Normally S1, which is associated with closure of the atrioventricular valves, and S2, which is associated with closure of the semilunar valves, are audible. It is also normal to hear the S3 and S4 heart sounds in some horses. These low frequency heart sounds are accentuated with the bell of the stethoscope and occur during diastole. S3 is caused by rapid ventricular filling and S4 is caused by ventricular filling due to atrial contraction.
The “ lub” is the first heart sound, commonly termed S1, and is caused by turbulence caused by the closure of mitral and tricuspid valves at the start of systole. The second sound,” dub” or S2, is caused by the closure of aortic and pulmonic valves, marking the end of systole
In young people and athletes it is a normal phenomenon. In older individuals it indicates the presence of congestive heart failure. The third heart sound is caused by a sudden deceleration of blood flow into the left ventricle from the left atrium.
The fourth heart sound is produced by an increase in stiffness of the left ventricle due to scar tissue formation. This may be a manifestation of coronary heart disease. A fourth heart sound can also be caused by a greatly thickened left ventricular wall such as with essential hypertension or aortic stenosis.
The fourth heart sound (S4), when audible, is caused by vibration of the ventricular wall during atrial contraction. This sound is usually associated with a stiffened ventricle (low ventricular compliance), and therefore is heard in patients with ventricular hypertrophy, myocardial ischemia, or in older adults.
Diagnosing Heart Murmurs in Horses
The location of a murmur is where it is heard loudest, or the point of maximal intensity (PMI). This is generally described as left or right and apical or basilar. In horses, it may be possible to further differentiate the pulmonic valve area from the aortic valve area at the left heart base. Murmurs may also radiate to other areas of the thorax, but are softer there than at the PMI.
The timing of a murmur may be systolic, diastolic, or continuous. Since the period of diastole is usually longer than that of systole, the length of the murmur can help to determine the timing. It is also useful to palpate the carotid pulse while ausculting a murmur as a systolic murmur is coincident with the pulse and a diastolic murmur occurs without the pulse. The intensity or loudness of a murmur is graded on a scale of 1 to 6:
Very soft, focal murmur only detected in a quiet area after extended auscultation
Soft, focal murmur that is readily audible (softer than S1 and S2)
Moderately loud murmur with some radiation (similar intensity to S1 and S2)
Very loud murmur that radiates widely (louder than S1 and S2)
Very loud murmur with a palpable thrill
Very loud murmur with a thrill that is audible with the stethoscope off the chest wall
Causes of Heart Murmurs in Horses
Below is a list of possible causes of heart murmurs in horses with brief descriptions.
Aortic Regurgitation – A leak at the valve between the left ventricle and aorta usually caused by age related degeneration or infection of the valve
Mitrial Regurgitation – A leak at the valve between the left ventricle and atrium usually caused by age related degeneration or infection of the valve
Tricuspid Regurgitation – A leak at the valve between the right ventricle and atrium which may be seen in normal horses or may be caused by age related degeneration
Pulmonic Regurgitation – A leak at the valve between the right ventricle and pulmonary artery which may be seen in normal horses
**Physiologic Murmur **(low grade) – No heart abnormalities, normal blood flow is heard across the aortic, pulmonic valve, or occasionally the mitral valve
Ventricular Septal Defect – A congenital heart defect where there is a hole between the two ventricles (most common congenital heart defect of horses)
**PDA **(normal in first week of life) – An normal vessel in the developing fetus that connects the aorta and pulmonary artery, but should close shortly after birth
Aortic Stenosis (rare) – Congenital malformation of the aortic valve (between the left ventricle and aorta)
Pulmonic Stenosis (rare) – Congenital malformation of the pulmonic valve (between the right ventricle and pulmonary artery)
Aorto-cardiac fistula – acquired defect where a hole forms between the aorta just above the aortic valve and the right ventricle or atrium