Cardiac arrhythmias can be classified by site of origin - Sinus rhythms originate in the sinoatrial node, or SA node. - Atrial rhythms originate in the atria. - Ventricular rhythms originate in the ventricles. Sinus rhythm is the normal rhythm of the heart established by its natural pacemaker in the SA node. In a healthy heart, the SA node fires 60 to 100 times per minute resulting in a normal heart rate of 60 to 100 beats per minute. The most common variations in sinus rhythm include: - Sinus bradycardia: when the SA node fires LESS than 60 times per minute resulting in a slower heart rate of less than 60 beats per minute. Y - Sinus tachycardia: when the SA node fires MORE than 100 times per minute generating a faster heart rate of more than 100 beats per minute. Sinus bradycardia and sinus tachycardia can be normal or clinical depending on the underlying cause. For example, sinus bradycardia is considered normal during sleep, and sinus tachycardia may be normal during exercise. Cardiac arrhythmias originating in other parts of the atria are always clinical. The most common include: atrial flutter, atrial fibrillation, and atrioventricular nodal reentrant tachycardia. These are forms of supraventricular tachycardia or SVT. Atrial flutter or ALF is caused by an electrical impulse that spins around in a localized, self-perpetuating loop, commonly located in the right atrium. This is called a reentry circuit. For each cycle around the loop, there is a contraction of the atria. The atrial rate is regular and rapid – between 250 and 400 beats per minute. The ventricular rate, or heart rate, however, is slower, thanks to the refractory properties of the AV node. The AV node prevents some of the atrial impulses from reaching the ventricles. In this example, only one in three atrial impulses makes its way to the ventricles. Therefore, the ventricular rate is 3 times SLOWER than the atrial rate. The ventricular rate in ALF is usually regular, but may also be irregular. On an ECG, atrial flutter is characterized by the absence of normal P waves. Instead, flutter waves, or F waves, are present in sawtooth patterns . Atrial fibrillation is caused by multiple electrical impulses that initiate RANDOMLY from many ECTOPIC locations in and around the atria, usually near the roots of the pulmonary veins. These chaotic, unsynchronized electrical signals cause the atria to quiver or fibrillate instead of contracting. The atrial rate during atrial fibrillation can be extremely high, but most of the electrical impulses do not pass through the AV node to the ventricles, again due to the refractory properties of the AV nodal cells. Those that DO cross are IRREGULAR. The ventricular rate or heart rate is therefore irregular and can vary from slow – less than 60 – to fast – more than 100 – beats per minute. On the ECG, atrial fibrillation is characterized by the absence of P waves and narrow, irregular QRS complexes. The baseline may appear wavy or completely flat depending on the number of ectopic sites in the atria. In general, a greater number of ectopic sites results in a flatter baseline. Atrioventricular nodal reentrant tachycardia or AVNRT is caused by a small reentry circuit that DIRECTLY involves the AV node. Each time the impulse passes through the AV node, it IS transmitted to the ventricles. The atrial rate and the ventricular rate are therefore identical. The heart rate is regular and rapid, ranging from 150 to 250 beats per minute. Ventricular rhythms are the most dangerous. In fact, they are called LETHAL rhythms. Ventricular tachycardia or VT is mostly caused by a STRONG, SINGLE trigger site or circuit in one of the ventricles. It usually occurs in people with structural heart problems such as scarring from a previous heart attack or abnormalities in the heart muscle. The impulses that begin in the ventricles produce PREMATURE ventricular contractions (extrasystoles) that are regular and rapid, ranging from 100 to 250 beats per minute. On the ECG, VT is characterized by wide and strange QRS complexes. The P wave is absent. VT can occur in short episodes of less than 30 seconds and produces few or no symptoms. Prolonged VT lasting more than 30 seconds requires immediate treatment to prevent cardiac arrest. Ventricular tachycardia can progress to ventricular fibrillation. Ventricular fibrillation or VF is caused by MULTIPLE, weak ectopic sites in the ventricles. These chaotic and unsynchronized electrical signals cause the ventricles to quiver or fibrillate instead of contracting. The heart pumps little or no blood. VF can quickly lead to cardiac arrest. The ECG of a VF is characterized by irregularly shaped and random waves of variable amplitude, WITHOUT identifiable P wave, QRS complex or T wave. The amplitude decreases over time, from the initial coarse VF to the fine VF and ultimately to the flat line.