The precise mechanism of action of metoprolol in patients with suspected or definite myocardial infarction is not known.
In a large (1,395 patients randomized), double-blind, placebo-controlled clinical study, metoprolol was shown to reduce 3-month mortality by 36% in patients with suspected or definite myocardial infarction.
Patients were randomized and treated as soon as possible after their arrival in the hospital, once their clinical condition had stabilized and their hemodynamic status had been carefully evaluated. Subjects were ineligible if they had hypotension, bradycardia, peripheral signs of shock, and/or more than minimal basal rales as signs of congestive heart failure. Initial treatment consisted of intravenous followed by oral administration of metoprolol or placebo, given in a coronary care or comparable unit. Oral maintenance therapy with metoprolol or placebo was then continued for 3 months. After this double-blind period, all patients were given metoprolol and followed up to 1 year.
The median delay from the onset of symptoms to the initiation of therapy was 8 hours in both the metoprolol- and placebo-treatment groups. Among patients treated with metoprolol, there were comparable reductions in 3-month mortality for those treated early (≤ 8 hours) and those in whom treatment was started later. Significant reductions in the incidence of ventricular fibrillation and in chest pain following initial intravenous therapy were also observed with metoprolol and were independent of the interval between onset of symptoms and initiation of therapy.
In this study, patients treated with metoprolol received the drug both very early (intravenously) and during a subsequent 3-month period, while placebo patients received no beta-blocker treatment for this period. The study thus was able to show a benefit from the overall metoprolol regimen but cannot separate the benefit of very early intravenous treatment from the benefit of later beta-blocker therapy. Nonetheless, because the overall regimen showed a clear beneficial effect on survival without evidence of an early adverse effect on survival, one acceptable dosage regimen is the precise regimen used in the trial. Because the specific benefit of very early treatment remains to be defined however, it is also reasonable to administer the drug orally to patients at a later time as is recommended for certain other beta blockers.
Metoprolol Tartrate Injection, USP is indicated in the treatment of hemodynamically stable patients with definite or suspected acute myocardial infarction to reduce cardiovascular mortality when used in conjunction with oral metoprolol maintenance therapy. Treatment with intravenous Metoprolol Tartrate Injection, USP can be initiated as soon as the patient’s clinical condition allows (see DOSAGE AND ADMINISTRATION, CONTRAINDICATIONS, and WARNINGS).
Metoprolol is contraindicated in patients with a heart rate < 45 beats/min; second- and third-degree heart block; significant first-degree heart block (P-R interval ≥ 0.24 sec); systolic blood pressure < 100 mmHg; or moderate-to-severe cardiac failure (see WARNINGS).
These adverse reactions were reported from treatment regimens where intravenous metoprolol was administered, when tolerated.
Central Nervous System: Tiredness has been reported in about 1 of 100 patients. Vertigo, sleep disturbances, hallucinations, headache, dizziness, visual disturbances, confusion, and reduced libido have also been reported, but a drug relationship is not clear.
Cardiovascular: In the randomized comparison of metoprolol and placebo described in the CLINICAL PHARMACOLOGY section, the following adverse reactions were reported:
| | Metoprolol | Placebo |
| Hypotension (systolic BP < 90 mmHg) | 27.4% | 23.2% |
| Bradycardia (heart rate < 40 beats/min) | 15.9% | 6.7% |
| Second- or third-degree heart block | 4.7% | 4.7% |
| First-degree heart block (P-R ≥ 0.26 sec) | 5.3% | 1.9% |
| Heart failure | 27.5% | 29.6% |
Respiratory: Dyspnea of pulmonary origin has been reported in fewer than 1 of 100 patients.
Gastrointestinal: Nausea and abdominal pain have been reported in fewer than 1 of 100 patients.
Dermatologic: Rash and worsened psoriasis have been reported, but a drug relationship is not clear.
Miscellaneous: Unstable diabetes and claudication have been reported, but a drug relationship is not clear.
Early Treatment: During the early phase of definite or suspected acute myocardial infarction, initiate treatment with metoprolol as soon as possible after the patient’s arrival in the hospital. Such treatment should be initiated in a coronary care or similar unit immediately after the patient’s hemodynamic condition has stabilized.
Begin treatment in this early phase with the intravenous administration of three bolus injections of 5 mg of metoprolol each; give the injections at approximately 2-minute intervals. During the intravenous administration of metoprolol, monitor blood pressure, heart rate, and electrocardiogram.
In patients who tolerate the full intravenous dose (15 mg), initiate metoprolol tablets, 50 mg every 6 hours, 15 minutes after the last intravenous dose and continue for 48 hours. Thereafter, the maintenance dosage is 100 mg orally twice daily.
Start patients who appear not to tolerate the full intravenous dose on metoprolol tablets either 25 mg or 50 mg every 6 hours (depending on the degree of intolerance) 15 minutes after the last intravenous dose or as soon as their clinical condition allows. In patients with severe intolerance, discontinue metoprolol (see WARNINGS).