Mechanisms

PSVT often begins with:

• sinus node acceleration
• sinus pauses with junctional escape beats
• premature atrial contraction (PAC)
• premature ventricular contraction (PVC)

Take a moment to review Rnceus Interactive's animated description of abnormal cardiac conduction.

The common mechanisms of PSVT are:

• Increased automaticity
  • automaticity arises from the spontaneous depolarization of pacemaker sites located throughout the heart.
  • under normal circumstances, the pacemaker cells of the SA node provide overdrive suppression of other potential pacemaker sites, and are in turn able to control the heart rate within its normal expected limits.
  • when there is an increase of the automaticity of the atrial fibers, the SA node is no longer able to control the normal heart rate and PSVT has the opportunity to occur.
    • increased automaticity can occur due to altered cardiac pathophysiology (Ebstein's anomaly, corrected transposition of the great vessels) or pharmocologic influences (medications containing sympathomimetic amines, caffeine, illicit drug use).
• Rentry within the cardiac conduction system occurs when the primary impulse repeatedly activates the conduction system by encountering excitable cardiac tissue. The mechanism of reentry is further broken down into:
  • Reentry with an accessory pathway, e.g., Wolff-Parkinson-White (WPW) syndrome.
    • this mechanism uses the same conduction pathway over and over again.
    • also may be known as ordered reentry or circus movement.
  • Reentry without an accessory pathway
    • this mechanism uses many different conduction pathways which are all different in size and location.
    • also may be known as random reentry or AV nodal reentry.

View a Movie of Normal Cardiac Conduction & Contraction from Rnceus Interactive

Instant Feedback:

Where there is increased automaticity of the atrial fibers (as with a child with PSVT), the SA node is no longer able to control the heart rate within its normal expected limits.

TRUE or FALSE