Tachycardia

Automaticity

The rate at which the sinus node discharges usually is faster than other latent or subsidiary automatic cardiac pacemakers. Subsidiary pacemakers can become dominant in the settings of acidosis, ischemia, sympathetic stimulation, and use of certain drugs. Normal automaticity can be suppressed by pacing but generally resumes after pacing stops.

Abnormal automaticity can be due to cell damage and abnormal depolarization. The partial depolarization and failure to reach or maintain the normal maximum diastolic potential may induce automatic discharge. Examples include accelerated junctional rhythm (i.e., nonparoxysmal junctional tachycardia), accelerated ventricular rhythms, certain atrial tachycardias, some ventricular tachycardias (VTs) in patients without structural heart disease, exercise-induced VT, and VT during the first several hours of acute myocardial infarction (MI).