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Propofol - A Complete Overview
Chemical Structure
Propofol is chemically known as 2,6-diisopropylphenol - a benzene ring with a hydroxyl group (OH) and two isopropyl side chains at the 2 and 6 positions. It belongs to the alkylphenol class of compounds. It is highly lipid-soluble and insoluble in water, which is why it is formulated as a white lipid emulsion.
Formulation
The standard commercial formulation (1% propofol) contains:
- 1% propofol
- 10% soybean oil
- 1.2% purified egg phospholipid (emulsifier)
- 2.25% glycerol (tonicity agent)
- Sodium hydroxide (pH adjustment to ~7)
- EDTA (added later for bacteriostatic activity)
It appears as a milky white, slightly viscous liquid. A 2% formulation is available in Europe. Stable at room temperature, not light-sensitive, and may be diluted with 5% dextrose in water.
A water-soluble prodrug called fospropofol (Lusedra) was FDA-approved in 2008. It is metabolized by alkaline phosphatases in the liver to release active propofol. Unlike propofol, it does NOT cause pain on injection, but can cause perineal paresthesias and pruritus.
Mechanism of Action
Propofol primarily acts by positive allosteric modulation of GABA-A receptors:
- It enhances the inhibitory effect of GABA by binding to a specific site on the GABA-A receptor
- This prolongs chloride channel opening, causing hyperpolarization and neuronal inhibition
- It also inhibits NMDA receptors and modulates slow calcium ion channels
This GABAergic potentiation accounts for its sedative, hypnotic, and anesthetic properties.
Pharmacokinetics
| Parameter | Detail |
|---|---|
| Route | IV only |
| Onset | 30-60 seconds (rapid due to high lipid solubility) |
| Duration (single bolus) | ~10 minutes |
| Distribution | Highly lipid-soluble; large volume of distribution |
| Metabolism | Hepatic (conjugation to glucuronide and sulfate) + extrahepatic metabolism |
| Elimination clearance | ~1.94 L/min |
| Excretion | >60 hours post-anesthetic (as metabolites in urine); <1% unchanged in urine; only 2% in feces |
Three-compartment model describes propofol pharmacokinetics. Elderly patients (>80 years) require half the dose of young patients (<20 years) for both pharmacokinetic and pharmacodynamic reasons.
Drug interactions affecting propofol PK:
- Midazolam reduces metabolic clearance from 1.94 to 1.61 L/min (blood propofol levels rise ~25%)
- Alfentanil decreases elimination clearance from 2.1 to 1.9 L/min
Clinical Uses
- Induction of general anesthesia - IV bolus dose: 1-2.5 mg/kg (adults); 2-3 mg/kg (children - higher due to larger Vd and faster clearance)
- Maintenance of anesthesia - Infusion: 100-200 mcg/kg/min (titrated to BIS, surgical stimulus)
- Procedural sedation - Outside/inside OR
- ICU sedation - For mechanically ventilated patients
- Monitored anesthesia care (MAC)
- Antiemetic - Subhypnotic doses (10-20 mg IV) reduce PONV
- Antipruritic - Against cholestatic pruritus
- Refractory status epilepticus - Due to anticonvulsant properties
Dose reductions needed in: elderly, ASA III-IV patients, patients premedicated with opiates or benzodiazepines, and cardiac surgery patients.
Effects on Body Systems
Central Nervous System
- Sedation, hypnosis, amnesia at subhypnotic doses
- Produces a sense of well-being (euphoria); hallucinations and sexual fantasies have been reported
- Decreases ICP by 30-50% (useful in neurosurgery), but also reduces CPP significantly
- Reduces cerebral metabolic rate (CMR) and cerebral blood flow (CBF) by 48-79%
- Preserves CO2 responsiveness and cerebral autoregulation
- EEG: initial increase in alpha rhythm → shift to gamma/theta → burst suppression at concentrations >8 mcg/mL
- BIS: 50% of patients unresponsive to verbal command at BIS 63; 95% at BIS 51
- Anticonvulsant properties (via GABA agonism, NMDA inhibition) but paradoxically can also trigger seizures during induction/emergence
- Reduces intraocular pressure by 30%
Cardiovascular System
- Most significant adverse effect: hypotension
- Causes vasodilation (decreased SVR), direct myocardial depression, and reduced preload
- More pronounced in elderly, hypovolemic, and sick (ASA III-IV) patients
- Combining with opioids worsens hypotension
- Negative inotrope at high doses
- Generally decreases heart rate (but less than barbiturates)
Respiratory System
- Dose-dependent respiratory depression - apnea common after induction boluses
- Blunts airway reflexes (facilitates LMA insertion without neuromuscular blockers)
- Bronchodilation (useful in asthmatics)
- Reduces tidal volume and respiratory rate
Other Effects
- Antiemetic - even sub-anesthetic concentrations reduce nausea
- Does NOT trigger malignant hyperthermia (safe in susceptible patients)
- No effect on neuromuscular junction (does not affect twitch tension or EMG), yet intubating conditions after propofol alone can be acceptable
- Inhibits phagocytosis and killing of Staphylococcus aureus and E. coli - risk of infection if vials are contaminated
Adverse Effects / Side Effects
| Effect | Details |
|---|---|
| Pain on injection | Common (30-90% with propofol); less with fospropofol |
| Hypotension | Most significant; especially in elderly/hypovolemic |
| Apnea | After induction boluses |
| Propofol Infusion Syndrome (PRIS) | Rare but life-threatening - metabolic acidosis, rhabdomyolysis, cardiac failure, renal failure; associated with prolonged high-dose infusions (>4 mg/kg/h for >48h), especially in ICU patients |
| Bacterial contamination | Fat emulsion supports microbial growth; strict aseptic technique required; opened vials must be used within 6-12 hours |
| Hypertriglyceridemia | With prolonged ICU infusions |
| Involuntary movements | Dystonic/choreiform movements, opisthotonos |
| Sexual disinhibition | Hallucinations, inappropriate sexual behavior |
| Egg/soy allergy | Contains egg phospholipid and soybean oil - use with caution in allergic patients (but actual risk is controversial) |
| FDA warning (2016) | Potential risks to fetal brain development with prolonged/repetitive exposure in children under 3 and in pregnant women in 3rd trimester |
Propofol Infusion Syndrome (PRIS)
A rare but potentially fatal complication characterized by:
- Metabolic/lactic acidosis
- Rhabdomyolysis
- Hyperkalemia
- Renal failure
- Hepatomegaly and lipemia
- Cardiac failure (new right bundle branch block, ST changes)
Risk factors: high dose (>4 mg/kg/h), prolonged duration (>48 h), critically ill patients (especially children and those with traumatic brain injury), high-fat/low-carbohydrate diet, inborn errors of mitochondrial metabolism.
Abuse Potential
Propofol can be addictive. In ICU patients, 20-40% require upward dose titration over time (tolerance). Case reports document lethal self-administration among healthcare workers who have easy access. It is not currently a DEA-controlled substance in the US (unlike fospropofol, which was classified Schedule V in 2009).
Contraindications / Cautions
- Known allergy to propofol components
- Hypovolemia (relative - correct volume first)
- Severe cardiac dysfunction
- Mitochondrial disease (risk of PRIS)
- Children <3 years for prolonged sedation (FDA warning)
- Pregnancy - category B but use with caution; avoid prolonged exposure
- Pancreatitis with severe hypertriglyceridemia
Sources: Miller's Anesthesia, 10th Edition (2023), pp. 2458-2490; Morgan & Mikhail's Clinical Anesthesiology, 7th Edition; Roberts & Hedges' Clinical Procedures in Emergency Medicine.
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