Chapter 194. Inamrinone (Formerly Amrinone)

Pharmacology. Inamrinone is a positive inotropic agent with vasodilator activity. It is not a beta-adrenergic receptor agonist, and its exact mechanism of action is unknown. It appears to work by inhibiting myocardial cell phosphodiesterase activity, thereby increasing cellular concentrations of cyclic AMP. Cardiac afterload and preload are reduced owing to a direct vasodilator effect.

Indications. Inamrinone may be useful for patients with beta blocker, mixed beta and alpha blocker (eg, labetalol), or calcium antagonist overdose when intravenous fluids, atropine, beta agonists, glucagon, high-dose insulin with supplemental dextrose, and lipid emulsion therapy have failed to restore cardiac output and blood pressure.

Contraindications. Known hypersensitivity to inamrinone or sulfites (used as a preservative).

Adverse effects

1. Hypotension may result from direct vasodilator effects, especially in patients who are volume-depleted. Give adequate intravenous fluids before and with inamrinone administration.

2. The formulation contains sodium metabisulfite as a preservative, which can cause acute allergic-like reactions in patients (especially those with asthma) who are sensitive to sulfites.

3. Inamrinone may aggravate outflow obstruction in patients with hypertrophic subaortic stenosis.

4. Inamrinone affects platelet survival time, resulting in a dose- and time-dependent thrombocytopenia.

5. Use in pregnancy. FDA Category C (indeterminate). Animal studies are conflicting, and there are no good human data (See Introduction in Section III). Use only if the benefit justifies the potential risk (eg, a severe beta blocker or calcium antagonist overdose unresponsive to other measures).

6. Name changed from amrinone to inamrinone to prevent confusion with amiodarone.

Drug or laboratory interactions. The positive inotropic effects of inamrinone are additive with those of other inotropic agents, including digitalis glycosides. These drugs can be used together, but the patient should be monitored for cardiac dysrhythmias.

Dosage and method of administration

1. The initial dose is 0.75 mg/kg as a bolus over 2–3 minutes. This is followed by an infusion at 5–10 mcg/kg/min.

2. The manufacturer recommends that the total daily dose not exceed 10 mg/kg. However, up to 18 mg/kg/d has been given to some patients.

Formulations

1. Inamrinone lactate (Inocor, others), 5 mg/mL in 20-mL ampules containing 0.25 mg/mL sodium metabisulfite as a preservative.

2. Suggested minimum stocking levels to treat a 100-kg adult for the first 8 hours and 24 hours: inamrinone lactate, first 8 hours: 500 mg or five ampules (5 mg/mL, 20 mL each); first 24 hours: 1000 mg or 10 ampules (5 mg/mL, 20 mL each).