Atrial fibrillation, which is a common condition, occasionally occurs in normal individuals without evidence of structural heart disease, although is usually associated with abnormalities such as hypertensive or valvular heart disease, coronary artery disease, cardiomyopathies, pulmonary embolism, pericarditis, myocarditis, alcohol abuse, thyrotoxicosis, and chronic obstructive pulmonary disease. When seen in the setting of an acute anterior myocardial infarction, atrial fibrillation may be associated with higher morbidity and mortality as it may indicate larger areas of myocardial damage. There may be associated pericarditis, higher intra-atrial pressures, or atrial infarction.
Atrial fibrillation is completely irregular on the ECG, and P waves are not present. There is usually an undulating rhythm at a rate of 600 beats/min. Physical examination may be helpful by demonstrating fibrillatory jugular venous waves and a variable first heart sound. Even though this rhythm is highly irregular, it is believed to be re-entry in origin with the occurrence of a random sequence of depolarization. In patients without atrioventricular node disease, the ventricular response is usually 120 to 180 beats/min. The QRS complex is narrow unless aberrant ventricular conduction or a bundle branch block is present. Occasionally, Ashman's phenomenon will occur, particularly with a rapid ventricular response. This is seen when a short RR interval is preceded by a long RR interval and is usually of a right bundle branch block configuration. The long RR interval increases the refractory period of the following beat, thus accounting for the aberrancy ( Fig. 5) The QRS may also be broadened in patients who have Wolff-Parkinson-White syndrome when depolarization is conducted down the accessory bypass tract. This rapid conduction can lead to ventricular rates in excess of 300 beats/min and can degenerate into ventricular fibrillation.
Atrial fibrillation with aberrancy can be difficult to distinguish from ventricular tachycardia. However, carefully plotting the RR intervals will be helpful because atrial fibrillation is grossly irregular while ventricular tachycardia is usually regular, although subtle irregularities may be seen.
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