What are calcium channel blockers?

As part of your NAPLEX exam, you are expected to have a rounded knowledge of general and clinical pharmacology. Here, we review the fundamental details about calcium channel blockers that you need to know.

Calcium channel blockers can be classified as follows:

• Dihydropyridines
  • amlodipine (Norvasc)
  • nifedipine (Procardia, Adalat)
  • felodipine (Plendil)
• Non-dihydropyridines
  • verapamil (Calan, Isoptin)
  • diltiazem (Cardizem)

Calcium channel blockers are an important class of medicines involved in the treatment of many common and serious conditions.

For example – they are used in the treatment and management of:

• Hypertension – nifedipine and amlodipine are used either as first or second-line drugs to reduce risk of stroke, heart attack and death from cardiovascular disease.
• Stable angina – all calcium channel blockers can be used for this indication.
• Supraventricular tachycardias – verapamil and diltiazem are used to control cardiac rate in patients with conditions such as atrial fibrillation, atrial flutter and supraventricular tachycardia.

Of course, indications are not limited to these.

For example, nifedipine has also been used to treat Reynaud’s phenomenon and it has also been used as a tocolytic; a drug used to suppress premature labor so that other drugs, such as dexamethasone, can be used to promote fetal lung maturation.

Mechanism of action

Of course, calcium channel blockers work to reduce calcium entry into both vascular and cardiac cells – lowering intracellular calcium concentration and causing relaxation and vasodilation of arterial smooth muscle.

Calcium channel blockers reduce myocardial contractility and suppress cardiac conduction. This effect is pronounced at the atrioventricular (AV) node – which slows ventricular rate.

By reducing cardiac activity, contractility and afterload, calcium channel blockers have the effect of reducing myocardial oxygen demand – thereby acting as a prophylaxis against angina.

Non-dihydropyridines – such as verapamil – are more cardioselective than dihydropyridines – such as nifedipine – which are more selective for vasculature.

Side effects

Calcium channel blockers are associated with their own range of prospective side effects. These include:

• Ankle swelling
• Gingival hyperplasia
• Facial flushing
• Dizziness / headache
• Palpitations
• Constipation

Constipation is significantly more likely with verapamil, though may occur with any of the medicines listed above. More seriously, these drugs have the potential to cause bradycardia, AV block and cardiac failure.

Calcium channel blockers are also associated with side effects not listed in this guide.

Clinical Pharmacology

Here are some of the key factors of clinical pharmacology of calcium channel blockers you need to know:

• Calcium channel blockers should not be used alongside beta blockers – except under strict supervision – because the combination increases the risk of heart failure, bradycardia and systole.
• Non-dihydropyridine drugs should be used with caution in patients with poor left ventricular function as it increases the risk of heart failure.
• Similarly, they should be avoided in cases of AV nodal conduction delay.
• Dihydropyridine drugs should be avoided in cases of unstable angina – as myocardial oxygen demand increases as a product of reflex cardiac contractility and tachycardia.
• Calcium channel blockers have different half-lives. For example – amlodipine carries a high half-life of around 42 hours, allowing once daily dosing. Felodipine isn’t too far behind, at 25 hours. In contrast, nifedipine has a half-life of 2-3 hours, verapamil of 3-8 hours and diltiazem 6-8 hours. However, drugs – like diltiazem – are also available in extended-release formulations that permit once daily dosing.

Calcium channel blockers remain an important medicinal class used in the treatment of a variety of common and serious conditions. As part of your NAPLEX or clinical pharmacy exam, you can expect to encounter clinical pharmacology questions on this vital drug class.

Above, we put together the fundamental details you need to know. Build upon this detail in the weeks and months ahead to ensure that, when you do face questions on this topic, you remain more than prepared. If you’d like to test your knowledge of calcium channel blockers, register now to receive exclusive access.