Miscellaneous Antiarrhythmic Drugs

Adenosine

Endogenous derivative of purine nucleotide.

 Mechanism of Action

Adenosine acts on the adenosine receptors. They are of various types:

A1, A2A, A2B, A3

Adenosine acts mainly on A1 (atria, SA node, AV node) and A2A (coronary blood vessels) receptors.

The antiarrhythmic effect is due to its effect on A1 receptors, acting as agonist. By blocking these receptors, decrease in AV conduction and decreased firing of SA and AV nodes occurs. As a result dilatation of blood vessels occurs (esp. coronary arteries).

When A1 receptors are blocked, inhibition of:

1. Adenyl cyclase
2. Activity of heart/blood vessels occurs

 Pharmacokinetics

Cannot be given by oral route. Always given I/V.

It has short half life, because they is rapidly metabolized by adenosine deaminase. The effects last only for about 15 seconds.

Adenosine is mainly used for treatment of supraventricular arrhythmias, but has no effect on ventricular arrhythmias (ultimately effects due to K+ channels flux causing hyper polarization and decreasing the activity of the heart.

A1 mediated channels are absent in the ventricles. So has no effects there.

Adenosine may also be used to diagnose the cause of arrhythmias on ECG. If it is effective in relieving the condition, it is supraventricular arrhythmia. If not, then it is ventricular arrhythmia. Thus adenosine can be used to differentiate between arrhythmias.

Adverse Effects:

1. Flushing
2. Headache –due to vasodilatation
3. Bradycardia

 Contraindications

1. Bradycardia
2. Heart block
3. Sick sinus syndrome
4. Asthma –methyl xanthines are used which block adenosine receptors, otherwise may precipitate attack due to contraction of bronchi and bronchioles.

 Drug Interactions

Caffeine

Theophylline

Methyl xanthine

Act by blocking adenosine receptors

Dose

Given I/V. Dose range is initially 3-6 mg in 2 minutes. Dose may be doubled after 2 minutes. But maximum dose must not exceed 165 μg/kg body weight.

Digoxin

Digoxin may be used in supraventricular arrhythmias.

Magnesium

Not commonly used. Can be used in supraventricular arrhythmias. Given sometimes by I/V route.

Mechanism of Action:

Mg++ competes with Ca++ in the cells, causing decreased entry of Ca++, which decreases the activity of heart and blood vessels.

Most commonly adenosine is used in emergency.

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Anti-arrhythmic drugs

Sodium channel blockers (Class 1 antiarrhythmics)

Beta Adrenergic Blockers (Class II antiarrhythmics)

Potassium channel blockers (Class III antiarrhythmics)

Calcium channel blockers (Class IV antiarrhythmics)

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