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CELLULAR ELECTROPHYSIOLOGIC MECHANISMS OF CARDIAC ARRHYTHMIAS

Advances in Supraventricular Tachycardia

Cellular Mechanisms of Cardiac Arrhythmias

  • Cardiac arrhythmias stem from abnormalities in impulse initiation or conduction.

  • Abnormalities can arise from :

    • Automaticity: spontaneous generation of impulses

    • Triggered activity: afterdepolarizations requiring a preceding action potential

  • Electrocardiogram (ECG) recordings help visualize these issues, while microelectrode techniques explore transmembrane events at the cellular level.

  • All arrhythmias reflect critical changes in cellular electrophysiology.

Types of Arrhythmias

Abnormal Impulse Initiation

  • Defines when an electrical impulse starts in individual or groups of cardiac cells.

  • Causes of Impulse Initiation:

    1. Automaticity

    2. Triggered Activity

  • Automaticity can be normal (in the sinus node) or abnormal (in other heart regions).

Automaticity

  • Normal Automaticity: Present primarily in the sinus node; can be overtaken by latent pacemakers under certain conditions.

  • Abnormal Automaticity: Occurs due to significant changes in transmembrane potentials.

  • Mechanism involves phase 4 depolarization leading to action potentials.

  • Inhibition of latent pacemakers occurs via overdrive suppression from dominant impulse initiation in the sinus node.

Triggered Activity

  • Two types of afterdepolarizations:

    • Early afterdepolarizations occur during repolarization, potentially leading to secondary action potentials due to ongoing inward current.

    • Delayed afterdepolarizations occur after full repolarization when transient inward currents trigger further action potentials.

  • Risk factors include hypoxia and certain drug interactions.

Abnormal Impulse Conduction and Reentry

  • Reentry: Arises when excitatory impulses circulate in the heart due to slowed conduction and unidirectional block.

  • Several structural or functional pathways can showcase this mechanism, including anatomical obstacles and aberrant conduction in purkinje fibers.

  • An excitable gap may facilitate successful reentry.

  • Reentrant Mechanisms:

    • Leading circle mechanism

    • Anisotropic reentry due to varying conduction velocities across heart tissue

Clinical Relevance

  • Recognizing the mechanisms of arrhythmias is essential for treatment.

  • Understanding how overdrive pacing influences arrhythmias can help with therapeutic strategies.

References

  • A extensive list of previous research and studies that contextualize these findings.