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Propofol Pharmacology and Total Intravenous Anesthesia (TIVA)

Pharmacology of Propofol

Chemical Properties

Propofol (2,6-diisopropylphenol) is formulated as a 1% or 2% lipid emulsion containing soybean oil, glycerol, and egg lecithin. It has a pH of approximately 7 and a white, milky appearance. Because the emulsion supports bacterial growth, strict aseptic technique is required, and unused drug should be discarded within 6-12 hours of opening. Pain on injection is common and can be mitigated with lidocaine co-administration.

Mechanism of Action

Propofol acts primarily as a positive allosteric modulator of GABA-A receptors, enhancing chloride channel opening. It also has additional effects at glycine receptors, the endocannabinoid system, and sodium channels. It produces hypnosis, anxiolysis, and amnesia but provides minimal analgesia.

Pharmacokinetics

Propofol has a distribution half-life of 2-8 minutes, reflecting rapid redistribution from the vessel-rich group. The elimination half-life is much longer at 4-23 hours, prolonged by its large volume of distribution and slow return from peripheral compartments. The context-sensitive half-time increases with infusion duration but remains relatively short — approximately 25 minutes after a 3-hour infusion and approximately 50 minutes after an 8-hour infusion — making it favorable compared to other IV agents. Clearance occurs through hepatic metabolism (CYP2B6, glucuronidation) plus extrahepatic metabolism in the lungs and kidneys; total clearance exceeds hepatic blood flow. The volume of distribution is very large (150-700 L) due to high lipophilicity, and the drug is 97-99% protein-bound to albumin.

Three-Compartment Model

Propofol's pharmacokinetics are described by a three-compartment model. The central compartment (V1) is 15-30 L. V2 (rapidly equilibrating) includes muscle and viscera. V3 (slowly equilibrating) is fat. Rapid redistribution from brain accounts for rapid awakening after a single bolus. With prolonged infusion, peripheral compartments become saturated and recovery depends more on elimination.

Clinical Pharmacology

Cardiovascular Effects

Propofol causes dose-dependent decreases in systemic vascular resistance through arterial and venous vasodilation, direct myocardial depression (negative inotropy), and blunting of the baroreceptor reflex (reducing compensatory tachycardia). The net result is significant hypotension, especially in elderly, hypovolemic, or cardiac-compromised patients. Strategies to mitigate this include slow injection rate, reduced dose, fluid preloading, and co-administration of vasopressors.

Respiratory Effects

Respiratory depression is dose-dependent, and apnea is common with induction doses. Propofol decreases both tidal volume and respiratory rate. It depresses airway reflexes, which is useful for LMA insertion, and has bronchodilatory properties that make it safe in asthma and reactive airway disease.

CNS Effects

Loss of consciousness occurs within one arm-brain circulation time (approximately 30-45 seconds). Propofol decreases cerebral blood flow, CMRO2, and ICP. It has anticonvulsant properties (though excitatory movements can occur during induction) and produces burst suppression on EEG at high doses. One of its most significant advantages is its antiemetic effect, which persists even at sub-hypnotic doses of 10-20 mg.

Other Effects

Propofol is one of the most antiemetic anesthetic agents available, through a mechanism that remains uncertain. It is anti-pruritic (useful for opioid-induced or neuraxial opioid-related pruritus), does not trigger malignant hyperthermia, and can cause green discoloration of urine from phenol metabolites (a benign finding). It may impair neutrophil and macrophage function, though the clinical significance of this immunomodulation is debated.

Dosing

Induction

Standard induction doses vary by patient population:

Maintenance (Infusion)

For general anesthesia, infusion rates of 100-200 mcg/kg/min (6-12 mg/kg/hr) are typical. For sedation (MAC sedation), 25-75 mcg/kg/min is used. Adjustments are made based on clinical response, BIS monitoring, and co-administered agents.

Sedation Bolus

For procedural sedation, 10-20 mg increments are given. The same sub-hypnotic dose (10-20 mg IV) is effective as an antiemetic.

Total Intravenous Anesthesia (TIVA)

Concept

TIVA involves complete avoidance of inhaled anesthetic agents, maintaining anesthesia with IV agents only — typically propofol combined with an opioid. It offers advantages over volatile-based anesthesia in specific settings.

Indications for TIVA

The absolute indication for TIVA is malignant hyperthermia susceptibility. The most common clinical indication is neuromonitoring requiring preserved MEPs and SSEPs. Other indications include open airway cases (tracheal surgery, rigid bronchoscopy), procedures with high PONV risk (propofol reduces PONV), patient preference or environmental concerns, and anticipated difficult airway with a spontaneous ventilation technique.

Common TIVA Regimens

The most common combination is propofol plus remifentanil, as both are ultra-short-acting and allow precise titration. Propofol plus sufentanil is an alternative for longer cases, and propofol plus fentanyl is acceptable but less titratable. Adjuncts may include ketamine, dexmedetomidine, lidocaine infusion, and muscle relaxant.

Target-Controlled Infusion (TCI)

TCI uses a computer-controlled infusion pump with pharmacokinetic models to achieve a target plasma or effect-site concentration. It is widely used outside the US but is not FDA-approved in the US, where manual rate adjustments are used instead.

Marsh Model

The Marsh model is weight-based, with a fixed V1 of 228 mL/kg and parameters that scale linearly with body weight. It targets plasma concentration, tends to produce a higher initial bolus due to the larger V1, and was originally developed for plasma targeting.

Schnider Model

The Schnider model incorporates age, weight, height, and lean body mass, with a fixed V1 of 4.27 L that is not weight-adjusted. Its parameters are more individualized. It targets effect-site concentration, produces a smaller initial bolus but achieves faster time to equilibrium, and is generally preferred for effect-site targeting in modern practice.

Manual TIVA Without TCI (US Practice)

In the US, a bolus plus infusion technique is used (such as the Roberts "10-8-6" rule or similar stepdown approaches). Processed EEG monitoring (BIS, SedLine) guides titration. Ensuring IV line patency is critical because TIVA without a patent IV creates a serious awareness risk. Key safety concerns include unrecognized IV disconnection, infiltration, or pump malfunction.

Propofol Infusion Syndrome (PRIS)

Definition

PRIS is a rare but potentially fatal complication of prolonged, high-dose propofol infusion. It was originally described in pediatric ICU patients but also occurs in adults.

Risk Factors

Risk factors include infusion rate exceeding 5 mg/kg/hr (greater than 83 mcg/kg/min), duration beyond 48 hours, pediatric patients (especially with respiratory infections), critical illness, catecholamine or steroid administration, and low carbohydrate intake.

Pathophysiology

The underlying mechanism involves impaired mitochondrial fatty acid oxidation and electron transport chain disruption, resulting in energy failure in cardiac and skeletal muscle.

Clinical Features (PRIS Pentad)

The five cardinal features are unexplained worsening metabolic acidosis (lactic acidosis), rhabdomyolysis (elevated CK, myoglobinuria), cardiac failure (arrhythmias, bradycardia progressing to asystole), renal failure, and hypertriglyceridemia/lipemia.

Management

Treatment requires immediate discontinuation of propofol, supportive care with hemodynamic support and renal replacement therapy, and a switch to alternative sedation (dexmedetomidine, benzodiazepines, barbiturates). ECMO may be required for refractory cardiac failure. Mortality is high, reaching 30-80% in severe cases.

Prevention

Prevention involves limiting infusion rates to below 4-5 mg/kg/hr in the ICU, limiting duration when possible, monitoring triglycerides, CK, and lactate regularly during prolonged infusions, and providing adequate carbohydrate nutrition. The risk during standard intraoperative use lasting hours rather than days is negligible.

<image>A graph showing the context-sensitive half-times of propofol, thiopental, midazolam, fentanyl, sufentanil, alfentanil, and remifentanil plotted against infusion duration (x-axis: 0-8 hours, y-axis: minutes). Propofol shows a moderate rise remaining under 40 minutes even after 8 hours. Remifentanil remains flat near 3-4 minutes. Thiopental and midazolam rise steeply. Annotations highlight why propofol and remifentanil are ideal TIVA agents.</image>

<image>A schematic diagram of a target-controlled infusion system showing: (1) a syringe pump connected to the patient IV, (2) a computer interface displaying the pharmacokinetic model (three-compartment model with V1, V2, V3 and rate constants), (3) the clinician-set target concentration (plasma or effect-site), and (4) the algorithm calculating bolus and infusion rate adjustments in real time to maintain the target. Arrows show the feedback loop between predicted concentration and pump output.</image>

<image>A clinical presentation diagram of propofol infusion syndrome showing a critically ill patient on mechanical ventilation with propofol infusion, surrounded by text boxes indicating the five cardinal features: metabolic acidosis (arterial blood gas showing pH 7.1, lactate 12), rhabdomyolysis (CK 50,000), cardiac arrhythmia (ECG showing Brugada-like pattern and widened QRS), renal failure (creatinine rising, dark urine), and hypertriglyceridemia (lipemic serum sample). A central arrow points to the mitochondria showing impaired fatty acid oxidation as the underlying mechanism.</image>

Clinical Pearls

Propofol's rapid redistribution and relatively short context-sensitive half-time make it the ideal IV hypnotic for both induction and maintenance. IV access must always be secured before TIVA induction — an unrecognized disconnected IV during TIVA is one of the most common causes of awareness under anesthesia. BIS or other processed EEG monitoring is strongly recommended during TIVA because there is no end-tidal agent concentration to confirm adequate depth. Propofol's antiemetic properties make it valuable for PONV-prone patients even as a sub-hypnotic bolus (10-20 mg) at emergence. Regarding egg and soy allergy, modern evidence suggests propofol is safe in most patients with these allergies (the relevant allergens in eggs are typically in the white, not the lecithin from the yolk), though severe anaphylactic-type egg allergy warrants caution. PRIS is almost exclusively an ICU phenomenon; standard intraoperative propofol infusions lasting hours carry negligible PRIS risk. In the US without TCI pumps, manual TIVA requires vigilance — processed EEG monitoring and verified infusion pump accuracy are essential.

References

• Schnider TW, et al. The influence of method of administration and covariates on the pharmacokinetics of propofol in adult volunteers. Anesthesiology. 1998;88(5):1170-1182.
• Marsh B, et al. Pharmacokinetic model driven infusion of propofol in children. Br J Anaesth. 1991;67(1):41-48.
• Hemphill S, et al. Propofol infusion syndrome: a structured literature review and analysis of published case reports. Br J Anaesth. 2019;122(4):448-459.
• Absalom AR, et al. Target-controlled infusion: a mature technology. Anesth Analg. 2016;122(1):70-78.
• Miller RD, et al. Miller's Anesthesia, 9th edition. Chapter on Intravenous Anesthetics.

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