Multifocal Atrial Tachycardia MAT is defined as a rapid, irregular rhythm with at least 3 dis- tinct morphologies of P waves on the surface ECG. It may be difficult to distinguish MAT from AF on physical examination or even on a single ECG tracing, so a 12-lead ECG is indicated to confirm the diagnosis. On the ECG, the atrial rate is >100 bpm (or >90 bpm, as defined in at least 1 report220 ). Unlike AF, there is a distinct isoelectric period between P waves. The P-P, P-R, and R-R intervals are variable. The mechanism of MAT is not well established. Although it is assumed that the variability of P-wave morphology implies a multifocal ori- gin, there are very few mapping studies of MAT.221 Similarly, the variability of the P-R interval may relate to decremental conduction through the AV node, as opposed to the origin of the P wave. Occasional responsiveness to verapamil suggests a triggered mechanism, but data are limited.222 MAT is commonly associated with underlying conditions, including pulmonary disease, pulmonary hypertension, coro- nary disease, and valvular heart disease,223 as well as hypomag- nesemia and theophylline therapy.224 The first-line treatment is management of the underlying condition. Intravenous mag- nesium may also be helpful in patients with normal magne- Antiarrhythmic medications in general are not helpful in suppression of multifocal AT.226 Management often involves slowing conduction at the AV nodal level to control heart rate. Verapamil has been shown to have some efficacy in Table 7. 