08. Arrhythmias

• arrhythmia = disturbance in rate or rhythm of heartbeat due to changes or loss of electrical integrity in cardiac tissue

  • atrial fibrillation is the most common
• R-R interval = calculate heart rate and identify irregular rhythms
• P-R interval = beginning of atrial depolarisation to beginning of ventricular depolarisation

  • AVN and atrial conduction slowing
• QRS duration = duration of ventricular depolarisation

  • can see ventricular conduction slowing
• Q-T interval = ventricular depolarisation to repolarisation

  • can see prolongation of ventricular action potential duration
• delayed after depolarisations:

  • after action potential is completed, membrane potential increases
  • if sufficient magnitude this can trigger action potentials and spread to the rest of the heart
• early after depolarisations:

  • caused by action potential prolongation
  • often associated with decreased potassium efflux during repolarisation
  • prolonged action potential allows L-type Ca2+ channels to recover and reactivate
  • shown as QT prolongation
• AVN conduction block can have prolonged P-R intervals, missing QRS complexes or complete dissociation between P waves and QRS complexes
• re-entry circuits are the establishment of self sustaining circuits within the heart
• shorter wavelength = increased likelihood of a re-entry circuit
• wavelength = conduction velocity x ERP
• the heart is more susceptible to re-entry circuits when the refractory period is short and conduction velocity is slow
• decreased ventricular conduction velocity = longer QRS
• decreased atrial conduction velocity = longer P wave or longer P-R interval