A heart attack in people is a condition where one or more of the blood vessels supplying the heart become occluded so that part of the heart muscle dies leading to sudden and potentially fatal heart failure.  This condition very rarely happens in dogs and cats.

In dogs and cats there are three common conditions that can lead to heart failure and death.

A. The valves start to leak making the heart inefficient leading to “failure”.

The dog and cat heart consists of four chambers – two smaller atria (the top of the heart) and two larger ventricles (the chambers which serve to pump blood out to the lung and body) that are separated by valves (see Figure 1).  When the valves become leaky, the one-way flow of the blood through the heart gets disrupted so that the heart pumps less blood out to the body with each beat because some blood gets pumped backwards through the leaky valve.  This ultimately causes the heart to enlarge and fail.

B. The coordinated beating of the heart gets disrupted so that it beats irregularly causing the heart to fail.

The “nervous system” of the heart that allows the heart to have a coordinated heart beat is called the conduction system of the heart.  This system begins at the pacemaker that is located in the wall of the right atrium.  When the pacemaker fires, electricity emitted from the pacemaker follows a specific path causing the heart muscle to contract.  Initially the electricity follows a path causing the atria to contract which fills the ventricles.  From there, the path goes through the area between the atria and the ventricles called the atrioventricular node.  Here the electricity is slowed to allow time for the ventricles to fill as the atria contract.  The electrical impulse then moves out through the muscular septum between the ventricles to the ventricular walls causing them to contract pushing blood out to the body.  When the conduction system becomes diseased, the heart beats irregularly in an uncoordinated fashion that is too fast or too slow causing the heart to fail.

 C. The heart muscle is unable to relax or contract properly causing it to fail.

The heart muscle must be able to relax to fill, and to contract to push blood out to the body.  Different diseases occur that prevent the heart muscle from being able to relax properly so that the ventricles can completely fill.  When a smaller than normal volume of blood fills the ventricle, the heart has to beat twice as fast to push out the correct volume of blood, eventually leading to heart failure.

When the heart can properly fill, but the heart muscle is so weak that it cannot push out the proper volume of blood from the ventricles, then heart failure occurs.  In either circumstance, being unable to fill, or being unable to contract, heart failure occurs.