Calcium slow channel blockers: physiology and anesthetic interactions.

The author reviews the role of calcium in the cardiovascular system and explains the pharmacologic and physiologic effects of the three major calcium channel blocking agents available today. The proposed interactions between calcium blockers and inhalational anesthetics are also elucidated. The anesthetist is well aware of the fatal consequences of a calcium abnormality when confronted with the metabolic syndrome malignant hyperther-mia. The clinical emergence of a new class of drugs, the calcium slow channel blocking agents, emphasizes the need to review the role of calcium in the cardiovascular system in general. With the recent Federal Drug Administration (FDA) approval of intravenous verapamil and oral nifedipine, it will not be long before many surgical candidates present for anesthetic evaluation on calcium channel blocking therapy. These agents will have a major impact on the therapy of numerous cardiovascular disorders including cardiac ar-rhythmias, myocardial ischemic states, hypertension and hypertrophic cardiomyopathy. 1 Consequently, the role of calcium in myocar-dial specialized conducting cells and smooth muscle contraction is elucidated in this article. A review of the three major calcium blocking agents and indications for their use, in turn, provides a preview of the type of patient the anesthetist could 564 encounter on calcium channel blocking therapy. Lastly, although still in the dawn of discovery, some proposed interactions between calcium channel blocking agents and inhalational anesthetics are explored. The membrane action potential The calcium ion is the functional link between the electrical excitation initiated by pacemaker cells in the heart and the actual mechanical contraction of contractile cardiac tissue. 1 The electrical activity of myocardial cells is dependent upon ionic movement across cell membranes. In the heart, that membrane is called the sarcolemma. Electrical stimulation of the cardiac cell results in a membrane action potential that is a five phase process (Table I). Phase 4-Membrane resting potential: The voltage difference between the inside and outside of a non-stimulated cardiac cell is the membrane resting potential,-90 millivolts. It is due primarily to the movement of the potassium ion. The con