Betablockers and Calcium Antagonists: Mode of Action in Angina pectoris

Summary

The different effects of betablockers and calciumantagonists on LV hemodynamics and coronary blood flow (CBF) are discussed based on studies performed with atenolol, propranolol and nifedipine. LV hemodynamics were recorded in coronary patients at rest, measuring LV volumes and pressures simoultaneously during angiography with the use of Millar tip manometers. CBF was assessed at rest and during atrial pacing up to ischemia (ST depression ± angina) without and with the drug using the precordial xenon clearance technique (gamma camera, CDC 1700 computer). Basic differences were present both for hemodynamics as for coronary dynamics: under betablockade LV contractility, cardiac output and ejection fraction decreased, enddiastolic volume increased and the pressure-volume relation was shifted upwards and flattened, indicating both an increase in diastolic wall stress and a decrease in passive elastic stiffness. During atrial pacing CBF decreased both in the normal as well as in the ischemic (poststenotic) zone, when compared to control, a result of the decrease in MVO₂ due to the reduced contractility. After Nifedipine LV function improved slightly, contractility and cardiac output increasing, and peripheral resistance decreasing; enddiastolic volume and pressure remained unchanged. After the drug, CBF during pacing showed a further (not sign.) increase in the ischemic zone and remained unchanged in the normal area, inspite of a sign, decrease in MVO₂ i.e. in the rate pressure product. Hence, calcium antagonists improve stable angina, i.e. ischemia due to increased O₂ demand (exercise, pacing) by increasing O₂ supply in the ischemic zone and decreasing demand through afterload reduction, whereas betablockers decrease both O₂ demand and supply, the latter by autoregulatory changes of CBF.

Today, the treatment of angina pectoris is based on nitrates, betablockers and most recently, on calcium antagonists, three different therapeutic principles with different indications. Their mode of action varies both with regard to their influence on myocardial oxygen consumption as well as on coronary blood flow. The following presentation, which concentrates mainly on the difference between betablockers and so-called calcium antagonists, describes the hemodynamic effects and the action on the coronary circulation of both groups of drugs in patients with coronary artery disease.