Although used commonly to treat supraventricular arrhythmias, beta blockers should be used cautiously in pulmonary hypertension (due to their negative inotropic and chronotropic effects)
However, at higher plasma concentrations, metoprolol’s cardioselectivity is less and it can inhibit β2-adrenergic receptors (located predominantly in the bronchial and vascular musculature)
Physiologic Effects
Decreased Blood Pressure
Possible Mechanisms of Decreased Blood Pressure (exact mechanism is not well-defined)
Competitive Antagonism of Catecholamines at Peripheral (Especially Cardiac) Adrenergic Neuron Sites: results in decreased cardiac output
Central Decreased Sympathetic Outflow to the Periphery
Suppression of Renin Activity
Negative Chronotropy: decreased resting and exercise heart rate (due to antagonism of catecholamine-induced increases in heart rate)
Negative Inotropy: decreased velocity and force of myocardial contraction
Decreased Myocardial Oxygen Consumption
Mechanisms
Negative Chronotropy
Negative Inotropy
Decreased Afterload: due to decrease in blood pressure
Metabolism
Hepatic: CYP2D6 Enzyme
CYP2D6 Enzyme Inhibitors Which May Increase the Plasma Concentration of Metoprolol
May especially occur when used in conjunction with other negative chronotropes which slow heart rate and atrioventricular nodal conduction (such as digoxin or calcium channel blockers)
Masking of Hypoglycemia-Induced Tachycardia (see Hypoglycemia)
Beta blockers may mask tachycardia (but not necessarily the diaphoresis or dizziness) which occur in hypoglycemia
Masking of Hyperthyroidism-Induced Tachycardia (see Hyperthyroidism)
Abrupt withdrawal of beta blockers in this setting may precipitate thyroid storm
Paradoxical Increase in Blood Pressure in Pheochromocytoma (see Pheochromocytoma)
When used alone and not in combination with alpha blockers, beta blockers may paradoxically increase blood pressure (due to attenuation of the beta receptor-mediated vasodilatation in skeletal muscle)
Therefore, in pheochromocytoma, beta blockers should only be initiated after alpha blockers have been initiated
Potentiation of the Hypertensive Response that Occurs with Clonidine Withdrawal (in Patients on Concomitant Clonidine and Beta Blockers)
In such cases, it is advised to withdraw the beta blocker at least several days before withdrawing the clonidine
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Detrimental effects of beta-blockers in COPD: a concern for nonselective beta-blockers. Chest. 2005 Mar;127(3):818-24 [MEDLINE]
Pulmonary hypertension: a contraindication to the use of {beta}-adrenoceptor blocking agents. Thorax. 2010 May;65(5):454-5. doi: 10.1136/thx.2008.111955 [MEDLINE]