A Grade 6 murmur is one that you can actually hear even with your stethoscope off of the chest wall by about an inch. Though
they are loud, they are not necessarily indicative of severe disease. For example, one can have loud, buzzing harmonic murmurs
of aortic regurgitation in a horse that might be graded 5-6/6, but they may be almost incidental, minor, trivial disease.
Therefore a loud murmur does not necessarily reflect a severe condition. The grading system is mainly a communications scale,
so that different doctors can communicate what they are hearing.
DVD in dogs versus DVD in horses
Mitral and aortic insufficiency
From human medicine, a normal mitral valve consists of two thin leaflets, located between the left atrium and the left ventricle
of the heart. These leaflets, shaped like parachutes, are attached to the inner wall of the left ventricle by a series of
strings called chordae. When the ventricles contract, the mitral valve leaflets close snugly, preventing backflow of blood
from the left ventricle into the left atrium. When the ventricles relax, the valves open to allow oxygenated blood from the
lungs to fill the left ventricle. In human patients with mitral valve prolapse, the leaflets and chordae are affected by a
process called myxomatous degeneration. In myxomatous degeneration, the structural protein, collagen, forms abnormally and
causes thickening, enlargement and redundancy of the leaflets and chordae. When the ventricles contract, the redundant leaflets
prolapse (flop backwards) into the left atrium, sometimes allowing leakage of blood through the valve opening, i.e. mitral
Although its etiology is unknown, degenerative lesions resulting in regurgitation of the atrioventricular (mitral, tricuspid)
and semilunar (aortic, pulmonary) valves are the most common cause of valvular disease in older horses, though the mitral
and aortic valves are affected more frequently than the tricuspid and pulmonary valves, Sage says. Lesions involve "degeneration
of the collagen in the fibrosa of the valve and proliferation of loose fibroelastic tissue in the spongiosa of the valve.
The lesions also have histiocyte, lymphocyte and fibroblast infiltration of the superficial layers of the endocardium. Degenerative
lesions appear as generalized or discrete fibrous thickening and rounding of the free borders of the valve leaflets that may
NH state veterinarian retires after 19 years
Reef and Sage note that murmurs of mitral regurgitation (MR) are the most commonly acquired valve lesions associated with
underlying cardiac disease in horses that result in clinical signs. They are usually holosystolic or pansystolic, band shaped,
grade 3/6 or louder; coarse or honking; with their point of maximal intensity at the left AV valve area radiating dorsally
and caudally. The clinical signs are increased respiratory rate and effort during exercise, excessive sweating, prolonged
recovery time after exercise, coughing, and atrial fibrillation — exercise intolerance. Post exercise, these signs may take
longer to recover to normal levels. At rest, horses with MR may not show clinical signs other than the murmur.
Echocardiography is used to further diagnose mitral regurgitation, quantify its severity and determine a prognosis. Spectral
and color flow Doppler are used to map the size and location of the regurgitant jet. According to Sage, if the jet is small,
of brief duration and limited to a small area behind the valve leaflets, then it is thought to be a normal physiological finding
and does not represent a manifestation of subclinical valvular disease. The regurgitant jet seen in horses with cardiac murmurs
compatible with mitral regurgitation seen on color flow Doppler is longer in duration than normal horses but similar in length,
width, area and time of onset.