Calcium channel blockers are medications that are primarily used to treat cardiac arrhythmias, hypertension, and angina pectoris.
Calcium channel blockers are used to a lesser extent for prophylaxis of migraines and for symptomatic treatment of Raynaud phenomenon, which is caused by reduced blood flow in tiny peripheral vessels.
Finally, calcium channel blockers also relax uterine smooth muscle, which is useful to prevent premature uterine contractions, and this can help delay preterm labor.
Now, calcium channel blockers can be administered orally or intravenously, and are divided into two main groups, dihydropyridines and non-dihydropyridines.
Dihydropyridines include amlodipine, nicardipine, nifedipine, felodipine, and clevidipine, which have a more potent action on the blood vessels, specifically the arterioles, than the heart.
As a result, they’re preferred to treat hypertension. Another dihydropyridine is nimodipine, which has the added benefit of being able to cross over the blood-brain barrier.
So, it can be used to prevent or treat cerebral vasospasm caused by an aneurysmal subarachnoid hemorrhage.
On the other hand, non-dihydropyridines include verapamil and diltiazem, which have a more potent action on the heart than the blood vessels.
Verapamil is highly selective for cardiac calcium channels; and, thus, is mainly used to treat angina pectoris and arrhythmias.
Diltiazem is equally good at blocking both cardiac and vascular calcium channels, and so it’s effective in hypertension and arrhythmias.
Now, once administered, calcium channel blockers block voltage-gated calcium channels, and they act by decreasing the amount of calcium entering the cell.
Normally, calcium is required for the contraction of both the cardiac muscles and the vascular smooth muscles. So, the decreased calcium reduces the muscle’s ability to contract, ultimately relaxing them.
Now, calcium channel blockers relax vascular, particularly arterial, smooth muscles, which helps reduce the blood pressure, and peripheral vascular resistance, or the afterload, which is the pressure that the heart must work against to eject the blood.
At the same time, in the heart, calcium channel blockers relax the coronary vessels, which improves the oxygen delivery to the heart; and they also reduce the force of contraction of the cardiac muscles, which reduces the oxygen demand of the heart.
Additionally, calcium channel blockers prevent the depolarization of cardiac pacemaker cells in the SA and AV nodes, which are responsible for generating and maintaining the heart rate
This way, these medications reduce the firing and conduction of the impulses through these nodes, eventually decreasing the heart rate.
Now, the vasodilatory action of calcium channel blockers may also lead to some side effects, including headache, dizziness, flushing of the skin, peripheral edema, and hypotension, which is more common with dihydropyridines.
Moreover, hypotension can trigger reflex tachycardia, in order to maintain adequate tissue perfusion. Finally, dihydropyridines can also cause gingival hyperplasia, the cause of which is still unclear.
On the other hand, non-dihydropyridines decrease the heart rate and contractility, so they can cause additional side effects like bradycardia, constipation, and hyperprolactinemia.
Now, due to their depressive action on the heart, non-dihydropyridines, and to a lesser extent, dihydropyridines, are contraindicated in clients with pre-existing bradycardia, heart block, and heart failure.
