risk of congenital heart defects. Veterinarians at the
University of California,
Davis, strongly urge owners not to
breed any animals with known congenital defects of any kind, including heart
defects. They also caution that it is probably wise to also avoid breeding
horses that have developed an acquired heart disorder relatively early in life,
as this might indicate an increased susceptibility for that condition in
offspring.
About the Author
Carla Wennberg, coach of the 2013 Intercollegiate Horse Show Association
reserve national champion western team from St. Andrews University in
Laurinburg, N.C., offers some advice to help you teach and reinforce
neck reining.
The
health of the planet is a big concern these days, and while it’s easy
to point the finger of blame toward big polluters such as factories,
refineries and concentrated animal feeding operations, individuals need
to own up to their part of the problem, too. Horse people in particular
have responsibilities we can’t shirk. Conserving our slice of nature,
however small or large, is important for our horses. There are several
inexpensive ways to make your existing stable and pasture more
eco-friendly without rebuilding your entire operation from scratch. 
2. Use Beneficial, Biological Measures
3. Grow Healthy Pastures