The aortic valve opens when the heart contracts (systole), allowing oxygenated blood to leave the left ventricle and be distributed around the body. The valve closes at the end of systole to prevent blood returning to the left ventricle during heart relaxation (diastole). This occurs passively as the pressure in the left ventricle drops below the pressure in the aorta causing the valve to snap shut. The aortic valve has three leaflets or cusps, named after the coronary arteries that emerge from the aorta (sinus of Valsalva) supplying blood to the left and right side of the heart. These are the left coronary cusp, right coronary cusp, and the non-coronary cusp. The aortic valve is referred to as a semilunar valve because the leaflet margins are attached to the arterial wall in the shape of a half-moon. Dilation of the artery above the leaflets (arterial sinuses) include the coronary artery ostia. These dilations allow the valve to “hide” during opening to facilitate unrestricted flow of blood from the left ventricle during systole.
The aortic valve is a tricuspid (three cusps or leaflets) valve. Although extremely rare, quadricuspid aortic valve has been described in horses. Below is a probable quadracuspid aortic valve.
Valve disease
The aortic valve is the most common valve that undergoes pathologic change in horses. The aortic valve can fail in two ways. Firstly, failure of the valve to fully open, defined as stenosis of the valve resulting in increased work of the ventricles to move blood from the left ventricle into the aorta. In humans the narrowing can occur below, at, or above the aortic valve. All conditions of stenosis are rare in horses, although supravalvular aortic stenosis was recently reported in a middle aged Warmblood mare. Murmurs associated with valvular stenosis are systolic.
Secondly, and far more importantly, degenerative valve thickening and distortion results in failure of the valve to close effectively during heart relaxation (diastole). This results in regurgitation. Infection of the valve leaflets can also cause regurgitation. The consequence of reverse flow of blood from the arterial system into the left ventricle is overloading of the left ventricle and compensatory eccentric hypertrophy. This is extremely common in horses, and typically occurs in response to aging. Interestingly, it is the left coronary cusp that is almost always affected. It has been reported that approximately 50% of horses aged over 10 years have some degree of aortic valve insufficiency. The disease appears to be less common in small ponies and more common in males.
Physical examination
In most horses the murmur is not associated with overt clinical signs and is picked up by veterinarians who are examining the horse for other reasons. This can include annual health assessments, pre-purchase examinations, injury assessments or colic episodes, endurance rides, and routine dental examinations. I work with several veterinary dentists who are now highly proficient at detecting cardiac murmurs in their patients.
Palpation of the arterial pulse is an important tool in the assessment of aortic regurgitation severity. Detectable increases in peripheral arteries is more reliable than grading the loudness of the murmur. Horses with significant aortic insufficiency will have decreased diastolic pressure because blood leaks back into the left ventricle during diastole thereby leading to failure to hold pressure in the arterial system. Conversely, the systolic pressure is often increased as the ventricle beats harder in response increased volume. This is due to Frank-Starling’s Law. The net result is a large difference difference between systolic and diastolic pressures, producing bounding or hyperkinetic pulses. This can be verified using a tail cuff sphygmomanometer.
A difference between systolic and diastolic pressure of more than 60 mmHg reflects moderate to severe aortic valve regurgitation.
The Murmur
The murmur associated with aortic valve regurgitation can commonly be heard on both sides of the thorax but is significantly louder on the left side. It is best identified by auscultation in the 4th intercostal space on the left side of horse slightly beneath the point of the shoulder under the triceps muscle. At the start of diastole the velocity of the blood regurgitating into the ventricle is highest, and it gradually trails off as diastole continues. This creates the classical diastolic decrescendo murmur of aortic regurgitation. The murmurs can produce a range of unusual sounds, from geese honking through to wheezing. This is because the returning blood can cause the open mitral valve and the septum to vibrate.
A left sided holodiastolic murmur is assumed to be aortic valve regurgitation unless proven otherwise.
The Consequences
The left ventricle will adapt over time to the increased volume of blood it needs to deal with, leading to compensatory eccentric hypertrophy. The normal diastolic inflow of oxygenated blood from the lungs via the left atrium is superimposed by blood leaking back into the left ventricle through the faulty aortic valve. This increased volume (volume overload) causes tension on the ventricular wall resulting in greater contractility and increased stroke volume. This is described by Frank-Starling’s Law. This also can cause the aortic root to dilate. Another adaptation is an increase in left ventricular size and change in ventricular shape. The top of the left ventricle is restricted by the mitral annulus, a fibrous ring where the leaflets of the mitral valve attach. The result is a change in shape from a cone or funnel to a globe, hence the term “globoid” shape.
If the volume overload of the left ventricle continues then ultimately the fibrous annulus of the mitral valve will begin to stretch, leading to secondary leakage across the mitral valve during systole. This results in a second systolic cardiac murmur and left atrial dilation. (See section on mitral valve regurgitation)
Unfortunately, death in horses with severe aortic valve regurgitation can be due to the secondary mitral valve regurgitation. As regurgitation increases and the limits of left ventricular remodelling have been reached the heart begins to fail, leading to exercise intolerance, increased respiratory rate, and arrhythmias.
Assessment
Assessment of a moderate to severe murmur should be pursued if the horse continues to be ridden. In general this applies to murmurs that are at or greater than 3/6. This not only provides a baseline assessment of severity but also can provide reassurance to riders regarding safety for both horse and rider.
Initial assessment is with ultrasound. There are several items to be considered, including:
- Enlargement of the left ventricle, left atrium and aorta
- Pre-ejection period and left ventricular ejection time
- Diastolic runoff in the aorta (aortic diameter early diastole – aortic diameter late diastole)
- Size of the regurgitant jet
- Continuous wave doppler to assess duration, pressure half time, and velocity time integral.
Assessment of diastolic function is critical in assessing disease progression. This can be achieved using advanced ultrasound techniques, such as spectral or colour tissue doppler imaging. Essentially we compare the peak early diastolic velocity (E wave) to the peak late diastolic velocity (A wave) of the left ventricular free wall. In horses with aortic regurgitation we see a lower E wave velocity and a higher A wave velocity.
Horses with aortic regurgitation can usually continue to be ridden. However animals with moderate regurgitation that are still being ridden ideally should be assessed with an exercise ECG. At rest these horses have a more forceful heart contraction than normal in order to maintain adequate blood flow to the periphery (increased stroke volume). During exercise contractility of the ventricle increases further potentially cause an oxygen deficit to the myocardium and arrhythmias.
Prognosis and follow-up
As mentioned above aortic valve regurgitation is common in horses older than 10 years of age. In many of these animals progression may be negligible or minimal, and continued exercise is therefore permissible. Biannual reassessment of AR is recommended until it is established that the condition is stable. Palpation of peripheral pulse pressures is more reflective of disease progression than auscultation.