The Heart

The heart is a pump responsible for maintaining blood supply to the body. It has four chambers. The two upper chambers (the right atrium and left atrium) are the chambers that receive blood as it returns from the body via the veins. The lower chambers (the right and left ventricle) are the chambers responsible for pumping the blood out to the body via the arteries. Like any pump, the heart has an electrical system that controls how it functions.

Normal heart rhythm

In order for the heart to do its work (pumping blood throughout the body), it needs a sort of spark plug or electrical impulse to generate a heartbeat. Normally this electrical impulse begins in the upper right chamber of the heart (in the right atrium) in a place called the sino-atrial (SA) node.

The SA node is the natural pacemaker of the heart. The SA node gives off electrical impulses to generate a heartbeat in the range of 60 to 100 times per minute. If you are exercising, doing strenuous work or are under stress, your heart rate will be faster. When you rest or sleep your heart rate will slow down. If you take certain medications, your heart rate may be slower.

From the Sinus Node, the electrical impulse is relayed along the heart’s conduction system. It spreads throughout both the right and left atria causing them to contract evenly. When the impulse spreads over the right atrium it reaches the atrio-ventricular (AV) node. This is a very important structure in the heart because it is the only electrical connection between the top chambers and the bottom chambers. It is therefore the only way in which an electrical impulse can reach the pumping chambers (the ventricles). The impulse spreads through the AV node and down into the lower chambers or ventricles of the heart. This causes them to contract and pump blood to the lungs and body.

How does Atrial Fibrillation occur?

Atrial fibrillation is due to the development of electrical short circuits inside the top chambers of the heart. Usually these short circuits begin in the top chamber on the left (left atrium). They are usually triggered by abnormal electrical activity located within the veins that drain blood from the lungs back to the heart (pulmonary veins).

These rapid short circuits have several consequences:

1. The short circuits drive the pumping chambers rapidly and erratically. This produces palpitations, shortness of breath, and tiredness. In some people it can also cause dizzyness and chest pain. It usually reduces your exercise or physical capacity. It is NOT however a life-threatening rhythm disturbance.

2. The short circuits result in ineffective pumping of the upper chambers. This leads to slow blood flow in both of these upper chambers (the left and right atria). This can rarely cause blood clots and possibly stroke. For this reason many (but not all) patients with atrial fibrillation will require a blood thinner. This may be aspirin, warfarin or one of the newer medications (Pradaxa, Xarelto or Eliquis).

Why does Atrial Fibrillation occur?

In many people who develop atrial fibrillation the cause is not known. Usually the heart is otherwise sound. In some people atrial fibrillation can develop due to other conditions such as high blood pressure, prior heart attack, weak heart, leaky heart valves, obesity or sleep apnoea. There may be a familial tendency to atrial fibrillation.