Continuing Education Activity

Metoprolol is FDA-approved to treat angina, heart failure, myocardial infarction, atrial fibrillation/flutter, and hypertension. Off-label uses include supraventricular tachycardia and thyroid storm. Both oral and intravenous preparations are available. There are immediate and extended-release preparations available orally. There is controversy regarding the selection of beta-blockers in the management of the above conditions. There is also conflicting evidence regarding the optimal choice of a particular beta-blocker in treating each specific disease. This activity will highlight the mechanism of action, adverse event profile, pharmacology, monitoring, and relevant interactions of metoprolol, pertinent for members of the interprofessional team in the treatment of patients with conditions where it is of clinical value.

Objectives:

• Identify the mechanism of action of metoprolol.
• Describe the adverse effects of metoprolol.
• Summarize the contraindications of metoprolol.
• Employ interprofessional team strategies for enhancing care coordination and communication to advance the safe use of metoprolol and improve outcomes.

Indications

Metoprolol is FDA-approved to treat angina, heart failure, myocardial infarction, atrial fibrillation/flutter, and hypertension. Off-label uses include supraventricular tachycardia and thyroid storm. Both oral and intravenous preparations are available. There are immediate and extended-release preparations available orally. There is controversy regarding the selection of beta-blockers in the management of the above conditions. There is also conflicting evidence regarding the optimal selection of a particular beta-blocker in treating each specific disease.[1] The role of beta-blockers as initial therapy for hypertension, particularly in the absence of compelling indications, has been questioned.[2][3] 

Several randomized trials in the 1980s showed a mortality benefit for beta-blockers in acute myocardial infarction.[4][5][6] The Metoprolol Atherosclerosis Prevention in Hypertensives (MAPHY) trial specifically showed a benefit of metoprolol over diuretics regarding sudden cardiac death as well as myocardial infarction.[7] A large randomized trial of over 50,000 patients in the 1990s showed metoprolol to reduce mortality and re-infarction when used chronically after myocardial infarction.[8] Beta-blockers have demonstrated prognostic benefit and reduce mortality in the treatment of chronic heart failure. The carvedilol or metoprolol European trial (COMET) enrolled patients with stable heart failure and compared each drug head-to-head. Carvedilol was associated with a statistically significantly lower risk of all-cause death.[9] However, recent trials have had conflicting evidence on whether selective beta-1 blockers such as metoprolol have any benefit over other beta-blockers like carvedilol.[1]

Mechanism of Action

Metoprolol is a cardioselective beta-1-adrenergic receptor inhibitor that competitively blocks beta1-receptors with minimal or no effects on beta-2 receptors at oral doses of less than 100 mg in adults. It decreases cardiac output by negative inotropic and chronotropic effects. Metoprolol does not exhibit membrane stabilizing or intrinsic sympathomimetic activity. Administration of metoprolol to normal subjects results in a reduction in heart rate and cardiac output; this appears to be related to the dose and concentration of the drug. Metoprolol is mainly lipophilic, and distribution is typical of a basic lipophilic drug. Based on animal studies, it appears to be almost completely absorbed from the gastrointestinal (GI) tract when taken orally.

There is significant hepatic first-pass elimination, which results in around 50% of the oral dose reaching the systemic circulation. It is 11% bound to serum albumin. The half-life of metoprolol is about 3 to 4 hours in most patients for non-extended release tabs. Metoprolol excretion principally occurs via the kidneys.[10] Metoprolol succinate produces more level drug concentrations as compared to metoprolol tartrate, which has more peak-to-trough variation. However, despite these differences in pharmacokinetics, studies have concluded that both agents produce similar clinical effects, both acute and chronic.[11]

Administration

Metoprolol may administration can be either oral or intravenous. The immediate-release oral formulation is to be administered with or immediately following food intake. The administration of the short-acting formulation of metoprolol tartrate is usually twice daily. It may be effective for hypertension when taken once per day; however, lower doses may not control blood pressure for 24 hours. Patients may take the extended-release metoprolol succinate formulation without regard to meals.[12]

Adverse Effects

The primary adverse effects of metoprolol include heart failure exacerbation, fatigue, depression, bradycardia or heart block, hypotension, bronchospasm, cold extremities, dizziness, decreased libido, diarrhea, tinnitus, decreased exercise tolerance, glucose intolerance, and may mask hypoglycemia. Abrupt cessation of the drug may lead to a withdrawal syndrome that could cause angina or myocardial infarction. Tachycardia and hypertension are both common in withdrawal syndrome.[13]

Contraindications

Metoprolol is contraindicated in patients with sick sinus syndrome, second or third-degree heart block (in the absence of pacemaker),  decompensated heart failure, hypotension, and documented hypersensitivity to the drug or components. Also, caution is necessary for patients with a history of noncompliance as the abrupt cessation of the drug can lead to withdrawal syndromes, including angina and myocardial infarction. Patients who have ingested cocaine or methamphetamine have traditionally had a contraindication to the use of selective beta-blocker such as metoprolol. This observation has its basis on case reports, and there is no strong evidence that they cause any deleterious effects. Regardless, agents such as labetalol, which have alpha and beta activity, or calcium channel blockers should be used in these cases. Metoprolol is a pregnancy risk factor Category C drug.[14]

Monitoring

The onset of action for oral immediate-release tablets is within 1 hour, with a duration of effect variable depending on the dose given. The extended-release preparation has a similar onset of action and a duration of up to 24 hours. Metabolism of metoprolol is hepatic with excretion in the urine. If there is renal impairment, there is no dosage adjustment necessary. If there is a history of hepatic impairment, slowly increasing titrated doses to effect should be used. With the use of intravenous (IV) administration, cardiac monitor, including ECG heart rate and blood pressure, should be constant. Oral administration heart rate rhythm and blood pressure require monitoring.[14]

Toxicity

Treatment will vary based on the amount of metoprolol amount taken, comorbidities, age, and other co-investments. On arrival, assess ABCs and monitor appropriate blood work, including coingestants, ECG, large-bore IVs, and pregnancy status if female. Consult poison control/toxicology early in the course. Treatment choices include volume resuscitation, activated charcoal, whole bowel irrigation, nasogastric lavage, atropine, glucagon, calcium gluconate/calcium chloride, high-dose insulin, vasopressors, Intralipid, transcutaneous, or transvenous pacemaker. Cardiac status and a current fluid balance will guide volume resuscitation. Activated charcoal is typically given 1 g/kg and usually only has efficacy if dosed within 1 to 2 hours of ingestion.[15] If the patient has any altered mentation, caution is necessary to the possibility of aspiration. Whole bowel irrigation should be a consideration for extended-release preparations or large quantity ingestion.

Nasogastric lavage is usually ineffective, except for large quantity ingestions. The clinician may consider atropine use, although it is typically ineffective in moderate-to-severe overdoses. Calcium administration to increase intracellular calcium at a dose of 60 mg/kg over 5 to 10 minutes of calcium gluconate. Calcium chloride at a dose of 10 to 20 mL of a 10% solution is an option if central access is obtained. Glucagon dosing is 50 mcg/kg as a bolus with titration of drip. High-dose insulin at a dose of 1 unit per kilogram bolus followed by 1 unit per kilogram per hour drip.[16]

Administration with dextrose with a drip titrating to euglycemia as well as potassium repletion as needed. Vasopressors with epinephrine or norepinephrine titrated rate and blood pressure. Intralipid IV lipid emulsion therapy can serve as a lipid sink that extracts the drug from the myocyte. It may also provide free fatty acids as a substrate. It should be noted that the use of this medication will affect some laboratory monitoring.[17] Consider a transcutaneous or transvenous pacemaker. Extracorporeal membrane oxygenation (ECMO) should be considered for refractory cases.[18] If the clinician is concerned about intentional overdose, they should order a mandatory psychiatric evaluation. There may also be a need for the possibility of co-ingestants and treatment of those as well. For non-extended or non-sustained-release preparations, 4 to 6 hours of observation without any derangement of mental status or vital signs is sufficient. Any extended-release or sustained-release preparation requires 12 to 24 hours of telemetry observation, depending on the preparation. Extra caution should be considered in the pediatric population as very low amounts, including one pill or even one-half pill, which can cause cardiovascular collapse and death.[19]

Enhancing Healthcare Team Outcomes

Metoprolol is an effective beta-blocker for many cardiac disorders. However, prescribers of this drug should be aware that specific indications for this drug are still lacking. While research has shown the drug to lower mortality in CHF patients, the same applies to several other beta-blockers. To ensure the safe use of the drug, all members of the interprofessional healthcare team, including clinicians (MDs, DOs, NPs, and PAs), specialists, nurses, and pharmacists, should be very familiar with the indications, administration, and potential adverse effects and drug interactions of metoprolol, as well as all members of the beta-blocker drug classification, including which agents are specific vs. beta-1 specific. This will result in improved patient outcomes with fewer adverse events. [Level 5]

Metoprolol can be administered orally or IV and works quickly.