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Pharmacology of Ephedrine
1. Introduction and Source
Ephedrine is a naturally occurring alkaloid derived from the plant Ephedra sinica (also known as Ma Huang in traditional Chinese medicine). It is a non-catecholamine sympathomimetic amine and one of the oldest sympathomimetic drugs. Only l-ephedrine and racemic ephedrine are used clinically.
- Goodman & Gilman's The Pharmacological Basis of Therapeutics, p. 1855
2. Chemical Structure
Ephedrine has a phenylethylamine backbone - like amphetamine - but with an additional hydroxyl (OH) group on the beta carbon and a methyl group on the alpha carbon. Crucially, unlike catecholamines (e.g., epinephrine), it lacks catechol ring hydroxyl groups, making it:
- Orally bioavailable
- Resistant to COMT and MAO degradation
- Longer-acting
3. Mechanism of Action
Ephedrine is a mixed-acting sympathomimetic - it has both direct and indirect actions, with the indirect action predominating.
Indirect (Primary) Mechanism
- Ephedrine undergoes endocytosis into presynaptic postganglionic adrenergic nerve terminals (alpha and beta)
- Inside the nerve terminal, it displaces norepinephrine (NE) from synaptic vesicles
- The released NE then activates postsynaptic alpha and beta adrenoceptors
- Drugs that block uptake into adrenergic nerves (e.g., cocaine) and drugs that deplete NE stores (e.g., reserpine) attenuate ephedrine's effects
Direct (Secondary) Mechanism
-
Ephedrine is also a direct agonist at alpha and beta adrenergic receptors
-
Direct beta-adrenoceptor stimulation limits the hypertensive effect produced by alpha-receptor activation
-
Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e, p. 970
-
Goodman & Gilman's, p. 1857
4. Pharmacological Effects
Cardiovascular System
- Increases heart rate, cardiac output, contractility, and systemic vascular resistance (SVR)
- Net effect: rise in systolic and diastolic blood pressure
- Effects mimic epinephrine but are less potent and have a longer duration of action
Respiratory System
- Activates beta-2 adrenergic receptors in bronchial smooth muscle → bronchodilation
- Used historically as a bronchodilator in asthma (now largely superseded by selective beta-2 agonists)
Urogenital System
- Stimulation of alpha-adrenergic receptors in bladder neck smooth muscle → increases urinary outflow resistance
- Used in urinary stress incontinence; conversely can cause urinary retention, particularly in men with BPH
CNS
-
Ephedrine is a potent CNS stimulant
-
Crosses the blood-brain barrier and promotes release of noradrenaline and dopamine in the CNS (including substantia nigra)
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Raises minimum alveolar concentration (MAC) of anesthetic gases
-
At high doses: anxiety, insomnia, tremor
-
Goodman & Gilman's, p. 1862; Morgan & Mikhail's Clinical Anesthesiology, 7e, p. 451; ROSEN's Emergency Medicine, p. 2958
5. Pharmacokinetics (ADME)
| Parameter | Details |
|---|---|
| Administration | Oral, intravenous (IV), intramuscular (IM) |
| Absorption | Well absorbed orally; effective after oral administration |
| Distribution | Crosses the blood-brain barrier (CNS stimulant) |
| Metabolism | NOT metabolized by COMT or MAO (lacks catechol ring); minimal hepatic metabolism |
| Elimination | Excreted largely unchanged in urine |
| Half-life (t½) | 3 to 6 hours |
| Duration of action | Several hours (much longer than epinephrine) |
- Goodman & Gilman's, p. 1862
6. Tachyphylaxis
A clinically important feature of ephedrine is the development of tachyphylaxis with repeated dosing:
-
Repeated administration depletes presynaptic NE stores rapidly
-
Ephedrine is then released from synaptic vesicles as a "false neurotransmitter", which has minimal adrenergic activity
-
This is in contrast to epinephrine, which acts directly on receptors and does NOT develop tachyphylaxis
-
To offset tachyphylaxis, subsequent doses must be incrementally increased
-
Morgan & Mikhail's, p. 451; Barash's Clinical Anesthesia, p. 970
7. Clinical Uses
| Indication | Notes |
|---|---|
| Hypotension during anesthesia | Most common clinical use intraoperatively; treats acute hypotension with bradycardia |
| Spinal/neuraxial anesthesia-induced hypotension | Particularly in obstetric patients (though phenylephrine now preferred - see below) |
| Bronchospasm / Asthma | Historical use; now replaced by selective beta-2 agonists |
| Urinary stress incontinence | Due to alpha-1 stimulation at bladder neck |
| Nasal decongestant | Topical or oral use for nasal congestion |
| CNS stimulant / performance enhancement | Used in herbal preparations (Ma Huang); banned in sports since 2003 |
Dosing (IV):
-
Adults: Bolus of 2.5 to 10 mg IV
-
Children: 0.1 mg/kg IV bolus
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Available as 25 mg/mL or 50 mg/mL ampules
-
Morgan & Mikhail's, p. 451
Ephedrine vs. Phenylephrine in Obstetrics
For many years, ephedrine was the preferred vasopressor in obstetric anesthesia because, unlike direct alpha-1 agonists, it did not decrease uterine blood flow in animal studies. However, phenylephrine is now widely preferred in obstetric neuraxial anesthesia due to:
- Faster onset and shorter duration (easier titration)
- Fewer adverse effects on fetal pH compared to ephedrine
- Morgan & Mikhail's, p. 451
8. Adverse Effects / Untoward Effects
| Adverse Effect | Mechanism |
|---|---|
| Hypertension | Excessive alpha/beta stimulation |
| Tachycardia / palpitations | Beta-1 stimulation |
| Insomnia, anxiety, tremor | CNS stimulation |
| Urinary retention | Alpha-1 stimulation at bladder neck (especially with BPH) |
| Tachyphylaxis | Depletion of presynaptic NE stores |
| Stroke, MI, death | In susceptible patients, especially with cardiovascular disease |
Usual or higher-than-recommended doses can cause significant adverse effects in patients with unrecognized underlying cardiovascular disease.
9. Contraindications / Cautions
- Hypertension
- Coronary artery disease / ischemic heart disease
- Benign prostatic hyperplasia (risk of urinary retention)
- Concurrent use with MAO inhibitors (risk of hypertensive crisis)
- Patients on cocaine or reserpine (altered response)
- Hyperthyroidism
10. Drug Interactions
| Drug | Interaction |
|---|---|
| Cocaine | Blocks NE reuptake transporter → blocks ephedrine uptake into nerves → attenuates effect |
| Reserpine | Depletes vesicular NE stores → attenuates indirect effect |
| MAO inhibitors | Impaired NE degradation → potentiates pressor response, risk of hypertensive crisis |
| Beta-blockers | Blunt cardiac/bronchodilatory effects |
| General anesthetics (halothane) | Increased risk of arrhythmias |
11. Legal and Regulatory Status
- The FDA banned dietary supplements containing ephedra alkaloids due to cardiovascular risks (heart attack, stroke, death) - ROSEN's Emergency Medicine, p. 2554
- The Combat Methamphetamine Epidemic Act of 2005 regulates the sale of ephedrine (used as a precursor in methamphetamine synthesis)
- Ephedrine and pseudoephedrine can be chemically converted to methamphetamine
- Ephedrine is banned in all sports competitions since 2003 (World Anti-Doping Agency)
- ROSEN's Emergency Medicine, p. 2560
12. Comparison: Ephedrine vs. Epinephrine
| Feature | Ephedrine | Epinephrine |
|---|---|---|
| Type | Non-catecholamine | Catecholamine |
| Mechanism | Mixed (indirect + direct) | Direct only |
| Catechol ring | Absent | Present |
| Oral bioavailability | Good | Poor (degraded by COMT/MAO) |
| Duration of action | Hours (3-6 h t½) | Minutes |
| Potency | Less potent | More potent |
| CNS effects | Marked stimulant | Minimal |
| Tachyphylaxis | Yes (with repeat dosing) | No |
| Receptor profile | α, β1, β2 | α, β1, β2 |
Key Summary
- Ephedrine is a non-catecholamine, mixed-acting sympathomimetic acting on alpha and beta adrenoceptors
- Its predominant mechanism is indirect - displacement of NE from presynaptic vesicles
- Cardiovascular effects resemble epinephrine but are less potent and longer-lasting
- The hallmark pharmacological limitation is tachyphylaxis due to NE depletion
- Primary clinical use is vasopressor during anesthesia, particularly for hypotension with bradycardia
- Regulated due to potential for misuse, cardiovascular toxicity, and use in drug manufacture
Sources:
- Goodman & Gilman's The Pharmacological Basis of Therapeutics (p. 1855-1870)
- Morgan & Mikhail's Clinical Anesthesiology, 7e (p. 451)
- Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e (p. 970)
- ROSEN's Emergency Medicine Concepts and Clinical Practice (p. 2554-2560)
- Katzung's Basic and Clinical Pharmacology, 16e (Figure 9-5)
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