Sodium channel blocker
The following naturally-produced substances block sodium channels by binding to and occluding the extracellular pore opening of the channel:
Drugs which block sodium channels by blocking from the intracellular side of the channel include:
- Calcium has been shown to block sodium channels which explains the effects of hypercalcemia and hypocalcemia.
- Caffeine has been shown to inhibit Na+ current in ventricular cells of guinea pigs.
Class I antiarrhythmic agents interfere with the (Na+) channel. Class I agents are grouped by their effect on the Na+ channel, and by their effect on cardiac action potentials. Class I agents are called Membrane Stabilizing Agents. 'Stabilizing' refers to the decrease of excitogenicity of the plasma membrane effected by these agents. A few class II agents, propranolol for example, also have a membrane stabilizing effect.
Class Ia agents
Class Ia agents block the fast sodium channel, which depresses the phase 0 depolarization (i.e. reduces Vmax), which prolongs the action potential duration by slowing conduction. Agents in this class also cause decreased conductivity and increased refractoriness.
Procainamide can be used to treat atrial fibrillation in the setting of Wolff-Parkinson-White syndrome, and to treat wide complex hemodynamically stable tachycardias. Oral procainamide is no longer being manufactured in the US, but intravenous formulations are still available.
While procainamide and quinidine may be used in the conversion of atrial fibrillation to normal sinus rhythm, they should only be used in conjunction with an AV node blocking agent such as digoxin or verapamil, or a beta blocker), because procainamide and quinidine can increase the conduction through the AV node and may cause 1:1 conduction of atrial fibrillation, causing an increase in the ventricular rate.
Class Ib agents
Class Ib antiarrhythmic agents are sodium channel blockers. They have fast onset and offset kinetics, meaning that they have little or no effect at slower heart rates, and more effects at faster heart rates. Class Ib agents shorten the action potential duration and reduce refractoriness. These agents will decrease Vmax in partially depolarized cells with fast response action potentials. They either do not change the action potential duration, or they may decrease the action potential duration. Class Ib drugs tend to be more specific for voltage gated Na channels than Ia. Lidocaine in particular is highly frequency dependent, in that it has more activity with increasing heart rates. This is because lidocaine selectively blocks Na channels in their open and inactive states and has little binding capability in the resting state.
Class Ib agents are indicated for the treatment of ventricular tachycardia and symptomatic premature ventricular beats, and prevention of ventricular fibrillation.
Class Ic agents
Class Ic antiarrhythmic agents markedly depress the phase 0 depolarization (decreasing Vmax). They decrease conductivity, but have a minimal effect on the action potential duration. Of the sodium channel blocking antiarrhythmic agents (the class I antiarrhythmic agents), the class Ic agents have the most potent sodium channel blocking effects.
Class Ic agents are indicated for life-threatening ventricular tachycardia or ventricular fibrillation, and for the treatment of refractory supraventricular tachycardia (i.e. atrial fibrillation). These agents are potentially pro-arrhythmic, especially in settings of structural heart disease (e.g. post-myocardial infarction), and are contraindicated in such settings.
- Sodium Channel Blockers at the US National Library of Medicine Medical Subject Headings (MeSH)
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