ATRIAL fibrillation.

Abstract

Normal Heart Electrical Activity and What Goes Wrong in AF The heart is essentially a large muscular pump that drives blood around the body. To do this effectively, the heart’s chambers must be precisely controlled electrically (Figure 1). The normal heartbeat begins with the natural pacemaker (the sinoatrial [SA] node) in the top right heart chamber (the right atrium). Activity spreads across the atria, causing them to contract. The contraction of the atria propels extra blood into the ventricles, which are the main pumping chambers. The electrical signal is delayed by about one tenth of a second by a special structure called the atrioventricular (AV) node, and then spreads rapidly across the ventricles to make them contract. Extra filling of the ventricles by the atria is not vital, but does serve to “prime the ventricular pump” and improve overall heart function. Normal heart rhythm is termed sinus rhythm. Most people have a resting heart rate of between 60 and 80 beats per minute (Figure 2). In AF, the atria contract rapidly and irregularly at rates of 400 to 600 beats per minute. Fortunately, the AV node will not allow many signals through to the ventricles; only about 1 or 2 out of every 3 atrial beats passes to the ventricles. The ventricles beat too fast, however, at rates of 110 to 180 beats per minute.