Antiarrhythmics

Sino-Atrial Node

Controls the heartbeat rate (60-100 bpm)

Cardiac Action Potential

Cardiac Arrhythmias

- Electrical conduction system malfunction

- Abnormal rhythms cause the heart to pump less effectively

- Result from disorders of impulse formation, conduction or both

Arrhythmias occur when:

- Heart's natural pacemaker develops an abnormal rate or rhythm

- Normal conduction path is interrupted

- Another part of the heart takes overas pacemaker

Causes of arrhythmias

- cardiac ischemia

- excessive discharge or sensitivity to autonomic transmitters

- exposure to toxins

- drugs, smoking, alcohol, caffeine

- unknown etiologies

Tachycardia

unusually fast heartbeat

Bradycardia

unusually slow heartbeart

Atrial fibrillation

- atria quiver instead of contract normally

- risk that blood pools in aria = clots

- supraventricular tachycardia

Ventricular fibrillation

- life threatening condition in which the heart ceases to beat regularly and instead"quivers" or fibrillates very rapidly

- 350bpm or more

- type of ventricular arrhythmia

Extra Beats Arrhythmias

- premature atrial contractions

- premature ventricular contractions

Supraventricular tachycardias

- atrial fibrillation

- atrial flutter

- paroxysmal supraventricular tachycardia

Ventricular arrythmias

- ventricular fibrillation

- ventricular tachycardia

Goal of antiarrhythmic therapy

restore normal pacemaker activity and modify impaired conduction

Types of Antiarrhymtic Therapies

- Na+ or Ca2+ channel blockade

- Prolongation of effective refractory period

- Blockade of sympathetic effects on the heart

Class I Antiarrhythmics

- Sodium channel blockers

- slows depolarization in fast AP cells

- 1a: atrial and ventricular, PROCAINAMIDE & QUINIDINE

- 1b: acute tx of ventricular after MI, LIDOCAINE (IV)

- 1c: chronic supraventricular & vent., FLECAINIDE, PROPAFENONE

Fast AP cells =

atria, ventricles, His/Purkinje

Slow AP Cells =

sinus (SA) and AV nodes

PROCAINAMIDE & QUINIDINE

Class 1a antiarrythmic (sodium channel blockers); tx for atrial and ventricular arrhythmias (all purpose)

LIDOCAINE (solution injected IV)

Class 1b antiarrythmic (sodium channel blocker); For acute treatment of ventricular arrhythmias especially after MI

FLECAINIDE & PROPAFENONE

Class 1c antiarrythmic (sodium channel blocker); For chronic treatment of life-threatening supraventricular & vent. arrhythmias

adverse effect of class 1b antiarrhythmics

blurred vision, dizziness

Class II Antiarrhythmics

- Beta blockers = block sympathetic

- Decrease heart rate, contractility, and conduction velocity

- slows depolarization in slow AP cells

- Increase AP duration and effective refractory period

- Used for tachyarrhythmias, esp from inc SNS

- Ex) PROPANOLOL (INDERAL)

- Adverse effects: hypotension, bradycardia

PROPRANOLOL (Inderal)

- Class II Beta blocker

- Can block arrhythmias induced by exercise or apprehension

- Decrease heart rate, contractility, and conduction velocity

- Increase AP duration and effective refractory period

Class III Antiarrythmics

- Potassium Channel Blockers

- Prolongs repolarization phase by blocking outward K+ flux (prolongs AP)

- For treatment of ventricular tachycardia and Afib

- Ex) AMIODARONE (Cordarone)

AMIODARONE (Cordarone)

- Class III antiarrhythmic (K+ channel blocker)

- SE: corneal deposits, pulmonary fibrosis & pneumonitis, and can affect thyroid fxn

- tx ventricular tachy and Afib

Class IV Antiarrythmics

- Calcium Channel Blockers

- Prolong repolarization phase by blocking inward calcium current in slow AP cells

- Predominantly for treatment of atrial arrhythmias

- Ex) VERAPAMIL (ISOPTIN) & DILTIAZEM (CARDIAZEM CD)

VERAPAMIL (ISOPTIN) & DILTIAZEM (CARDIAZEM CD)

- Class IV antiarrythmic (Calcium Channel Blockers)

- prolonds repolarization phase in slow AP cells by blocking inward calcium current

- tx for atrial arrythmias

ADENOSINE

- DOC for paroxysmal supraventricular tachycardia

- IV only, extremely short half-life (seconds)

- Inc. K+ conductance and dec Ca2+ activity => hyperpolarization

- Reduces release of NE from nerve terminals

- Net effect = reduce automaticity

DIGOXIN (LANOXIN)

- Direct suppression of AV node, increase refractory period

- for atrial fibrillation

Magnesium

dec Ca2+ influx

ATROPINE

- increases rate & conduction velocity

- Tx for bradyarrythmias

- SE: dry mouth, mydriasis

ISOPROTERENOL

- Increases heart rate & contractility

- tx for bradyarrhythmias

- Used to maintain adequate heart rate & CO in patients with AV block

- SE: tachycardia, flushing

Antiarrhythmics Adverse Effects

- affect normal cardiac function, have the potential to be pro-arrhythmic

- Tachycardia, not necessarily life-threatening, may initiate a life-threatening ventricular arrhythmia when treated