These drugs interact with cell membranes to interfere with movement of calcium into cells through ion-specific channels (known as slow channels because their transition among the resting, activated, and inactivated states is delayed compared with fast sodium channels). Calcium channel blockers are a heterogeneous group of drugs with dissimilar structures and different electrophysiologic and pharmacologic properties. These drugs are most useful for the treatment of supraventricular tachydysrhythmias and coronary artery vasospasm (Table 15-6: Comparative Effects of Calcium Channel Blockers).
- Verapamil is the drug of choice for termination of supraventricular dysrhythmias, and it is also effective in slowing the heart rate in patients with atrial fibrillation and atrial flutter. There is a dose-dependent increase in the P-R interval on the electrocardiogram and a delay in conduction of cardiac impulses through the AV node.
- Caution must be exercised when treating patients with Wolff-Parkinson-White syndrome because verapamil may increase conduction velocity in the accessory tract.
- Unlike βantagonists, verapamil does not increase airway resistance in patients with obstructive pulmonary disease.
- Nifedipine is more effective than nitroglycerin for treatment of angina pectoris caused by coronary artery vasospasm.
- Vasodilation results in compensatory tachycardia, and cardiac output may increase as a result of afterload reduction.
- Administration of nifedipine is useful during anesthesia when there is evidence of myocardial ischemia associated with hypertension.
- Diltiazem is an effective coronary artery vasodilatory but a poor peripheral vasodilator; it may produce bradycardia.
- Nicardipine produces vasodilation of coronary arterioles without altering activity of the sinus node or conduction of cardiac impulses through the AV node.
- Nimodipine is a highly lipophilic drug that produces somewhat selective vasodilation of cerebral arteries, resulting in a favorable effect on the severity of neurologic deficits caused by cerebral vasospasm after subarachnoid hemorrhage.
- Felodipine and Clevidipine
- Felodipine is a daily drug that can be administered for blood pressure control.
- Clevidipine is an ultra-short-acting (ester hydrolysis) IV calcium channel blocker used for management of moderate to severe hypertension (alternative to nitroglycerin, nitroprusside and nicardipine).
- Calcium Channel Blockers and Anesthesia. These drugs may exhibit additive myocardial depressant effects with volatile anesthetics, which may also interfere with inward calcium movement. Opioids do not seem to alter the response to calcium channel blockers. Calcium channel blockers seem to augment the effects of both depolarizing and nondepolarizing muscle relaxants in a manner similar to those of “mycin” antibiotics.