Multifocal (or multiform) atrial tachycardia (MAT) is an abnormal heart rhythm, specifically a type of supraventricular tachycardia, that is particularly common in older people and is associated with exacerbations of chronic obstructive pulmonary disease (COPD). Normally, the heart rate is controlled by a cluster of pacemaker cells called the sinoatrial node (SA node). When different clusters of cells known as ectopic pacemakers, that are outside the SA node take over control of the heart rate, and the rate exceeds 100 beats per minute, this is called multifocal atrial tachycardia. A fast heart rate below 100, is technically not a tachycardia and is then termed multifocal atrial rhythm, also known as wandering atrial tachycardia. "Multiform" refers to the observation of variable P wave shapes, while "multifocal" refers to the underlying cause. Although these terms are used interchangeably, some sources prefer "multiform" since it does not presume any underlying mechanism.
Causes MAT usually arises because of an underlying medical condition. Its prevalence has been estimated at 3 per 1000 in adult hospital inpatients and is much rarer in paediatric practice; it is more common in the elderly, and its management and prognosis are both those of the underlying diagnosis. It is mostly common in patients with lung disorders, but it can occur after acute myocardial infarction and can also occur in the setting of low blood potassium or low blood magnesium. It is sometimes associated with digitalis toxicity in patients with heart disease. It is most commonly associated with hypoxia and COPD. Additionally, it can be caused by theophylline toxicity, a drug with a narrow therapeutic index commonly used to treat COPD. Theophylline can cause a number of different abnormal heart rhythms when in excess, and thus further predisposes COPD patients to MAT. Theophylline toxicity often occurs following acute or chronic overtreatment or factors lowering its clearance from the body.
Pathophysiology The P-waves and P–R intervals are variable due to a phenomenon called wandering atrial pacemaker (WAP). The electrical impulse is generated at a different focus within the atria of the heart each time. WAP is positive once the heart generates at least three different P-wave formations from the same ECG lead. Then, if the heart rate exceeds 100 beats per minute, the phenomenon is called multifocal atrial tachycardia.
Diagnosis Multifocal atrial tachycardia is characterized by an electrocardiogram (ECG) strip with three or more discrete P wave morphologies in the same lead, not including that originating from the sinoatrial node, plus tachycardia, which is a heart rate exceeding 100 beats per minute (although some suggest using a threshold of 90 beats per minute). Furthermore, there should be irregular PP intervals, and the baseline should be isoelectric between P waves. Other findings that are commonly seen, but are not diagnostic include irregular PR and RR intervals. Variation in PR intervals has not been included in the diagnostic criteria because the PR interval varies with the length of the preceding RP interval. Other diagnoses that may present with similar findings on electrocardiogram that should be included in the differential diagnosis include sinus tachycardia with frequent premature atrial contractions (this would have regular PP intervals), atrial flutter with variable AV node conduction (this would have regular PP intervals and flutter waves), atrial fibrillation (this would not have discrete P-wave morphologies), and wandering atrial pacemaker which would have a heart rate less than 100 beats per minute).
Additional workup If arrhythmia persists despite the treatment of underlying medical conditions it may be worth checking a complete blood count and serum chemistry for signs of infection, anemia, or electrolyte abnormalities such as hypokalemia and hypomagnesemia.
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