Tachyarrythmia - Mechanisms

• Enhanced Automaticity

  • Enhance automaticity is associatied with an increase in the slope of phase 4 depolarization results in

    • As a result of the increase in phase 4 slope the cell reaches threshold more often per minute resulting in higher heart rate.

    • Factors that increase automaticity include

      • mechanical stretch

      • beta-adrenergic stimulation

      • hypokalemia

      • Ischemia can induce abnormal automaticity, i.e. automaticity that occurs in cells not typically exhibiting pacemaker activity.

• Triggered Automaticity

  • Triggered automaticity occurs when a second depolarization occurs prematurely.

    • One type of triggered automaticity is a delayed afterdepolarization (DAD).

      • If this late depolarization reaches threshold (a) second beat(s) may occur.

    • Factors that predispose to delayed afterdepolarizations include:

      • excessive adrenergic activity

      • digitalis toxicity

      • high intracellular Ca²⁺

  • A second type of triggered automaticity is Early Afterdepolarization (EAD) which is associated with significant prolongation of the action potential duration.

    • In this case, during a prolonged phase 3 repolarization, the repolarization is interrupted by a second depolarization.

    • Factors that predispose to Early Afterdepolarizations include

      • bradycardia

      • low extracellular K⁺

      • certain drugs, including some antiarrhythmics

  • Torsades de pointes, a polymorphic ventricular arrhythmia- associated with

    • Prolongation of cardiac repolarization (prolonged Q-T interval)

    • Possibly induced by early afterdepolarizations.

  • • The antiarrhythmic drug quinidine gluconate can cause this arrhythmia. Many other drugs can also cause this effect.

  • Reentry is the most common cardiac conduction abnormality leading to arrhythmias.

Category: Pharmacology Notes