![]() It also can be the result of vasovagal hypertonia, an unusually active vagus nerve. Sinus node dysfunction also can be caused by coronary artery disease, hypothyroidism, severe liver disease, hypothermia, typhoid fever or other conditions. The most common cause is scar tissue that develops and eventually replaces the sinus node. Sinus node dysfunction - This usually causes a slow heart rate (bradycardia), with a heart rate of 50 beats per minute or less.Some common types of cardiac arrhythmias include: ![]() They also can be classified according to their effect on the heart rate, with bradycardia indicating a heart rate of less than 60 beats per minute and tachycardia indicating a heart rate of more than 100 beats per minute. This causes the ventricles to contract and produces a heartbeat.Ĭardiac arrhythmias sometimes are classified according to their origin as either ventricular arrhythmias (originating in the ventricles) or supraventricular arrhythmias (originating in heart areas above the ventricles, typically the atria). The signal enters the muscles of the ventricles. ![]() It's made up of a series of modified heart muscle fibers located between the ventricles. Next the signal travels through the bundle of His (pronounced HISS). From the sinus node, the heartbeat signal travels to the atrioventricular node or "A-V node," which is located between the atria. It's the natural pacemaker located in the upper portion of the right atrium. Under normal circumstances, the signal for a heartbeat comes from the heart's sinus node. But this milder contraction occurs just before the ventricles contract, and it is not felt in the pulse. The heart's two upper chambers, called the atria, also contract to help fill the ventricles. And the pulse (felt at the wrist, neck or elsewhere) matches the contractions of the heart's two powerful lower chambers, called the ventricles. In normal adults, the heart beats regularly at a rate of 60 to 100 times per minute. A cardiac arrhythmia is any abnormal heart rate or rhythm.
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