Acute intervention with metoprolol in myocardial infarction

Metoprolol. An updated review of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy, in hypertension, ischaemic heart disease and related cardiovascular disorders

During the intervening years since metoprolol was first reviewed in the Journal (1977), it has become widely used in the treatment of mild to moderate hypertension and angina pectoris. Although much data have accumulated, its precise mechanisms of action in these diseases remain largely uncertain. Optimum treatment of hypertension and angina pectoris with metoprolol is achieved through dose titration within the therapeutic range. It has been clearly demonstrated that metoprolol is at least as effective as other beta-blockers, diuretics and certain calcium antagonists in the majority of patients. Although a twice daily dosage regimen is normally used, satisfactory control can be maintained in many patients with single daily doses of conventional or, more frequently, slow release formulations. Addition of a diuretic may improve the overall response rate in hypertension. Several controlled trials have studied the effects of metoprolol administered during the acute phase and after myocardial infarction. In early intervention trials a reduction in total mortality was achieved in one moderately large trial of prolonged treatment, but in another, which excluded patients already being treated with beta-blockers or certain calcium antagonists and where treatment was only short term, mortality was significantly reduced only in 'high risk' patients. Overall results with metoprolol have not demonstrated that early intervention treatment in all patients produces clinically important improvement in short term mortality. Thus, the use of metoprolol during the early stages of myocardial infarction is controversial, largely because of the requirement to treat all patients to save a small number at 'high risk'. This blanket coverage approach to treatment may be more justified during the post-infarction follow-up phase since it has been shown that metoprolol slightly, but significantly, reduces the mortality rate for periods of up to 3 years. Metoprolol is generally well tolerated and its beta 1-selectivity may facilitate its administration to certain patients (e.g. asthmatics and diabetics) in whom non-selective beta-blockers are contraindicated. Temporary fatigue, dizziness and headache are among the most frequently reported side effects. After a decade of use, metoprolol is well established as a first choice drug in mild to moderate hypertension and stable angina, and is beneficial in post-infarction patients. Further study is needed in less well established areas of treatment such as cardiac arrhythmias, idiopathic dilated cardiomyopathy and hypertensive cardiomegaly.

The effect of exaprolol (MG 8823) on epicardial ST-segment changes in a feline model of acute myocardial ischaemia

A model is described (anaesthetized, open-chest cats subjected to acute coronary artery occlusion) which allows the effects of drug interventions to be determined on one major electrocardiographic index of myocardial ischaemia. Epicardial ST-segment changes were continuously recorded from five individual sites on the surface of the left ventricle. Coronary artery occlusion (left anterior descending branch) resulted in marked and consistent elevations of the ST-segment in all sites in nearly all experiments. These changes started within 1 min of the onset of ischaemia and reached a peak at between 30 and 60 min; thereafter there was a gradual reduction over the next 4 h. The one significant haemodynamic effect of coronary artery occlusion was an increase in left ventricular (LV) end-diastolic pressure (LVEDP). Ventricular ectopic activity was not pronounced in this model (about 50 ectopic beats over the initial 30 min post-occlusion period). Exaprolol (1.0 mg kg-1, intravenously) a potent beta-adrenoceptor blocking agent with 'membrane stabilising activity', when given 1 h after the onset of ischaemia, reduced heart rate and LV dP/dtmax and increased LVEDP. These effects were prolonged (i.e. little recovery in heart rate 3 h after administration). Exaprolol decreased total ST-segment elevation immediately after administration; this was significantly different from the effect of intravenous saline and lasted for at least 3 h. The effects appeared to be greater at sites of less pronounced ischaemia. Intramyocardial temperature records were taken to indicate a reduction in blood flow to the ischaemic region; however the alleviation of epicardial ST-segment elevation suggests an improved myocardial oxygen demand:supply ratio. Reperfusion was unsuccessfully attempted after a 4 h occlusion period; reperfusion after a shorter period (30 min) resulted in ventricular ectopic activity but no fibrillation.

The effect of beta-adrenoceptor blocking agents, with differing ancillary properties, on the arrhythmias resulting from acute coronary artery ligation in anaesthetized rats

The effects of several beta-adrenoceptor blocking agents, [+), (-) and (+/-)-oxprenolol, p-oxprenolol, practolol, propranolol and timolol) were investigated on the ventricular arrhythmias occurring within the first 30 min of acutely ligating the main left coronary artery in anaesthetized rats. The degree of cardiac and vascular beta-adrenoceptor blockade was also assessed. All the compounds exhibited antiarrhythmic activity under these conditions. The degree of cardiac beta-adrenoceptor blockade required for this protection was less for the cardioselective agents, p-oxprenolol and practolol, than for the non-selective beta-adrenoceptor blocking agents. A comparison of the two isomers of oxprenolol demonstrated that the (-)-isomer markedly suppressed ischaemic arrhythmias (ventricular ectopic beats, incidence and duration of ventricular tachycardia and duration of ventricular fibrillation) more effectively than the (+)-isomer. Compounds possessing intrinsic sympathomimetic activity (ISA) caused less marked haemodynamic changes (in equivalent beta-blocking doses) than those that did not possess this ancillary property. The membrane stabilizing activity of oxprenolol and p-oxprenolol did not appear to contribute to the antiarrhythmic activity of these agents; however, the membrane stabilizing activity of propranolol may contribute to its effectiveness. In all the drugs studied, the main pharmacological property required to suppress early postischaemic arrhythmias is blockade of cardiac beta-adrenoceptors.