How do Class IB antiarrhythmics affect ECG intervals?

Class IB antiarrhythmic agents do not prolong repolarization and do not block potassium channels; on the contrary, they cause some shortening of phase 3 and the effective refractory period. Unlike other classes, drugs of this group do not slow normal sinus rhythm and do not affect cardiac pacemakers, acting exclusively on the ventricles.

Why is lidocaine administered intravenously only and not prescribed as tablets?

Lidocaine undergoes intensive presystemic metabolism (hepatic 'first-pass' effect). When administered orally, it is almost completely inactivated in the portal circulation.

Do Class IB antiarrhythmics affect the normal sinus heart rhythm?

No. Due to very rapid association and dissociation kinetics (less than 1 second), these drugs do not slow normal sinus rhythm, selectively blocking only high-frequency premature impulses.

Which Class IB drug is the agent of choice for digitalis toxicity?

Phenytoin. It effectively eliminates ventricular arrhythmias caused by cardiac glycosides while preserving their beneficial positive inotropic effect.

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Class IB Antiarrhythmic Drugs

Antiarrhythmica classis IB

For medical students2 min readUpdated 2026-10-10

Class IB antiarrhythmic drugs are selective sodium channel blockers that act primarily in the ventricles of the heart. They effectively eliminate ectopic rhythms without depressing myocardial contractility or slowing the normal sinus rhythm.

LocalizationPurkinje fibers and ventricular contractile cardiomyocytes

LidocaineDrug of choice for arrhythmias associated with myocardial infarction

MexiletineStructural analog of lidocaine with high oral efficacy

PhenytoinDrug of choice for cardiac glycoside toxicity

Mechanism of Action and Electrophysiology

Class IB drugs selectively block membrane sodium channels in their inactivated state. They are characterized by very rapid binding and dissociation kinetics (less than 1 second).

As a result, the rate of processes does not exceed the duration of the action potential, which provides important clinical features:

No effect on normal sinus rhythm (heart rate is not slowed).
Selective suppression of high-frequency premature impulses.
Shortening of phase 3 repolarization and the effective refractory period due to potassium efflux.
No direct effect on calcium currents.

Pharmacological Profile and Indications

The targets of these pharmacological agents are exclusively ventricular structures. They do not reduce myocardial contractility and do not affect pacemakers in the sinoatrial node.

The main indication for use is ventricular rhythm disorders, particularly ventricular extrasystoles. Drugs of this group are indispensable in situations where pathological impulses must be suppressed without the risk of additional depression of the heart muscle.

Key Representatives: Lidocaine, Mexiletine, and Phenytoin

The group includes several clinically significant medications with different pharmacokinetics:

Lidocaine (xylocaine) — combines antiarrhythmic and local anesthetic properties. It has a high 'first-pass' hepatic metabolism effect, so it is administered exclusively parenterally (intravenous infusion). The main indication is ventricular arrhythmias during myocardial infarction.
Mexiletine — a structural analog of lidocaine. Due to altered kinetics, it is well absorbed orally (bioavailability up to 100%) and acts for up to 12–16 hours.
Phenytoin (diphenylhydantoin) — originally an antiepileptic drug, recognized as the drug of choice for arrhythmias caused by overdose or intoxication with cardiac glycosides, while preserving their positive inotropic effect.

Side Effects and Safety Profile

Despite good tolerability, the use of Class IB agents requires monitoring due to the risk of adverse reactions:

CNS effects: drowsiness, confusion, seizure activity; for phenytoin — ataxia, nystagmus, and diplopia.
Cardiovascular system: arterial hypotension, depression of atrioventricular conduction, bradycardia, and arrhythmogenic effects.
Specific manifestations: chronic phenytoin use can cause hyperplastic gingivitis (overgrowth of gum tissue).

Mnemonic

Lidocaine saves the ventricles during a heart attack (IV only), Mexiletine is taken at home (oral), and Phenytoin rescues from glycosides.

Frequently asked questions

How do Class IB antiarrhythmics affect ECG intervals?: Class IB antiarrhythmic agents do not prolong repolarization and do not block potassium channels; on the contrary, they cause some shortening of phase 3 and the effective refractory period. Unlike other classes, drugs of this group do not slow normal sinus rhythm and do not affect cardiac pacemakers, acting exclusively on the ventricles.
Why is lidocaine administered intravenously only and not prescribed as tablets?: Lidocaine undergoes intensive presystemic metabolism (hepatic 'first-pass' effect). When administered orally, it is almost completely inactivated in the portal circulation.
Do Class IB antiarrhythmics affect the normal sinus heart rhythm?: No. Due to very rapid association and dissociation kinetics (less than 1 second), these drugs do not slow normal sinus rhythm, selectively blocking only high-frequency premature impulses.
Which Class IB drug is the agent of choice for digitalis toxicity?: Phenytoin. It effectively eliminates ventricular arrhythmias caused by cardiac glycosides while preserving their beneficial positive inotropic effect.

Go deeper

Mechanism of inactivated sodium channel blockade at the cellular level
Zero-order kinetics and elimination features of phenytoin
Impact of liver pathologies on lidocaine clearance and half-life
Comparative analysis of bioavailability and administration routes of mexiletine and lidocaine
Differential diagnosis of central nervous system and cardiovascular adverse effects