The mitral valve controls the flow of blood through the left side of the heart, before it is pumped around the rest of the body in the aorta. It prevents the blood from re-entering the left atrium.
In some dogs, as they get older, this valve becomes thickened and leaky. This means that when the heart contracts, blood goes back into the left atrium, as well as into the aorta. This causes turbulence in the blood flow and can be heart through a stethoscope as a heart murmur. The worse the back flow of blood, the louder the murmur will become. Murmurs are graded from 1-6 with 1 being barely audible and 6 being able to be heard without a stethoscope.
Mitral valve disease is particularly common in small breed dogs, with Cavalier King Charles Spaniels being highly affected (most dogs will develop the condition, many will have an audible heart murmur before 10 years of age).
As the disease progresses, the pressure within the left atrium increases as more and more blood in pushed back. This will lead the atrium to increase in size – this increase in size is important in deciding when to start treatment.
Congestive heart failure happens when not enough blood is being pumped into the aorta because too much of it is pushed back into the left atrium instead. This leads to an increase in pressure within the lungs, which causes fluid to leak out into the lung tissue. This causes breathing difficulties and coughing. The lack of blood flow to the rest of the body causes weakness and in severe cases, organ failure.
Treatment is aimed at:
- improving the function of the heart using pimobendan
- reducing the pressure in the blood vessels which the heart is working against using an ACE-inhibitor such as benazepril
- reducing build-up of fluid in the lungs using diuretics such as furosemide or spironolactone