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).
Due to this, 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 sites of action of these pharmacological agents are exclusively ventricular structures. They do not decrease myocardial contractility and do not affect pacemakers in the sinoatrial node.
The main indication for prescription is ventricular rhythm disorders, particularly ventricular extrasystoles. Drugs of this group are indispensable in situations where it is necessary to suppress pathological impulse generation without the risk of additional depression of the heart muscle.
Group representatives: Lidocaine, Mexiletine, and Phenytoin
The group includes several clinically significant drugs with different pharmacokinetics:
- Lidocaine (xylocaine) — combines antiarrhythmic and local anesthetic properties. It has a high first-pass hepatic effect, and therefore is administered exclusively parenterally (intravenous infusion). The main indication is ventricular arrhythmias in myocardial infarction.
- Mexiletine — a structural analog of lidocaine. Due to altered kinetics, it is well absorbed when taken orally (bioavailability up to 100%) and acts for up to 12–16 hours.
- Phenytoin (diphenylhydantoin) — originally an antiepileptic drug, which is 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:
- Central nervous system: somnolence, confusion, seizure reactions are possible, and for phenytoin — ataxia, nystagmus, and diplopia.
- Cardiovascular system: arterial hypotension, depression of atrioventricular conduction, bradycardia, and arrhythmogenic effect.
- Specific manifestations: prolonged use of phenytoin can lead to hyperplastic gingivitis (overgrowth of gingival tissue).
Briefly: answers to topical 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.
Lidocaine saves the ventricles during infarction (IV only), Mexiletine is taken at home (orally), and Phenytoin saves from glycosides.
Class and exam questions
Why is lidocaine administered intravenously only rather than prescribed in tablets?
Lidocaine undergoes intensive presystemic metabolism (first-pass hepatic effect). When administered orally, it is almost completely inactivated in the portal circulation.
Do Class IB antiarrhythmics affect normal sinus rhythm?
No. Due to very rapid association and dissociation kinetics (less than 1 second), the 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.
What else to study on this topic
- Mechanism of inactivated sodium channel blockade at the cellular level
- Zero-order kinetics and elimination characteristics 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 side effects
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