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Using the E max model, the maximal beta 1 -blocking effect has been estimated to produce a 30% reduction in exercise heart rate. Beta 1 -blocking effects in the range of 30Ð 80% of the maximal effect (corre-sponding to approximately 8Ð 23% reduction in exercise heart rate) are expected to occur at metoprolol plasma concentrations ranging from 30Ð 540 nmol/ L. The concen-tration-effect curve begins reaching a plateau between 200Ð 300 nmol/ L, and higher plasma levels produce little additional beta 1 -blocking effect. The relative beta 1 -selec-tivity of metoprolol diminishes and blockade of beta 2 -adrenoceptors increases at higher plasma concentrations. Although beta-adrenergic receptor blockade is useful in the treatment of angina, hyper-tension, and heart failure there are situations in which sympathetic stimulation is vital. In patients with severely damaged hearts, adequate ventricular function may depend on sympathetic drive. In the presence of AV block, beta-blockade may prevent the neces-sary facilitating effect of sympathetic activity on conduction. Beta 2 -adrenergic blockade results in passive bronchial constriction by interfering with endogenous adrenergic bronchodilator activity in patients subject to bronchospasm and may also interfere with exogenous bronchodilators in such patients. In other studies, treatment with TOPROL-XL produced an improvement in left ventric-ular ejection fraction. TOPROL-XL was also shown to delay the increase in left ventricular end-systolic and end-diastolic volumes after 6 months of treatment. Text Continues Below
Hypertension The mechanism of the antihypertensive effects of beta-blocking agents has not been elucidated. However, several possible mechanisms have been proposed: (1) competi-tive antagonism of catecholamines at peripheral (especially cardiac) adrenergic neuron sites, leading to decreased cardiac output; (2) a central effect leading to reduced sympathetic outflow to the periphery; and (3) suppression of renin activity. Page: << Prev | 1 | 2 | 3 | 4 | 5 | Next >>
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