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Anti-arrhythmic Drugs: Class I

Anti-Arrhythmic Drugs: Class I

Action Potenial

We draw a simplified diagram to show the phases of a normal action potential in the ventricles. To track the flow of key ions into and out of the cell, indicate the extracellular fluid and intracellular fluid of a ventricular myocyte; write Phases 0-4 along the top.

Phase 0 = Fast sodium channels open and ions quickly move into the cell, which causes rapid depolarization.

– Class 1 antiarrhythmics affect the slope of this line.

Phase 1 = Potassium channels open, which allows potassium ions to move out of the cell and cause early repolarization.
Phase 2 = The plateau phase; potassium continues to move out of the cell, and calcium move into it.
Phase 3 = As more potassium channels begin to open, and calcium channels close, Phase 3 is characterized by rapid repolarization.
Phase 4 = Resting phase, during which “leaky” potassium channels maintain resting membrane potential.
Effective refractory period is the time when new action potentials cannot be triggered.

Effects of Class 1 anti-arrhythmics on this curve

These drugs are fast-sodium channel blockers; they also have local anesthetic properties.

They slow the rate of Phase 0 depolarization and conduction velocity.*

Class 1 drugs are further classified by their effects on the duration of action potentials.

– Class 1a drugs prolong the duration of the action potential – Class 1b drugs shorten its duration – Class 1c drugs have minimal effect

Details

Class 1a

Used to treat atrial and ventricular arrhythmias.
Moderate sodium channel blockers, and also block some potassium channels.

They prolong the action potential duration, effective refractory period, QRS complex, and QT interval.*

Examples and their most common side effects:

– Procainamide is associated with hypotension; long term use is typically avoided because the drug can cause lupus-like syndrome, characterized by arthralgia and purpura. It is generally considered the third best choice in myocardial infarction arrhythmias (behind amiodarone and lidocaine). – Quinidine is associated with gastrointestinal side effects, cinchonism (headache, dizziness, and tinnitus), and thrombocytopenia. – Disopyramide are due to its anticholinergic effects (for example, constipation, urine retention, dry mouth, blurred vision). Can worse glaucoma, and is not suitable for patients with heart failure.

All Class 1a drugs can precipitate torsades de pointes.

Class 1b

Used to treat ventricular arrhythmias.
They are weak sodium channel blockers, and are selective for ischemic tissue and depolarized Purkinje fibers and ventricular myocytes.

These drugs shorten the action potential duration and effective refractory period, but have no effect on the QRS complex or QT interval.*

Lidocaine is the prototypical class 1b drug; it is second in line to treat arrhythmias associated with myocardial infarction (behind amiodarone).

– Lidocaine has low toxicity, but, in vulnerable patients or when given in high doses or administered too rapidly, it is associated with neurological, and, more rarely, cardiovascular side effects. – Neurological effects include paresthesia (tingling, “pins and needles,” usually in the hands and feet), dizziness, and confusion. – Cardiovascular effects include sinus bradycardia, arrhythmias, and shock.

Phenytoin is often considered an honorary member of class 1b anti-arrhythmias; it is used to treat arrhythmias caused by digitalis.

Class 1c

We'll list characteristics for the prototypical drug, Flecainide; as we’ll see, however, not all class 1c drugs have the same effects.
Used to treat supraventricular arrhythmias.
Strong sodium-channel blockers that also block some potassium channels.

Flecainide has no effect on the duration of action potentials or the effective refractory period; it prolongs the QRS complex with little effect on the QT interval.*

Common side effects are dizziness, headaches, and visual disturbances, which may include blurred vision or difficulty focusing, or the appearance of flashing lights or spots.
Propafenone has similar side effects; it can exacerbate heart failure (propafenone also blocks beta channels and has negative inotropic effects).

– Unlike flecainide, it is associated with prolonged QT intervals and effective refractory periods.

Moricizine is another class 1c anti-arrhythmic, but it is no longer used in the United States.

Class 1c drugs are pro-arrhythmic; they are not suitable for patients with structural heart disease.*

For References, please see our full tutorial on Class I anti-arrhythmic drugs.

Anti-arrhythmic Drugs: Class I

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