Anti-Arrythmics

 

Anti-arrhythmic drugs are categorised by their effects on the cardiac action potential, rather than the type of drug itself.

 

Class I

Class I drugs bind to voltage gated Na+ channels. They have increased affinity either for the activated or inactivated state of the channel.

 

Class IA

These drugs block active Na+ channels, thereby increasing the action potential duration, the effective refractory period and the duration of the QT interval.

Procainamide

This is a class IA anti-arrythmic.

Quinidine

This is used for rhythm control in SVT as it maintains sinus rhythm.

It is also used for ventricular arrhythmias and the treatment of malaria (similar to quinine).

Class IBLidocaine, mexiletine

These drugs have a higher affinity for the inactivated state of the Na+ channel.

They lengthen the refractory period duration in cardiac cells.

As they bind the inactivated state, they have more action in depolarised cells where many sodium channels will be in the refractory period.

This is found in areas affected by myocardial infarction, meaning they can prevent premature beats in this more excitable tissue.

They are therefore used for preventing ventricular arrhythmias in MI patients.

Lidocaine is one of the least cardiotoxic anti-arrhythmics.

Lidocaine is given intravenously and mexiletine orally. 

Class IC – Flecainide, propafenone

These agents block all morphological forms of the voltage-gated sodium channel.

They cause widening of the QRS complex and prolongation of the PR interval.

They significantly prolong the action potential at the AV node.

They need to be used carefully as they are also pro-arrhythmic.

 

Flecainide

This is used to treat AV nodal reciprocating tachycardia.

It can be used to manage arrhythmias associated with an accessory pathway, such as Wolff-Parkinson-White syndrome.

Used for chemical cardioversion in atrial fibrillation patients without LV dysfunction.

Side effects

Pro-arrhythmic effects

Asthenia

Visual disturbances

Contraindications
  • LV dysfunction, heart failure
  • Significant valve disease
  • 2nd or 3rd degree heart block, bundle branch block

Class II

 

Beta-blockers – Propranolol, bisoprolol

These act by inhibiting sympathetic stimulation to the heart, slowing the heart rate.

They are used as rate control to decrease the heart rate in atrial fibrillation.

They prevent tachyarrhythmias in tissue with increased excitability, e.g., after MI.

They can prevent tachyarrhythmias due to conditions which increase sympathetic activity, e.g., hyperthyroidism.

Side effects

Bronchoconstriction

Fatigue

Sleep disturbances/nightmares

Coldness of extremities

Impotence

Decreased hypoglycaemia awareness

Class III

Amiodarone

This blocks the outward K+ current which inhibits repolarisation.

It also has class I actions by blocking inward Na+ currents and limited class II actions.

It has a very long half-life and is the most commonly prescribed anti-arrhythmic and the first-line drug in ventricular arrhythmias.

It is also used as pharmacological cardioversion (rhythm control) in atrial fibrillation.

Side effects

Corneal microdeposits [EYE]

Hypo/hyperthyroidism (as made of iodine) [THYROID]

Lengthens QT interval, transient fall in BP with IV use [HEART]

Lower lobe pulmonary fibrosis [LUNGS]

Liver fibrosis/hepatitis [LIVER]

Constipation, vomiting, altered taste [GI TRACT]

Sleep disorders, movement disorders [NEURO]

Blue/grey skin deposits (grey-man syndrome), photosensitivity reaction [SKIN]

Thrombophlebitis –> hence given into central veins [VEINS]

CYP450 enzyme inhibitor so enhances effect of warfarin + increases digoxin levels [ENZYMES]

 

Due to these effects, it is important to constantly monitor patients:

– Before starting treatment –> check TFTs, LFTs, serum K+ and CXR

– Then every 6 months –> check TFTs + LFTs

Sotalol

This is a non-selective β-adrenoceptor antagonist that also has class III actions.

It is used in the management of SVT and ventricular tachycardia.

Side effects

Associated with QT interval lengthening leading to torsade de pointes

Class IV

 

Calcium channel blockers – Verapamil, diltiazem

These are cardioselective L-type calcium channel blockers, which act primarily in nodal tissue due to their high use dependence.

They prolong nodal conduction and the refractory period, slowing the heart rate.

They are commonly used for rate control in atrial fibrillation.

Side effects

Sinus bradycardia (transient asystole) – reversed by atropine

Hypotension

Ankle swelling

Constipation (verapamil)

Class V

Adenosine

Adenosine binds P1 receptors which are Gi coupled. This leads to hyperpolarisation of the AV node which causes a transient heart block that slows the heart rate.

It is used for treating supraventricular tachycardias and has a half-life of 8 seconds.

It is contraindicated in asthma as can cause bronchospasm.

Side effects

Arrhythmia (can cause severe bradycardia and asystole)

Chest pain/discomfort and a bad feeling of apprehension

Contraindicated in asthma as can cause bronchospasm

Other anti-arrhythmics

Digoxin

Digoxin also increases vagal activity which helps to slow the heart down.

It can be used in rate control for atrial fibrillation as well as heart failure.

Atropine

This is a muscarinic antagonist which blocks the parasympathetic effects of Ach.

It shifts the balance towards the sympathetic nervous system.

It is used first line to increase heart rate in bradycardia and Ca2+-blocker overdose.

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