Pharmacology: Cardiac Arrhythmia Drugs Flashcards
What is the generalised action of class 1 drugs?
They block Na channels. This slows conduction of phase 0 but has minor effects on the action potential duration. 1A has moderate affects on phase 0 1B has no change in phase 0 1C has marked effects on phase 0
What is the action of class 2 drugs?
Beta-blockers prevent entry of calcium during phase 2 of the action potential.
This diminishes the phase 4 depolarisation
What is the action of class 3 drugs?
Block K channels.
Therefore the depolarisation occurs more slowly - the refractory period is prolonged so the cell cant be excited for long. Increases the overall action potential duration.
What is the action of class 4 drugs?
Block calcium channels
There is a decrease in the phase 4 spontaneous depolarisation. There is no change in the duration of the action potential.
What is the action of class 4 drugs on the slow action potential?
Calcium channels blockers
Decrease the gradient of the slope 0 which lengthens the refractory period
Describe the ways in which abnormal impulse generation can arise
Automatic rhythms:
Enhance normal - sinus tachycardia where there is increase APs from the SA node.
Ectopic focus - APs arise from sites other than the SA node.
Triggered rhythms: (abnormal depolarisation)
Delayed afterdepolarisation - arises from the resting potential
Early afterdepolarisation - arises from the plateau eg before the repolarisation is complete
Describe the abnormal conduction patterns
Conduction block: the impulse is not conducted from the atria to the ventricles. Can be 1st 2nd or 3rd degree.
Re-entry rhythms:
There is a unidirectional block where the impulse can travel retrograde
Abnormal anatomy:
Accessory pathway called the bundle of Kent, leads to WPW
What is the principle action of drugs used to treat arrhythmias caused by abnormal generation?
Decrease the phase 4 slow in the pacemaker cells. This slows conduction and increases the threshold.
What is the principle action of drugs used to treat arrhythmias caused by abnormal conduction?
- decrease the conduction velocity by decreasing slope of phase 0
- increase the effective refractory period so the cell wont be excited again
Name the class 1A agents
Procainamide
Quinidine
Disopyramide
What are the specific actions of class 1A agents?
- decrease conduction by decreasing phase 0 of AP
- increase refractory period by enhancing Na inactivation
- decrease automaticity by decreasing slpe of phase 4
- increasing threshold
What are the effects of 1A agents on ECG?
Increased QRS
Prolonged PR interval
Prolong QT interval
What are the uses of class 1A agents?
Not commonly used.
Quinidine - maintain sinus rhythm in AF and atrial flutter and prevent recurrence
Procainamide - IV treatment of supraventricular and ventricular rhythms
What are the side effects of class 1A agents?
hypotension reduced CO dizziness confusion insomnia seizure (high doses) GI effects common Lupus-like syndrome esp procainamide Can actually be pro-arrhythmic
Name the class 1B agents
Lidocaine - IV only
Mexiletine - oral
What are the specific actions of 1B agents?
AP duration slightly decreased
Increased threshold of APs
Decrease phase 0 in fast beating or ischaemic tissue but not normal tissue
What is the effect of 1B agents on ECG?
None in normal.
In fast or ischaemic tissue - increased QRS
What are 1B agents used for?
ventricular tachycardia esp during ischaemia
What are the side effects of 1B agents?
Safer than 1A due to less risk of pro-arrhythmia
CNS - drowsiness, dizziness
Gi upset common
Name the class 1C agents
Flecainide
Propafenone
Can be used orally or IV
What are the effects of 1C agents?
- substantial decrease of phase 0
- decreased automaticity by increasing threshold
- increase AP duration and refractory period esp in rapidly depolarising atrial tissue (AF)
What are the effects of 1C agents on ECG?
Increased PR interval
Increased QRS
Increased QT