Wednesday, September 07, 2005

Calcium channel blockers

Calcium channel blockers are a class of drugs effectting the muscle of the heart and the muscles of the rest of the body. The main action of calcium channel blockers is to lower the blood pressure. Most calcium channel blockers decrease the force of contraction of the myocardium. This is known as the negative inotropic effect of calcium channel blockers. They are avoided in patients with cardiomyopathy because of the negative inotropic effects.

Many calcium channel blockers also slow down the conduction of electrical activity within the heart, by blocking the calcium channel during the plateau phase of the action potential of the heart. This causes a lowering of the heart rate and may cause heart blocks. This is known as the negative chronotropic effect of calcium channel blockers. The negative chronotropic effects of calcium channel blockers make them a commonly used class of agents in individuals with atrial fibrillation or flutter in whom control of the heart rate is an issue.

Calcium channel blockers work by blocking voltage-sensitive calcium channels in the heart and in the blood vessels. This prevents calcium levels from increasing as much in the cells when stimulated, leading to less contraction.

This decreases total peripheral resistance by dilating the blood vessels, and decreases cardiac output by lowering the force of contraction. Because resistance and output drop, so does blood pressure.

Unlike with beta-blockers, the heart is still responsive to sympathetic nervous system stimulation, so blood pressure can be maintained more effectively.


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