CALCIUM CHANNEL BLOCKERS
In addition to their antianginal and antiarrhyth-mic effects , calcium channel blockers also reduce peripheral resistance and blood pressure. The mechanism of action in hypertension (and, in part, in angina) is inhibition of calcium influx into arterial smooth muscle cells.
Verapamil, diltiazem, and the dihydropyridine family (amlo-dipine, felodipine, isradipine, nicardipine, nifedipine, and nisoldipine) are all equally effective in lowering blood pressure,and many formulations are currently approved for this use in the USA. Clevidipine is a newer member of this group that is formu-lated for intravenous use only.
Hemodynamic differences among calcium channel blockers may influence the choice of a particular agent. Nifedipine and theother dihydropyridine agents are more selective as vasodilators and have less cardiac depressant effect than verapamil and diltiazem. Reflex sympathetic activation with slight tachycardia maintains or increases cardiac output in most patients given dihydropyridines. Verapamil has the greatest depressant effect on the heart and may decrease heart rate and cardiac output. Diltiazem has intermediate actions. Doses of calcium channel blockers used in treating hypertension are similar to those used in treating angina. Some epidemiologic studies reported an increased risk of myocardial infarction or mortality in patients receiving short-act-ing nifedipine for hypertension. It is therefore recommended that short-acting oral dihydropyridines not be used for hypertension. Sustained-release calcium blockers or calcium blockers with long half-lives provide smoother blood pressure control and are more appropriate for treatment of chronic hypertension. Intravenous nicardipine and clevidipine are available for the treatment of hypertension when oral therapy is not feasible; parenteral vera-pamil and diltiazem can also be used for the same indication. Nicardipine is typically infused at rates of 2–15 mg/h. Clevidipine is infused starting at 1–2 mg/h and progressing to 4–6 mg/h. It has a rapid onset of action and has been used in acute hyperten-sion occurring during surgery. Oral short-acting nifedipine has been used in emergency management of severe hypertension.