A 84-year-old woman with a long history of chronic obstructive pulmonary disease and asthma is admitted to the ICU with pneumonia:

• Multifocal atrial tachycardia (MAT), 138/min
• Left axis deviation
• Non-specific ST and T wave abnormality

The computer program misdiagnosed the arrhythmia as “atrial fibrillation”. In fact, this error is not uncommon, as in most cases of MAT the rhythm is very irregular. Notice the changing pattern of the P waves due to the multifocal nature of this arrhythmia.