Atrial fibrillation is the most frequently reported heart rhythm abnormality in horses. It involves the two upper chambers (atria) of the heart. Its name comes from the fibrillating or quivering of the heart muscles of the atria, instead of a coordinated contraction. Normally, the heart contracts and pumps blood with a regular rhythm, for example, at a rate of 40 beats per minute in a healthy horse, and there is a beat every second. The heart may beat faster or slower with a shorter or longer interval between beats, but at any one rate the interval between beats is constant. This regular rhythm occurs as a result of regular electrical discharges, or currents, that travel through the heart and cause the muscle of the heart to contract. In atrial fibrillation, the electrical discharges are irregular and rapid and, as a result, the heart beats irregularly.
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