Some dogs will be born with heart problems (congenital heart disease) but this article will focus on diseases that dogs get as they get older (acquired heart disease).
The heart
In order to understand heart disease, it is important to understand how the heart works. The heart receives blood from the body through from the biggest vein, the vena cava. The first chamber it enters is the right atrium. The right atrium pumps the blood into the right ventricle, via a valve called the tricuspid valve, which stops the blood leaking back the way it came. The right ventricle pumps blood around the lungs, via a blood vessel called the pulmonary artery. The pulmonary veins take blood away from the lungs, where it re-enters a chamber of the heart called the left atrium. The left atrium pumps blood though the mitral valve into the left ventricle, which then pumps blood to the rest of the body via the aorta.
With each beat of the heart, the two atria contract together, then the two ventricles, after which the heart relaxes while it refills.
Disease and failure
There is a difference between heart disease and heart failure. In heart disease, the heart has some sort of problem, but it may still function well. In heart failure, the heart is no longer working well enough for all the body's needs to be addressed. Forward failure refers to a lack of sufficient blood flow to the body, which can lead to weakness and collapse. Backward failure refers to a backing up of blood in the veins, which can lead to fluid leaking out into the lungs and other tissues.
What can go wrong?
The two most common acquired heart diseases in dogs are endocardiosis and dilated cardiomyopathy. In both these diseases, but for different reasons, blood leaks back the way it came from the left ventricle into the left atrium. The left atrium gets bigger as a result, which can tickle the trachea (windpipe), causing a cough. The blood from the left atrium backs up into the lungs causing fluid to leak into the lungs (pulmonary oedema) which can also cause a cough or breathing difficulties. This is known as left sided heart failure, because it is the left ventricle and left atrium that aren't functioning properly. Right sided heart failure because of problems in the right ventricle and right atrium can lead to a build up of fluid in the main circulation. Fluid can leak out into the abdomen (ascites) or into the space around the lungs (pleural effusion). Another heart disease called pericardial effusion, in which fluid builds up around the heart and squashes it, also leads to right sided heart failure.
Endocardiosis
Endocardiosis is also known as myxomatous degeneration of the mitral valve. With age, particularly in small breeds (the Cavalier King Charles Spaniel is particularly prone), but sometimes in large breed dogs too, the valve can degenerate (it looks like a shrivelled leaf). This causes the valve to leak. A small leak usually doesn't cause a problem, but the leak tends to get worse over age. Although the tricuspid valve can also be affected, the signs of disease due to the leak in the mitral valve disease are usually worse, leading to coughing and breathing difficulties because of fluid building up in the lungs. Also the natural pacemaker of the heart is found in the left atrium, so as it stretches, it can misfire, causing an irregular heart beat (arrhythmia).
Dilated cardiomyopathy
In this condition, the heart muscle becomes weaker over time. This leads to forward failure, in which the heart is not pumping blood around the body with sufficient force. This results in weakness. Backward failure also occurs, because insufficient blood is moved on to cope with the blood entering the heart. Both sides of the heart are usually affected in this condition., so signs of left and right-sided heart failure can occur. In addition, the heart muscle, which conducts the electrical impulse to tell the heart to beat, is disrupted, and this can also lead to arrhythmias. The arrhythmias seen in dilated cardiomyopathy tend to be more serious than those seen in endocardiosis.
Pericardial effusion
In this condition, fluid builds up between the heart, and the tough inelastic sac that surrounds it. This can be due to a tumour in the heart, or may be "idiopathic" which means the cause is not known. Although the heart itself is healthy, it can't function properly because the pressure in the pericardial space squashes it. This leads to forward failure, causing signs of weakness, and also a predominantly right sided backward failure.
Heart disease is a common condition in older dogs. Although it is a serious cause of ill health, there are some very effective treatments available. The next article will discuss how heart disease in dogs is treated.
Further reading
- Hearty Dog
- Abbot, Topics in Cardiology (2010)
Alex Gough - I am a qualified veterinary surgeon with 15 years experience, the last 9 of which have been working in referrals, seeing cases sent to be ...
