Bradyarrhythmias - SA node disorders

Question 1

What are the phases of the pacemaker action potential?

Answer 1

• Phase 0
• Phase 3
• Phase 4
• Note:*
• In pacemaker action potential, there is no phase 1 and 2

Question 2

Events in phase 4 pacemaker action potential

Answer 2

• Slow spontaneous diastolic depolarization due to I_f (“funny current”)
  
  • I_f channels responsible for a slow, mixed Na⁺/K⁺ inward current; different from I_Na in phase 0 of ventricular action potential

Question 3

Events in phase 0 pacemaker action potential

Answer 3

• Upstroke
  
  • Opening of voltage-gated Ca²⁺ channels
    
    • Fast voltage-gated Na+ channels are permanently inactivated because of the less negative resting potential of these cells
  • Results in a slow conduction velocity that is used by the AV node to prolong transmission from the atria to ventricles

Question 4

Events in phase 3 pacemaker action potential

Answer 4

Inactivation of the Ca²⁺ channels and ↑ activation of K⁺ channels → ↑ K⁺ efflux

Question 5

Upstroke in pacemaker action potential compared with atrial and ventricular myocytes

Answer 5

The action potential upstrokes (phase 0) are slow compared with atrial or ventricular myocytes, being mediated by calcium rather than sodium current

Question 6

Why is the SA node the pacemaker of the heart?

Answer 6

Cells in the SA node exhibit the most rapid phase 4 depolarization and thus are the dominant pacemakers in a normal heart

Question 7

Physiologically, bradycardia results from:

Answer 7

• Failure of impulse initiation; or
• Failure of impulse conduction