Antiarrythmic Drugs: Types, Uses and Side Effects

Antiarrythmic medications are those which are given to prevent and treat the abnormal hearbeats ( arrythmias). These arrythmias occur when there is a disturbance in the electrical activity in the heart. The reason can be many diseases like Coronary Artery disease, Heart attacks, electrolyte defects , anonymous rhythm formation or any infections. The different kind of arrythmias that requires the treatment are- Atrial fibrillation, atrial flutter , ventricular tachycardia etc. There are different classes of Antiarrythmic drugs.

Antiarrythmic Drugs Types

1. Class I antiarrythmics - Sodium Channel blockers
2. Class II antiarrythmics - Beta blockers
3. Class III antiarrythmics- Potassium Channel Blockers
4. Class IV antiarrythmic - Calcium channel blockers

Class I ( Sodium Channel blockers)

• They slow or block conduction especially in the depolarized cells by decreasing the slope of phase 0 depolarization.
• They have increased action at faster rate.
• Drugs usually dissociate from the receptor in the resting state, so increased Heart Rate leads to short diastole and so has less time to dissociate.
• Effects IC>IA>IB

Class IA: Quinidine, Procainamide, Disopyramide

Mechanism of Action

They act by moderate Na+ channel blockade causing increase in AP duration, QT interval and effective refractory period in ventricular action potential.

Clinical Use

• Atrial and Ventricular arryhtmias 
• Ectopic SVT and VT

Adverse Effects

• Procainamide causes Reversible SLE like syndrome.
• Disopyramide causes Heart failure.
• Quinidine causes Cinchonism ( headache and tinnitus)
• Thrombocytopenia
• Torsades de pointes due to increased QT interval.

Class IB: Lidocaine, Phenytoin, Mexiletine

Mechanism of Action

These drugs act by weakly blocking sodium channels leading to decrease in the AP. They mainly affect the ischemic tissue and depolarised Purkinje and ventricular tissue.

Clinical Use

• Acute Ventricular arrythmias ( especially post MI)
• Digitalis induced arryhtmias

Adverse Effects

• CNS stimulation/depression
• Cardiovascular depression

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Class IC: Flecainide and Propafenone

Mechanism of Action

They act by causing strong channel blockade. They significantly prolong the ERP in AV nodes and accessory bypass tracts. They have no effect on ERP in purkinje and ventricular tissue . There is minimal effect on AP duration.

Clinical Use 

It is used in the treatment of Supraventricular Tachycardia including Atrial fibrillations. They are also used as a last resort in refractory VT.

Adverse Effects

It is a Proarrythmic, so it is contraindicated post-MI ( In structural and ischemic heart disease).

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Class II Antiarrythmics

Beta blockers come under this category . The drugs are: Metoprolol, Propanolol, Esmolol, Atenolol, Timolol and Carvedilol.

Mechanism

These drugs act by decreasing the SA and Av nodal activity by decreasing cAMP and decreasing Calcium currents. They also suppress the abnormal pacemakers by decreasing the slope of phase 4. They also increase the PR interval by acting on AV nodes.

Clinical Use

It is used in the treatment of Supraventricular Tachycardia. They are also used for ventricular rate control for atrial fibrillation and atrial flutter.

Adverse Effects

• Impotence, exacerbation of COPS and asthma.
• Cardiovascular effects are - Bradycardia, AV block and Heart failure.
• Metoprolol can cause dyslipidemia, so the clinicians should also evaluate the Lipid profile of the patient while the patient is on metoprolol.
• Propanolol can exacerbate the vasospasm in vasospastic angina.
• Beta blockers ( except non selective alpha and beta antagonists carvedilol and labetolol) cause unopposed alpha agonism if given alone for pheochromocytoma or for cocaine toxicity. 
• Beta blocker overdose is treated with saline, atropine and glucagon.

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Class III Antiarrythmics

The drugs are Potassium channel blockers. The names are: Amiodarone, Ibutilide, Dofetilide and Sotalol.

Mechanism of Action

They cause blockage of potassium channels and hence increases the AP duration and QT interval. They also increase the Effective refractory period.

Clinical Use

They are used in the treatment of Atrial fibrillation, Atrial flutter and ventricular tachycardia (amiodarone and sotalol).

Adverse Effects

• Sotalol- Torsades de pointes, Excessive beta blockade
• Ibutilide- Torsades de pointed
• Amiodarone - Hepatotoxicity 
• Pulmonary fibrosis
• Hypothyroidism or hyperthyroidism
• Corneal deposits , blue/gray skin deposits resulting in photodermatitis as they act as haptens.
• Neurologic effects
• Constipation
• Cardiovascular effects are bradycardia, heart block and heart failure.

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Class IV antiarrythmics

The drugs are Calcium channel blockers . The names are Verapamil and Diltiazem.

Mechanism of Action

They act by blocking Calcium channels leading to decrease conduction velocity and increase ERP and PR interval.

Clinical Use

• They are used for rate control in atrial fibrillation
• They are also used for prevention of nodal arrythmia. ( Eg. SVT)

Adverse Effects

• Constipation
• Flushing
• Edema 
• Cardiovascular effects ( Heart Failure, AV block and Sinus node depression)

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Other Antiarrythmics

Adenosine

It causes increased efflux of K+ ions out of cells causing hyperpolarization of cell, decreased ICa and decreasing AV node conduction.

Clinical Use

Drug of choice for treatment of SVT. It is very short acting and its effect is blunted by theophylline and caffeine.

Magnesium

Effective in treating torsades de pointes and digoxin toxicity.

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