Cardiac Arrhythmias: Class IV: Ca channel blockers

• most frequently used to slow conduction through AV node in…

  • atrial fibrillation

  • WPW

  • AVNRT

• when patients present with atrial fibrillation and rapid ventricular response, they are commonly started on IV diltiazem to control heart rate until oral therapy can begin

• unlike β-blockers, do not decrease mortality post MI, so don’t use in this setting

• side effects—hypotension, bradycardia

• effects on AP 70

  • here you see the action potential of the AV node, and this is where Ca-channel blockers have their greatest effect

  • they slow the phase 0 depolarization and decrease the prolongation phase 2

  • finally, change phase 4 diastolic current

digoxin

• binds and inhibits Na-K-ATPase and enhances vagally-mediated slowing of AV node

• useful adjunct to Ca-channel blockers or β-blockers, but can’t be used by itself for rate control in atrial fibrillation

• rate control effects are overridden by sympathetic stimulation during exercise

adenosine

• very short-acting agent. blocks AV node for 5-10 seconds, but this can break AVNRT or AVRT

• for diagnostic purposes, it’s important to know that adenosine will not terminate reentrant tachycardias that don’t involve the AV node. these include…

  • atrial flutter

  • atrial fibrillation

  • ventricular tachycardia

Category: Pharmacology Notes