Antianginal Drugs

Voltage-gated L-type calcium channels are the main route of calcium entry into both vascular smooth muscle cells and cardiac pacemaker and conduction tissue. In vascular smooth muscle, calcium entry triggers the contraction that maintains vascular tone; blocking the channel causes the muscle to relax, producing vasodilation. In the sinoatrial and atrioventricular nodes, calcium entry drives the pacemaker depolarization that generates and conducts the heartbeat; blocking the channel here slows heart rate and slows conduction through the atrioventricular node. In ventricular muscle, calcium entry also contributes to the force of contraction, so channel blockade can reduce contractility as well.

Although all calcium channel blockers act on the same channel, individual drugs differ markedly in how selectively they target vascular tissue versus cardiac tissue. Dihydropyridines are highly vascular-selective: their dominant effect is peripheral and coronary vasodilation, with minimal direct effect on heart rate or contractility at usual doses. Non-dihydropyridines, which include verapamil and diltiazem, have much more balanced effects on both vascular and cardiac tissue, producing vasodilation alongside meaningful heart rate and conduction slowing.

This single distinction in tissue selectivity explains nearly everything that differs between the two subclasses clinically: which hemodynamic lever each one pulls, which combinations are safe, and which adverse effects are most likely.

Both verapamil and diltiazem produce peripheral and coronary vasodilation alongside negative chronotropy, or heart rate slowing, and negative dromotropy, or slowed conduction through the atrioventricular node. Because vasodilation and heart rate reduction happen together rather than separately, these agents do not produce the reflex tachycardia seen with short-acting dihydropyridines: heart rate tends to fall or stay unchanged rather than rise.

Verapamil has the most balanced vascular and cardiac effects of the two agents, producing the most pronounced heart rate and conduction slowing as well as a clinically significant reduction in contractility. Constipation is common with verapamil, resulting from the same calcium channel blockade acting on intestinal smooth muscle. Diltiazem is somewhat more vascular-selective than verapamil, producing meaningful but generally milder cardiac rate-limiting effects and is better tolerated overall.

Both agents are metabolized through the same hepatic enzyme pathway responsible for metabolizing many other commonly used drugs, so both carry potential for clinically relevant drug interactions that should be considered when either is added to a patient's regimen.

Peripheral edema, typically affecting the ankles, is the most common adverse effect of dihydropyridines. The mechanism is purely hemodynamic: arteriolar dilation occurs without a matching degree of venodilation, raising pressure in the capillary bed and pushing fluid into the surrounding tissue. This is not caused by sodium or fluid retention, which is an important distinction from edema due to heart failure or kidney disease, and it does not respond to a diuretic in the way fluid-overload edema would.

Because verapamil and diltiazem meaningfully reduce contractility, both are contraindicated in heart failure with reduced ejection fraction: reducing the calcium available for contraction further impairs a heart muscle that is already failing. Amlodipine, by contrast, has minimal effect on contractility at therapeutic doses and is considered safe in this same population. This is a direct, testable consequence of the tissue-selectivity principle introduced in Section 1.