Multifocal atrial tachycardia is diagosed when an irregular atrial rhythym is over 100 beats per minute.  It is caused by multiple competing atrial pacemaker sites.  There need to be at least three different P wave morphologies to diagnose MAT.  The PR intervals may vary also.   It is nearly always seen in very sick patients, often with chronic obstructive pulmonary disease and/or respiratory failure.