Some dogs with dilated cardiomyopathy and left A-V valve disease get a life threatening arrhythmia called atrial fibrillation.
It originates from a part of the left atrium wall which gets damaged when it enlarges. The damaged area fires off its own electrical current causing the atrium to contract very rapidly (sometimes as much as 200-300 times a minute).
Each time the damaged atrium contracts, it tries to sends an electrical signal to the ventricles below telling it to contract as "its their turn now". Not every electrical impulse gets through to the ventricle as a number of these extra charges arrive at the ventricle when it is in a "none-responsive" state (refractory period).
The refractory period may only be a split part of a second, and follows a normal ventricle contraction. Once it has passed, the extra electrical signals from the damaged atrium above can trigger off a ventricle contraction.
In effect, the atria are doing their own rapid irregular heart beat and 8 times out of 10 their extra contractions cause the ventricles to contract.
As a result, we see the ventricles pumping at a much faster and irregular rate than in a dog with no atrial fibrillation.
If a dog has either A-V valve disease or dilated cardiomyopathy, the ventricles are already having a hard time.
Cardiac output is reduced in A-V valve disease and dilated cardiomyopathy, so oxygen rich blood does not reach vital organs e.g. brain, liver, kidneys.
But don't forget the heart which is the most dynamic oxygen hungry organ (next to the brain). Any reduction in output means less oxygen for the heart muscles, so they can't pump as well as they would like.
The last thing the diseased ventricles want is atrial fibrillation causing them to contract even faster.
Atrial fibrillation can very quickly cause the ventricles to fail and lead to sudden heart failure and death.
On exmination, the heart rate is very rapid and irregular- often over 200 beats a minute (nearly 3-4 beats a second). There is often evidence of left sided heart failure - coughing, weakness, pale gums, weak thready rapid pulse.
A normal ECG has one P wave (atrial contraction) followed by one QRS wave (ventricle contraction) and a normal heart rate of approx 100-140.
Atrial fibrillation shows rapid irregular spaced ventricle heartbeats (QRS waves) and no regular P waves.
Treatment of Atrial Fibrillation
The classic drug used to treat atrial fibrillation is Digoxin (Lanoxin). It increases the length of the refractory period (mentioned above). As a result, even less of the atrial contractions can get through to the ventricle to make them contract.
Digoxin blood levels need to be monitored as overdosage is common and can cause serious problems.
If the ventricles are still beating at a much too high a rate and the heart failure is under control with medications, vets may add in a second drug (Beta blocker) to slow the ventricles down.