written by: Mark D. Kittleson, DVM, PhD, Diplomate ACVIM (Cardiology)

The Heart

The feline heart, like the human heart, is a dual pump consisting of the left heart and right heart.

Venous (deoxygenated) blood is returned from the cat's circulatory system to the right side of the heart, where the right ventricle pumps it through the pulmonary arteries to the lungs for oxygenation.

The left side of the heart receives the oxygenated blood from the lungs and the left ventricle pumps it into the aorta for circulation throughout the cat's body.




Each side of the heart has an upper chamber, called an atrium, and a lower, main pumping chamber, called a ventricle.
The tricuspid valve prevents blood from flowing backward from the right ventricle to the right atrium when the ventricle contracts. The mitral valve performs the same function on the left side of the heart.

Papillary muscles in the ventricular chambers connect to these valves via stringlike fibers called chordae tendineae; these structures prevent the valves from being pushed backward into the atria when the ventricles contract.




Hypertrophic cardiomyopathy is a heart (cardio-) muscle disease (myopathy). There are several forms of heart muscle disease seen in cats but with this cardiomyopathy the muscular walls of the left ventricle thicken (hypertrophy) on their own due to an inherent disease in the heart muscle.

The left ventricular walls may also hypertrophy secondary to (as the result of) other diseases (e.g., systemic hypertension, hyperthryoidism), which can make the diagnosis difficult at times.

Hypertrophic cardiomyopathy (HCM) is diagnosed when thickening of the left ventricular walls is diagnosed using cardiac ultrasound (i.e., echocardiography) and any other disease that could cause hypertrophy is excluded using other diagnostic tests. Along with the thickening of the left ventricular walls, the papillary muscles (small muscles within the heart that anchor some of the heart valves) are also almost always enlarged. Papillary muscle enlargement can cause an abnormality in mitral valve function called systolic anterior motion (SAM) of the mitral valve that may precede, as well as accompany, significant ventricular wall hypertrophy. SAM is the usual cause of a heart murmur in a cat with HCM.

As HCM progresses, it can alter the structure of the heart and impair its functioning in several ways:

Left ventricle:

  • Ventricular chamber size may be reduced, making it unable to fill with as much blood
  • Ventricular wall stiffness usually increases, which:
    • impairs the ability of the ventricle to relax. This prevents it from filling efficiently, increasing pressure in the ventricle during relaxation (diastole). This then causes blood to back up into the vessels of the lungs causing subsequent leakage of fluid from the capillaries into and around the lungs (pulmonary edema and pleural effusion, respectively). This is congestive heart failure.
    • may cause low frequency vibrations that can be heard as an extra heart sound (a gallop rhythm) in some HCM patients

When the left ventricle is unable to fill with a normal volume of blood, it follows that less than a normal amount of blood is pumped out to the body with each heartbeat. If the blood supply to other vital organs is inadequate, the heart rate may increase as the body attempts to compensate. If the blood flow to the kidneys is sufficiently decreased over time, the release of a hormone that increases blood volume may be stimulated, which in turn can increase the filling pressure on the left side of the heart and contribute to congestive heart failure.

Left atrium:

The left atrium may enlarge due to increased pressure caused by the stiffened left ventricle's inability to fill normally. Atrial enlargement can slow blood flow going through it, which may in turn cause blood clots to form in the atrium. Clots that dislodge are pumped into the systemic circulatory system where they become lodged and so block the flow of blood. Rear leg paralysis, a classic example, occurs when a large clot lodges where the descending aorta (the major artery to the body) branches to go to the rear legs. This situation is commonly referred to as a saddle thrombus or, more appropriately, thromboembolus. Smaller clots can go many places in the body to cause damage.

Mitral valve:

  • May be pulled forward (systolic anterior motion, or SAM, of the mitral valve) in the left ventricular chamber, partially obstructing the outflow path to the aorta, which carries blood out of the left ventricle to the body.
  • May, at the same time, be distorted such that blood is allowed to flow backward, or regurgitate, into the left atrium. When the valve opens during the relaxation phase (diastole) to allow filling of the left ventricle, blood might then be dumped suddenly into the ventricle. This may contribute to the vibrations that cause a gallop rhythm (an extra heart sound).
  • This distortion or change in valve position causes a systolic (during contraction) heart murmur. It is common for this murmur to be dynamic (i.e., change over time with changes in the stimulation of the heart).

Copyright © 1997-2009 Mark D. Kittleson, DVM, PhD, Diplomate ACVIM (Cardiology)