The Dog's Heart

Shane Jackson MA, Vet MB, MRCVS



We all know that the heart can be thought of as a pump, pushing blood around the body.  It can be divided into left and right side; the left side accepts oxygenated blood from the lungs and pumps this to the body and the brain, whereas deoxygenated blood enters the right side of the heart and is pumped to the lungs.  The right side is thought of as a low pressure system since the blood has to go a relatively short distance till it is back at the heart, whereas the left side is a high pressure system.  Consequently the left side is bigger and stronger than the right.  There are four chambers to the heart: left and right atria and left and right ventricles.  Heart valves are found between each atrium and ventricle and at the exit of each ventricle as the blood is starting its journey to either the body or the lungs.  The valves prevent blood flowing in the wrong direction as the heart contracts.

Normally the only audible sound of the heart is the classic ‘lub-dub’ of the valves closing with each beat.  However, when disease is present, of whatever form, the smooth flow of the blood is disturbed.  Turbulence is produced in the fluid and this becomes audible, often drowning out the normal heart sounds: the ‘lub-dub’ becomes ‘whoosh-whoosh’.  The loudness, the place over the heart where the murmur can be heard best and the timing of the murmur in the heart beat cycle will vary depending on the type of heart disease present.

Most murmurs are acquired, usually later in life, but some dogs have congenital murmurs.  These result from developmental defects while the pup was growing in the uterus, or within a few days of birth.  Congenital heart murmurs can produce very severe symptoms, such as sudden death (seen for example in aortic stenosis – a fibrous ring around the valve leading from the left ventricle to the body), or as poorly dogs who can lead a normal life if treated with care (such as pulmonary stenosis which is a malformation of the valve leading to the lungs from the right ventricle).

Of acquired disorders, the vast majority result either from chronic disease of the valves themselves, or from disease of the heart muscle, or a mixture of the two.  The commonest form of valvular problem is thickening of the valves or endocardiosis.  This is a progressive change which often starts early in life, though mild signs may not be noticed until the dog is much older.  In the period between changes to the heart valves starting and signs becoming noticeable, the heart and the circulation are able to adapt and change to compensate.  The precise reasons for the onset of the changes is unclear, though smaller breeds are often affected, indicating a breed predisposition.  The left side of the heart is more commonly damaged but any valve is at risk.  Another valve problem arises when blood borne infections, originating potentially from septic focus in the body, settle on the lining of the heart leading to inflammation.  The left side valves are more commonly affected.

The causes of heart murmur (and there are many more causes than I have mentioned above) will have a major influence on the progression of the disease and the severity of the symptoms.  Where a murmur is present and symptoms have been noticed, the dog may be said to be in heart failure. Left sided failure is graded into stages.  In the mildest form, very few symptoms are seen, even when the dog is exercised, but breathing difficulties, coughing and exercise intolerance become more noticeable, even at rest, as the failure becomes more serious.  In the severest cases, dogs are at risk of sudden death, but the progression of the disease can take many years.  Sometimes the progression is so slow that the more severe symptoms are never seen.  In right sided heart failure accumulation of fluid in the abdomen producing a pot belly is the usual feature.  There may be spleen and liver enlargement and the pressure of the fluid on the diaphragm can lead to breathlessness.