The cardiovascular system

Chapter 8

The cardiovascular system

Chapter contents


In horses, the prevalence of cardiovascular disease is low although it can cause poor performance (q.v.) or cardiac failure and is frequently implicated in sudden death. Cardiovascular diseases can be classified on anatomic, pathologic or functional criteria. A functional approach serves the clinician well as it defines clinical manifestations and, often, appropriate management. A functional classification of equine heart disease is listed in Table 8.1.

Volume overload is the most common type of heart failure in the horse and occurs when valvular regurgitation or an intra- or extra-cardiac shunt leads to one or both ventricles receiving excessive quantities of blood. There is activation of the autonomic nervous system and the renin–angiotensin–adrenergic system, progressive ventricular dilatation and, in response, ventricular hypertrophy. Ultimately cardiac dysfunction and irreversible changes within the myocardium ensue.

Pressure overload occurs when there is an anatomic or physiologic obstruction impeding ventricular ejection. In response to increased afterload, the myocardial cells in the affected ventricle increase in diameter as myofibrils are laid down in parallel within them. Myocardial failure occurs as cellular function deteriorates. Lesions resulting in pressure overload such as aortic or pulmonic stenosis are uncommon in horses and this form of heart failure is rare.

Both volume and pressure overload ultimately lead to failure of the affected ventricle. Accumulation of fluid and congestion in the vascular compartments develops “upstream” to the affected ventricle. Left-sided heart failure leads to congestion in the pulmonary circulation, pulmonary edema and respiratory signs ( Box 8.1). Right-sided heart failure leads to fluid accumulation in the systemic circulation, signs of venous distension and ventral edema. There can be effusions in the pleural and peritoneal cavities and, rarely, bowel edema and diarrhea ( Box 8.1). Usually, failure of one ventricle will lead to failure of the other, so that biventricular failure ensues and the clinical presentation includes both left- and right-sided signs ( Box 8.1).

Primary myocardial (pump) failure implies that there is a primary defect in myocardial contractility leading to a reduction in stroke volume. The low cardiac output reduces perfusion and can lead to multiple organ dysfunction (see Box 8.1). This is seen in association with severe cardiomyopathies. Clinical signs produced by low cardiac output will also occur as myocardial failure develops in volume or pressure overload. Where the venous return is severely compromised, e.g. by effusive pericarditis (q.v.), low output heart failure also occurs.



The horse has four heart sounds, although in some horses only two or three may be audible.

The first heart sound occurs when the left and right atrioventricular valves close at the onset of mechanical systole. It is a loud and low-pitched sound, audible throughout the cardiac silhouette, on both sides of the thorax. Its point of maximal intensity is over the left apex and it is followed immediately by the arterial pulse. Splitting of the first heart sound occurs uncommonly in normal horses.

The second heart sound is coincident with closure of the semilunar valves and reversal of blood flow in the great arteries at the conclusion of ventricular systole. The aortic and pulmonic valves may not close synchronously and physiologic splitting of the second heart sound is often found.

The third heart sound is not present in every horse. It is associated with termination of rapid ventricular filling, and it is usually heard best around the left atrioventricular (AV) valve area.

The fourth, or atrial, sound is almost always audible. It is produced by atrial systole with flow of blood from the atrium to the ventricle. It is usually heard best dorsally over the heart base on the left side. The fourth heart sound occurs immediately before the first heart sound so that it can be mistaken for splitting of the first sound.

The horse’s heart is almost totally located under the muscles of the shoulder in the third, fourth and fifth intercostal spaces. The left AV valve is auscultated in the left fifth intercostal space, at the caudal border of the triceps muscle and midway between the points of the shoulder and the elbow; sounds associated with the aortic valve are heard in the left fourth intercostal space deep to the triceps muscle just ventral to the point of the shoulder; and sounds associated with the pulmonic valve are in the left third intercostal space at the level of the point of the elbow and deep to the triceps muscle. In the right hemithorax, the right AV valve is auscultated in the fourth intercostal space at the midpoint between the shoulder and the sternum.


Cardiac murmurs are found in up to 80% of normal Thoroughbreds, and are also frequently auscultated in other breeds of horses and ponies. They are prolonged sounds occurring during periods of the cardiac cycle that are usually silent. Murmurs arise when normal laminar flow is disrupted. Physiologic (functional) murmurs are present where there is no cardiac pathology, particularly associated with left ventricular ejection. Although often found in normal horses, they also occur if blood viscosity is lowered in anemia or hypoproteinemia. However, because cardiac murmurs can accompany congenital cardiac disease or valvular insufficiency, it is important that physiologic murmurs are distinguished from pathologic murmurs.

The criteria used to describe murmurs are described in Box 8.2. If all five characteristics of a murmur are considered carefully, the clinician is usually able to formulate a differential diagnosis and differentiate those murmurs that are physiologic (functional) from those that indicate that there is cardiac pathology and justify further diagnostic evaluation.

Box 8.2   Parameters used to describe and classify cardiac murmurs in the horse

1. Timing

Systolic: Between the first and second heart sounds

Diastolic: Between the second and first heart sounds

Pan: Obscuring the heart sounds

Holo: Begins/ends immediately after/before the heart sounds

Early or mid: Distinct from the heart sounds

2. Intensity

Grade 1: A soft murmur audible only after careful auscultation in a localized area of the thorax

Grade 2: A soft murmur that is clearly audible after a few seconds of auscultation

Grade 3: A moderately loud murmur that is audible over a wide area of the thorax, with no precordial thrill

Grade 4: A loud murmur that is immediately audible and heard over a wide area of the thorax, with no precordial thrill

Grade 5: The loudest murmur that becomes inaudible when the stethoscope is removed from direct contact with the thorax, which is always accompanied by a precordial thrill

Grade 6: A loud murmur that can still be heard when the stethoscope is removed from direct contact with the thoracic wall and is always accompanied by a precordial thrill

3. Quality

Frequency, e.g. harsh, coarse, soft, musical or honking

Shape, e.g. band-shaped, crescendo–decrescendo, decrescendo or machinery

4. Point of maximal intensity

Left AV valve: Left fifth intercostal space, midway between the levels of the points of the shoulder and elbow

Aortic valve: Left fourth intercostal space, at the level of the point of the shoulder

Pulmonic valve: Left third intercostal space, at the level of the point of the elbow

Right AV valve: Right fourth intercostal space, midway between the levels of the point of the shoulder and elbow

5. Radiation

Dorsal, caudo-dorsal, etc.: The direction of radiation corresponds to the direction of turbulent blood flow

Note: The caudal border of the triceps muscle overlies the fifth intercostal space.

Functional murmurs are quieter (grade 3 or less), soft and localized, do not have precordial thrills and do not obscure the heart sounds. Functional murmurs in systole caused by ventricular ejection are localized to the heart base in the third or fourth intercostal space; they are usually grade 3 or less and have a soft, blowing quality. Functional diastolic murmurs occur in early or mid diastole, are frequently musical or squeaky, and are auscultated over the left heart base or left AV valve area. The characteristics of functional murmurs and those associated with a variety of forms of cardiac pathology are compared in Table 8.2.


There are a wide variety of diagnostic aids available for use in the evaluation of equine cardiovascular disease. The selection of appropriate diagnostic procedures depends on the clinical presentation and the differential diagnosis. Table 8.3 summarizes some of the diagnostic techniques that are used in equine cardiology. Echocardiography is the principal means of evaluation of horses with cardiac murmurs where valvular pathology or congenital cardiac diseases are suspected. Normal values for various M mode echocardiographic parameters are listed in Table 8.4.

Electrocardiography, including radiotelemetric and ambulatory electrocardiography, is the principal means of investigation of arrhythmias. Non-specific aids are used to investigate the etiology of certain cardiac diseases and to assess the other body systems that may be affected secondary to cardiac disease.


There are few known causes of primary myocardial disease (cardiomyopathy) in the horse. The ionophore antibiotics (monensin, salinomycin), common additives in cattle and poultry feed, produce myocardial necrosis in the horse. Other proposed etiologies include viral and immune-mediated myocarditis. Nutritional deficiencies are common causes of cardiomyopathy in other species and, in the horse, vitamin E–selenium deficiency produces cardiomyopathy as well as skeletal muscle disease (white muscle disease) (q.v.).

Myocardial dysfunction is often secondary, resulting from extracardiac systemic disease or drug reactions. Significant reductions in myocardial contractility have been noted in horses with lymphosarcoma (q.v.) (without cardiac infiltration), electrolyte disturbances, severe metabolic derangements, endotoxemia and septicemia.

Myocardial disease is characterized by cardiac arrhythmias and/or ventricular dysfunction with ventricular dilatation and reduced myocardial contractility. The presenting signs depend on the severity of myocardial dysfunction and range from varying degrees of exercise intolerance to signs of congestive or low output heart failure in severe cases (see Box 8.1).

The diagnostic procedures that are useful in investigation of myocardial disease are listed in Table 8.3. Echocardiography is used to assess ventricular function while electrocardiography is used to categorize arrhythmias and can reveal S-T segment changes or changes in the duration of complexes. Prolongation of ECG intervals may be associated with primary myocardial disease, however this should alert the clinician to the possibility of electrolyte or metabolic imbalances.

Measurement of serum concentrations of cardiac troponins and CK-MB and HBDH (cardiac isoenzymes of creatinine kinase [CK] and lactate dehydrogenase [LDH], respectively) can be helpful as increases indicate active or recent myocardial damage. However, normal results have been found in horses with significant myocardial necrosis and in chronic myocardial disease. In addition, concurrent damage to other organ systems must be taken into consideration because CK-MB and HBDH are not cardiac-specific in the horse.

Feed analysis for ionophore antibiotics, assessment of acute and convalescent viral titers (e.g. influenza, herpesvirus [q.v.]) or virus isolation is appropriate for determining the etiology of myocarditis in individual cases. Myocardial biopsy has not been attempted successfully in the horse.

Appropriate therapy depends on the presence and significance of any arrhythmias, and the degree of myocardial dysfunction. Certainly, any electrolyte or metabolic imbalances, or systemic disease should be corrected or excluded as a cause of myocardial disease (q.v. arrhythmias for further details of management of those horses with clinically significant arrhythmias).

Therapy for myocardial dysfunction includes rest, diuretics (furosemide) and, if the horse is showing signs of heart failure, positive inotropic agents such as digoxin or, in emergency situations, dobutamine. Digoxin is contraindicated in ionophore poisoning. Activated charcoal or mineral oil may be beneficial in horses that are known to have ingested ionophore very recently.

The prognosis for myocardial disease varies, and is based on the severity of the myocardial insult, the extent of irreversible changes and the degree of ventricular dilatation. Although horses with arrhythmias are often tentatively diagnosed as suffering from “myocarditis” or “myocardial disease”, these horses can have very localized inflammation of the conducting tissue alone. Horses with a pathologic arrhythmia as the only abnormality have a better prognosis than those with accompanying myocardial dysfunction and ventricular dilatation. Thus, echocardiographic assessment of ventricular function is the most accurate means of formulating a prognosis.

Horses with recent onset of myocardial dysfunction, including those with ionophore poisoning, may recover completely and return to their previous level of performance. However, if the damage is extensive, irreversible myocardial fibrosis will ensue. These horses may survive the cardiac disease, but their athletic capability can be markedly reduced. In other cases, cardiac dilatation progresses over weeks to years and the animal eventually dies of congestive heart failure.


Cardiac arrhythmias are common in horses, both at rest and during exercise. They are often physiologic in nature, produced by normal neurohumoral mechanisms as a means of regulating the heart rate and modifying blood pressure. However, arrhythmias can also be pathologic, signifying myocardial disease. The equine clinician is frequently required to determine the significance of arrhythmias. A methodical approach allows precise characterization of the rhythm disturbance and its origin and thus provides the basis for assessment of the importance of the arrhythmia.

Diagnosis of arrhythmias

Careful cardiac auscultation will enable the detection of most cardiac arrhythmias in the horse and in some instances a tentative diagnosis can be made from auscultation alone. However, an ECG is required to diagnose definitively the type of arrhythmia present, and should always be performed if a pathologic arrhythmia is suspected (see Table 8.3). The base-apex lead is used routinely in the horse for assessment of the heart rate and rhythm because movement artifacts are minimized and waveforms are easily visualized.

For the base-apex lead placement is as follows:

Multiple leads can be necessary for the accurate diagnosis of some complex arrhythmias and the standard limb leads used in other species are adequate in the horse. PR, QRS and QT interval determinations are most commonly calculated from lead II ( Table 8.5).

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Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on The cardiovascular system
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