Catecholamine · Inotrope
Isoproterenol (Isuprel)
Also called Isuprel
Indications
- Bradycardia or AV block unresponsive to atropine, as a bridge to pacing (older labels; off-label in current ones)
- Pause-dependent torsades de pointes: raises the rate and shortens QT until pacing (off-label)
- Bradycardia in the denervated heart after transplant, where atropine fails (off-label)
- Electrical storm in Brugada syndrome (off-label)
- Distributive or low-output shock, and bronchospasm during anesthesia (current label)
- Electrophysiology studies and tilt-table testing (off-label)
Dosing
Isoproterenol drip card and mL/h converterStart: 2 mcg/min
Mechanism of action
Isoproterenol is a synthetic catecholamine and a nonselective β1 and β2 agonist with very low α affinity. Both receptors couple to Gs: adenylyl cyclase raises cAMP, which speeds sinus and escape pacemakers and AV conduction and, through protein kinase A, strengthens contraction. In bronchial and arteriolar smooth muscle the same cAMP rise causes relaxation, and with no α1 constriction to oppose it, SVR falls.
Pharmacology
- Cardiovascular: raises heart rate, AV conduction, contractility and cardiac output; shortens QT by speeding the rate
- Cardiovascular: β2 vasodilation lowers SVR and diastolic pressure; systolic may hold or rise, and MAP often falls
- Cardiovascular: higher O₂ demand with lower coronary perfusion pressure, so ischemia and arrhythmias, worst in coronary disease
- Respiratory: potent β2 bronchodilator; also dilates pulmonary vessels
- Metabolic: less hyperglycemia than epinephrine and more free fatty acid release (label); β2 shifts K⁺ into cells
Pharmacokinetics
| Onset | Immediate IV |
|---|---|
| Duration | Minutes after stopping; a little longer than epinephrine because it escapes MAO and neuronal uptake (label) |
| Half-life | 2.5–5 min IV |
| Protein binding | About 69%, mostly albumin |
| Metabolism | COMT (to 3-O-methylisoproterenol, a weak β antagonist) and sulfation in liver, lung and gut wall; a poor MAO substrate (label) |
| Elimination | Urine, mainly as sulfate conjugates, with some unchanged drug |
| Routes | IV infusion; oral doses are mostly lost to gut-wall sulfation |
Chemistry
- Synthetic catecholamine: norepinephrine with an isopropyl group on the amine; the bigger the N-substituent (norepinephrine, then epinephrine, then isoproterenol), the less α and the more β activity
- Racemic hydrochloride (label); the (R)-(−) isomer carries most of the activity
- Amine pKa about 8.6 (PubChem): ionized and hydrophilic at pH 7.4
- 0.2 mg/mL vials, pH 2.5–4.5; some products contain sodium metabisulfite, others edetate and citrate; protect from light, discard if pinkish or darker than slightly yellow, and keep away from alkali such as bicarbonate
Adverse effects and cautions
In the one-pagers
- Rhythm GalleryVentricular rhythms and wide-complex tachycardia: Torsades is polymorphic VT twisting around the baseline on a long QT, typically started by a short–long–short sequence. Risk rises with QTc over 500 ms, female sex, bradycardia, low K⁺ or Mg²⁺, heart failure and QT drugs (class IA and III antiarrhythmics, haloperidol, ondansetron, macrolides, fluoroquinolones, methadone). Sustained polymorphic VT g
- Temporary PacingIndications and the 2018 ACC/AHA/HRS guideline: Drugs: atropine IIa for SND and AV-nodal block, including in acute MI; beta agonists (isoproterenol, dopamine, dobutamine, epinephrine) IIb when ischemia is unlikely; aminophylline IIb in inferior MI, IIa after heart transplant or spinal cord injury; atropine after heart transplant III: Harm.
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Sources
- FDA label: isoproterenol, Amphastar (2022)
- FDA label: Isuprel, Hospira (2018)
- PubChem: isoproterenol
For education and exam preparation only. Follow your institution’s protocols. Typical adult values for study. Follow your local protocols and medical direction.