The dog heart is very similar to a human heart with 4 chambers (2 atria and 2 ventricles). Blood inside the heart flows from the atria to the ventricles below. The heart valves prevent blood going back up the way it just came from i.e. they work as a one way valve.
The 2 small thin walled chambers of the heart (atria) sit above the 2 large chambers (ventricles)
The right side receives oxygen poor blood from the body and pumps it out to the lungs to have oxygen placed in it (a low pressure setup as the lungs are "next door").
The left side receives oxygen rich blood from the lungs (again a low pressure environment as the lungs are "next door"). It then has to pump it all around the body from the head to the tip of the tail. To be able to pump blood that far, the left ventricle has extra strong and thickened muscle walls.
The most common valve problems we see are damaged left (mitral) and/or right atrio-ventricular (AV) valves.
Right AV valve disease
The right AV valve lies between the right atrium and right ventricle. Oxygen poor blood from the body arrives at the heart's right atrium via the cranial and caudal vena cavas (very large internal veins). It then enters the right atrium and flows through the open right AV valve into the right ventricle.
The right AV valve closes and when the right ventricle contracts, the oxygen poor blood is pumped out to the lungs via the pulmonary arteries connected to the right ventricle.
Diseased right AV valves "leak", so some of the oxygen poor blood that normally goes to the lungs is pumped back up through the leaky right AV valve into the right atrium.
The oxygen poor blood returning to the heart is met with some resistance as the right atrium chamber already has some "leaked" blood in it. This causes a "back log" of blood in the vena cavas trying to get into the heart.
As a result, organs start to swell. The most common thing to happen is for the liver to swell up so much that free fluid starts to leak off into the abdominal cavity (ascites).
Similarly, in some , dogs and cats, fluid can leak into the chest cavity from this back pressure (hydrothorax).
When the heart contracts, blood squirting through the leaking right AV valve into the right atrium is quite turbulent and causes a lot of noise (murmur) which a vet can hear when listening to the right side of the heart with a stethoscope.
Symptoms of right AV valve disease
- Swollen liver and/or spleen
- Free fluid in the abdomen (ascites)
- Free fluid in the chest cavity compressing the lungs causing breathing difficulties
- General ill health from the above problems e.g. off food, lethargic
- Feinting or collapsing
Left AV (mitral) valve disease
The left AV valve (also known as the mitral valve) lies between the left atrium and ventricle.
All this happens under low pressure as the lungs are "next door" to the heart. Blood only has to travel a short distance to get into the empty left atria and ventricle- almost gravity fed filling up of the left side of the heart.
The mitral valve then closes, and the left ventricle starts to contract very forcefully as it has to send the oxygen rich blood all around the body. There is a huge amount of pressure generated inside the ventricle which opens up the aorta valve allowing blood to "escape" the left ventricle into the aorta and go around the body.
The mitral valve has to stand up to a lot of pressure to prevent blood trying to get through it instead of the aortic valve when the heart contracts.
Any slight damage to the mitral valve results in a high pressure jet of blood squirting back up into the thin walled left atrium when the left ventricle contracts (a bit like water going through a narrowed down hose nozzle).
Suddenly, there is a lot of high pressure "extra" blood in the left atrium obstructing blood returning under low pressure from the lungs via the pulmonary veins.
Oxygen rich blood starts to "backup" in the lungs as it meets this resistance. Eventually, the pressure gets so great that fluid leaks out of the swollen pulmonary veins and leaks into the lung tissue (pulmonary oedema).
Because there is a reduction in the amount of oxygen rich blood leaving the heart (some has gone back into the left atrium through the leaking mitral valve), there are several consequences:
- The "greedy" brain says "On your bike, I need some more oxygen delivered up here" and releases adrenalin to get the heart pumping faster and harder.
- The heart muscles themselves normally take away approx. 15% of the total amount of oxygen rich blood leaving the heart to supply themselves with oxygen and energy to keep on pumping (a bit like putting petrol in the motor). This important supply of oxygen rich blood is collected just above the aortic valve (where oxygen rich blood is leaving the left ventricle on its way around the body). It is delivered to the heart muscles via the coronary arteries which originate just above the aortic valve.
Any reduction in oxygen rich blood leaving the heart means less of it going to the oxygen and energy hungry heart muscles.
The last thing the heart muscle wants to "hear" is the "greedy" brain telling it to pump harder and faster. It is already feeling the effects of a reduction in its own oxygen and energy supply.
- The increased effort means there is a lot more tension in the heart walls (ventricle muscle). The tiny branches of the coronary arteries feeding the muscles of the heart are "squished" by the increased surrounding tissue tension, so even less oxygen rich blood flows through them to feed the heart muscle.
- Eventually, some areas of the heart muscle end up with no blood supply and "die off" and hence stop contributing to the general work rate of the heart.
- Guess what, the "greedy" brain detects this reduction and releases even more adrenalin and the cycle goes on and on.
- Some of the damaged heart muscle areas can generate "extra heart beats"
a) Atrial premature contractions (APC's)
b) Ventricular premature contractions (VPC's)
If there are enough of these extra beats, they can be life threatening e.g. ventricular tachycardia or atrial fibrillation
- Some of the damaged heart muscle areas can also interfere with the normal electrical circuit going through the heart e.g. heart block.
Symptoms of left AV (mitral) valve disease
- Coughing due to fluid (oedema) in the lungs
- Collapsing and/or weakness when exerted
- Pale gums and poor refill times when pressed with a finger
- Weak pulse
- Rapid resting heart rate
- Rapid resting breathing rate (greater than 40 breaths a minute at home at rest)
- Sudden death
A vet will hear a murmur when listening to the heart valves. The murmurs are graded 1-6; 1 being only just detectable and 6 being able to be felt by placing a hand on the lower chest wall.
The history of the cough and clinical signs will alert a vet. To distinguish the cough from a sore throat or lower airway disease e.g. bronchitis, the resting heart rate is a big clue. If its rapid, its more likely to be heart problem.
Because there is a reduction in the flow of blood out of the heart, the pulse is weak and the colour of the gums is paler than normal.
To aid in diagnosis, vets use radiology and ultrasound
Left Heart Failure
- Enlargement of the left atrium and left ventricle
- The trachea is elevated upwards towards the thoracic spine
- In severe cases one of the main branches of the trachea can be "pinched off' as it is being squeezed between the heart below and spine above
- Fluid in the lungs (pulmonary oedema)
Right Heart Failure
- Enlargement of the right atrium and right ventricle
- Liver enlargement
- Fluid in the abdomen (ascites)
- Fluid in the chest cavity (more so in cats)
Ultrasound is a powerful diagnostic tool in the hands of a skilled operator. A vet can see the actual damaged valves and measure heart chamber sizes and how well they are contracting. Using colour flow Doppler, the leaking blood can be seen shooting through the leaking valves into the atria. Blood pressure measurement can be made to give an indication of how severe the problem is.
Causes of heart valve disease
- Genetic e.g. Cavalier King Charles Spaniel
- Normal wear and tear
- Bacterial infection of the heart valves (bacterial endocarditis)
1) Lower the blood pressure
Going back to the flow of blood within the left heart described above, imagine the following...
In the left ventricle just before contraction, say the aorta on the other side of the closed aortic valve has some thick honey in it.
When the ventricle contracts and tries to open up the aortic valve, there is a lot of pressure required to shift the "honey" out of the way and open the valve.
A much easier solution is to push the blood up through the leaking mitral valve into the left atrium where there is a low pressure environment i.e. very low resistance to flow of blood out of the ventricle.
A large jet of blood will shoot back up through the leaking mitral valves into the left atrium because it is much easier to send it there (being a low pressure environment) than out the aorta with the thick "honey" on the other side of the closed aortic valve.
Now, imagine we replace the honey in the aorta with water.
The ventricle now finds it much easier to open the aortic valve and send blood into the aorta.
This means much less blood shoots up through the leaking mitral valve into the left atrium which in turn means more blood can get into the left atrium from the lungs.
In other words, there is improved blood flow from the left side of the heart.
A very similar scenario happens on the right side of the heart except we are dealing with the pulmonary and right AV valves instead of the aortic and mitral (right AV) valves.
So how do we go about creating this scenario?
Vets use drugs which lower the blood pressure in the aorta and pulmonary arteries. It just like changing the "honey" to "water" - same thing.
The most common drugs are:
- ACE inhibitors e.g. Enalapril (Enalfor), Captopril (Capoten) and Benazepril (Vetace, Fortekor)
- Pimobendan (Vetmedin)
They work by dilating the major arteries and veins in the body, thereby allowing more blood to accumulate in these vessels and lower the overall pressure in them.
2) Remove excess fluid in the lungs and/or chest cavities
The standard drug is Frusemide (Lasix) which removes fluid via the kidneys (a diuretic).
If it fails to shift the fluid a second diuretic is added to the regime. My favourite is Hydrochlothaizide (Dithiazide) as I find another one often used by vets called Spironolactone lacks "grunt".
3) Watch for over zealous use of diuretics
In a critically ill patient with lots of fluid buildup, we sue high doses of diuretics to shift the fluid out of the lungs. Once stabilised, the vet has ot be careful not to give too high a dose and cause dehydration of loss of potassium.
If using 2 diuretics, I add in a slow release potassium supplement to avoid low levels in the blood.
Blood test should be performed regularly to check the kidneys and liver are handling the medications ok.