Potassium Channel Blockers (Class III Antiarrhythmics)

Amiodarone

The main drug is amiodarone, which is most commonly used as antiarrhythmic. It is effective in a wide range of arrhythmias.

Chemical Nature

Resemble thyroxine, having properties of class 1, II and IV drugs.

Mechanism of Action

It mainly acts by blocking K+ channels. It increases the refractory period. It increases the duration without effecting phase 0 and resting membrane potential.

There are two types of K+ channels:

a. I_Kr

b. I_Ks

Inward potassium current or rectifier also called delayed potassium currents. 2 components are:

a. Rapid component -I_Kr

b. Slow component – I_Ks

The drug acts mainly by blocking I_Kr.Chronic administration also affects I_Ks, It blocks K+ channels. Also have some effects on alpha and beta receptors, blocking them non-competitively, having a small effect.

Pharmacokinetics

Drug is rapidly absorbed after oral route of administration. It has high volume of distribution (greater tissue binding). Since it is highly lipid soluble, it tends to deposit in the tissues, especially adipose tissue. It is metabolized in the liver and excreted in bile. It is slowly released from the tissue binding sites. Half life is longer (about 30 days, may even be up to 100 days).

Uses

Drug has broad antiarrhythmic effect (esp. ventricular arrhythmias)
Supraventricular arrhythmias
Reentrant tachycardia –as prolongs refractory period.

Adverse Effects

Since the drug causes prolongation of refractory period

QT interval is prolonged
Because of resemblance with thyroxine, drug may interfere with functions of thyroid, causing hypo or hyperthyroidism.
As has greater Vd, it gets bound and deposited in cornea, leading to visual problems and photophobia.
Has tendency to deposit in skin, may sensitize it to sunlight. Bluish discoloration occurs on exposed parts on prolonged administration (due to iodine content).
On prolonged administration pulmonary fibrosis might occur
Hepatic damage might occur as is metabolized in the liver.

Drug Interactions

With digoxin, plasma conc. of digoxin increases
Warfarin, similar to digoxin
With beta blockers and Ca++ channel blockers –both cause depression of heart, causing hypotension and bradycardia.

Dose

Start at loading dose, first given 200 mg for 1 week (twice daily). Maintenance dose is 200 mg once daily.

Bretylium

Bretylium is adrenergic neuron blocker and sympatholytic drug. It acts by inhibiting release of NE from adrenergic nerve terminals. It is mainly used for treatment of refractory ventricular arrhythmias esp. after MI and during surgery.