What Makes the Heart Beat?
The heart is made of four chambers. The two upper chambers are the atria, the two lower chambers are the ventricles. In addition to the upper and lower chambers, the heart also has right and left sides. The atria fill with blood and pass it to the ventricles. The right ventricle pumps poorly oxygenated blood to the lungs. The left ventricle pumps well oxygenated blood to the body (see ‘Normal Cardiac Circulation’).
What causes the heart muscle to contract? Heart muscle contracts after it is stimulated by an electrical impulse.
This starts within a small area of specialised tissue in the
right atrium called the sinus node. The sinus node initiates
the electrical impulse, which then spreads rapidly through
the muscles of both atria, stimulating them to contract.
The electrical impulse then spreads down to both ventricles
to stimulate them to contract. The electrical activity of the heart can be recorded on an electrocardiogram (ECG).
What is Atrial Fibrillation
Atrial fibrillation is a very common arrhythmia. It is characterised by a rapid, uncoordinated firing of electrical impulses from multiple sites in the atria (as opposed to the normal single firing from the sinus node). A small region of specialized tissue between the right atrium and right ventricle called the ‘atrioventricular node’ filters some of these impulses. However, the number of impulses that reach the ventricle is often still abnormally high, resulting in rapid and irregular heart rates. If the ventricular rate is not to fast, no clinical signs may be noted and no treatment is required. However, if the heart rate is rapid, the ventricles are not able to pump an adequate amount of blood to the body and lethargy is seen.
What Causes Atrial Fibrillation?
Atrial fibrillation usually occurs secondary to cardiac disease that has resulted in left or right atrial enlargement. There are many cardiac diseases that can cause this including mitral valve degeneration, dilated cardiomyopathy and, if left uncorrected, some congenital diseases. In giant breed dogs such as the Great Dane and Irish Wolfhound atrial fibrillation can occur without underlying cardiac enlargement. This syndrome is called ‘lone atrial fibrillation’ and is also recognised in people and horses.
How is Atrial Fibrillation Diagnosed?
Atrial fibrillation is usually suspected on the physical exam when a rapid, irregular heart rate is heard. Diagnosis requires an electrocardiogram (ECG). Once diagnosed, an echocardiogram (heart ultrasound) is recommended to look for underlying cardiac enlargement as the cause. Your pet may also require a Holter monitor (24 hr ECG) to assess the heart rate over the course of the day and the response to treatment.
How is Atrial Fibrillation Treated?
The majority of the time, atrial fibrillation is treated medically. The aim of medication is to slow the heart rate down to allow for better cardiac filling and output. Oral medication will not result in restoration of normal rhythm. Commonly used medications for this purpose include:
If the heart rate is not rapid, no treatment is required. In cases where they atrial fibrillation is recent and there is no evidence of cardiac enlargement, electrical cardioversion will be discussed. This procedure involves a short general anaesthetic and delivery of an electric shock to the heart to restore normal rhythm.
In most cases, as long as the heart rate can be controlled within a normal range, your pet should be able to maintain a good quality of life. Regular check ups are essential to maintain optimal treatment and quality of life for your pet.