13.7: Parenteral Anesthetics: Overview

Intravenous anesthetics are drugs administered parenterally to induce anesthesia or sedation. Propofol is a widely used agent formulated as a 1% emulsion in soybean oil, glycerol, and egg phosphatide. It induces rapid anesthesia primarily due to its rapid distribution from the bloodstream to target tissues and is metabolized in the liver. However, it can cause significant pain on injection and hypertriglyceridemia. Fospropofol, a water-based prodrug of propofol, lacks these adverse effects. Benzodiazepines like midazolam enhance GABA effects in the CNS, providing sedation and anxiolysis. Barbiturates such as thiopental induce anesthesia by producing hypnosis and have anticonvulsant properties. They decrease cerebral blood flow and intracranial pressure but may depress respiration and cause hypotension. Opioids like fentanyl provide analgesia during anesthesia by acting on opioid receptors in the CNS. Ketamine, a dissociative anesthetic, offers profound analgesia and dissociation, beneficial for patients at risk of hypotension and bronchospasm. However, it can induce hallucinations and vivid dreams. Etomidate, used for anesthetic induction in patients at risk for hypotension, has a rapid onset and is metabolized in the liver. Dexmedetomidine, a selective α2 adrenergic agonist, is employed for sedation in ICUs, providing sedation and analgesia without respiratory depression.

Adverse effects of intravenous anesthetics include myoclonic movements, cardiovascular instability, respiratory depression, nausea, vomiting, and inhibiting adrenal steroid synthesis. A rare but potentially fatal complication called propofol infusion syndrome is associated with prolonged, high-dose propofol use. Despite these potential side effects, intravenous anesthetics are invaluable for their rapid induction of anesthesia, suitability for short procedures or longer surgeries, and diverse therapeutic uses.

Parenteral anesthetics rapidly induce general anesthesia when infused intravenously.

Most agents target GABA_A receptors for sedation and hypnotic effects. However, ketamine induces dissociative anesthesia and analgesia by inhibiting NMDA receptors.

Propofol is a commonly used parenteral anesthetic, while its prodrug,  fospropofol, is used for sedation during diagnostics.

These drugs have a short elimination half-life, making them suitable for anesthesia induction and maintenance.

However, they pose adverse effects, including hypotension due to vasodilation and potential myocardial contractility depression. Propofol can cause respiratory depression, necessitating oxygenation, and ventilation monitoring.

Its prolonged use may trigger PRIS, a rare, potentially fatal condition marked by metabolic acidosis and hyperlipidemia.

Adjunct agents include benzodiazepines for sedation and amnesia and dexmedetomidine, an α₂ adrenergic agonist, for short-term sedation.

Additionally, opioid analgesics are used to manage postoperative analgesia and intraoperatively as part of a balanced anesthesia regimen.