Disorders of impulse formation

• altered normal automaticity—for examples, SA node

• abnormal automaticity: cells that don’t usually have automaticity exhibit it

• triggered activity

  • early afterdepolarizations (EADs)

  • delayed afterdepolarizations (DADs)

Altered normal automaticity

• occurs when normal pacemaker cells (SA, AV nodes) begin pacing at a non-physiologic rate

• example: junctional tachycardia in the AV node; this can overdrive the pacemaker cells and drive the heart too quickly

Abnormal automaticity

• occurs when automaticity arises from cells that are not the normal pacemaker cells in the heart

• example: accelerated idioventricular rhythm

  • ischemia creates a current of injury between normal and ischemic cells

  • cells that don’t usually fire begin to fire themselves

  • occurs 24 to 72 hrs after myocardial infarction

Triggered activity

• caused by oscillations in the membrane potential induced by the previous beats; these are classified by how closely they occur relative to the previous beat

• two types

  • early afterdepolarizations (EADs)

  • delayed afterdepolarizations (DADs)

Category: Cardiology Notes