Mechanism of antiarrhythmic action
The basis of our heart's rhythm is the cardiac pacemakers. They generate the so-called "calcium" action potentials. Calcium channel blockers (CCBs) interfere with this physiological process by decreasing automaticity and inhibiting conduction. This effect is predominantly realized in the nodes of the cardiac conduction system.
Due to this mechanism, Class IV drugs function as effective antiarrhythmics, but their spectrum of action is strictly limited: they help only in supraventricular arrhythmias.
Classification of calcium channel blockers
The CCB group is extremely heterogeneous. The sensitivity of the channels to drugs directly depends on the chemical structure of the substance molecule. L-type channels dominate in the heart and blood vessels, but others also exist (e.g., T-type). In total, there are 6 classes of CCBs depending on their selectivity and site of action:
- Selective Class I CCBs (phenylalkylamines): the main representative is verapamil. The target is predominantly L-channels of cardiomyocytes (heart).
- Selective Class II CCBs (dihydropyridines): these include nifedipine, amlodipine, and lacidipine. Their target is L-channels of vascular smooth muscle cells, which has a pronounced effect on blood pressure.
- Selective Class III CCBs (benzothiazepines): the representative is diltiazem. It provides uniform blockade of L-channels in both the heart and blood vessels.
- Class IV CCBs: the representative is cinnarizine. It is characterized by a predominant effect on cerebral blood vessels.
- Class V CCBs: the representative is prenylamine (also belongs to phenylalkylamines).
- Class VI CCBs: the representative is mibefradil. Its unique feature is the blockade of T-type calcium channels.
Pharmacology of verapamil
Verapamil (Verapamil) is the main drug of choice in the treatment of arrhythmias within this group. According to the chemical classification, it belongs to the phenylalkylamines.
- Pharmacokinetics: When administered orally, it demonstrates a high degree of absorption (90–92%) and is actively bound to plasma proteins (up to 90%). The maximum effect develops in 1.5–2 hours, and the half-life ranges from 3 to 7 hours. Elimination occurs via the kidneys and bile (unchanged and as conjugates).
- Specific side effects: Caused by the blockade of Ca²⁺ channels. Cardiac effects include bradycardia (slowing of the rhythm) and a decrease in contraction force. Conduction effects include suppression of atrioventricular (AV) conduction with the risk of developing complete AV block. Vascular effects may include arterial hypotension.
- Nonspecific effects: Include allergic reactions and gastrointestinal issues (constipation, nausea, vomiting).
Characteristics of diltiazem
Diltiazem (Diltiazem) is used less frequently as an antiarrhythmic agent. It belongs to the benzothiazepine group.
- Pharmacokinetics: The drug is well absorbed (up to 90%), but its bioavailability remains low (about 40%) due to a pronounced first-pass effect. The action begins 30 minutes after administration, peak plasma concentration is reached in 3 hours, and the half-life is 3–4 hours. It is metabolized by acetylation and excreted via the intestine (unchanged and as metabolites).
- Side effects: Mainly caused by vasodilation. Patients may experience headache and dizziness from the central nervous system. Reflex tachycardia and peripheral edema are possible from the cardiovascular system. An effect on the musculoskeletal system in the form of muscle weakness is also noted.
Effects on the myocardium and blood vessels
In addition to their antiarrhythmic action, CCBs have a systemic effect on hemodynamics, which is often considered a side effect when treating rhythm disorders.
- Effect on myocardial contractility: Blockade of calcium channels in working ventricular cardiomyocytes leads to a decrease in the force of cardiac contractions (negative inotropic effect). The clinical significance of this phenomenon is high—there is a risk of reduced cardiac pumping function.
- Vascular effects: The drugs act on angiomyocytes (vascular smooth muscle cells), causing their dilation (vasodilation). A logical consequence of this process is the development of arterial hypotension.
Briefly: answers to topical questions
What are the absolute contraindications to the use of verapamil?
Contraindications and limitations for the use of verapamil include:
- high-degree sinoatrial or atrioventricular block;
- sick sinus syndrome and bradycardia (HR <60 bpm);
- chronic heart failure and severe left ventricular dysfunction (LVEF <40%);
- age under 1 year;
- combination of atrial fibrillation with Wolff-Parkinson-White syndrome;
- use of drugs dependent on P-glycoprotein or CYP3A4.
With which drug groups is it strictly not recommended to combine verapamil due to the risk of cardiac arrest?
Co-administration of verapamil with beta-blockers is highly undesirable. Drug interactions also include combinations with drugs that have a negative inotropic effect, including sodium channel blockers, as well as with rate-lowering drugs.
Why are Class IV antiarrhythmic agents ineffective in ventricular arrhythmias?
Class IV antiarrhythmic agents are ineffective in ventricular arrhythmias because their therapeutic action is localized predominantly in the nodes of the cardiac conduction system. The physiological basis of their effect is the blockade of slow calcium channels. Pacemakers in the sinus and atrioventricular nodes generate "calcium" action potentials. Blockade of these channels reduces automaticity and conduction and increases the effective refractory period in the nodes, which is why the drugs are effective only in supraventricular arrhythmias. In working ventricular cardiomyocytes, calcium channel blockade does not exert an antiarrhythmic effect, but causes only a side effect—a decrease in the force of cardiac contractions (negative inotropic effect).
How to remember the selectivity of the first three classes: Verapamil — In the heart (Class I), Diltiazem — Divides equally (Class III, heart and vessels), Dihydropyridines — Pressure (Class II, vessels only).
Class and exam questions
In which arrhythmias are Class IV drugs effective?
They are effective exclusively in supraventricular arrhythmias. They do not affect ventricular rhythm disorders.
Why does diltiazem have low bioavailability despite good absorption?
Despite the fact that diltiazem is absorbed by 90%, its bioavailability is only 40% due to a pronounced first-pass hepatic effect.
How do calcium channel blockers affect blood pressure?
They act on angiomyocytes, causing dilation of blood vessels (vasodilation), which naturally leads to the development of arterial hypotension.
What is the main cardiac adverse effect of verapamil?
Verapamil causes a negative inotropic effect (decreased contraction force) and suppresses AV conduction, which creates a risk of developing complete AV block and reducing cardiac pumping function.
What else to study on this topic
- Chemical structure and formulas of verapamil and diltiazem (visual identification)
- Characteristics of T-type calcium channels and the mechanism of action of mibefradil
- First-pass effect using the pharmacokinetics of diltiazem as an example
- Mechanisms of development of reflex tachycardia during the administration of benzothiazepines
- Comparative characteristics of the selectivity of dihydropyridines and phenylalkylamines
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