Parenteral Anesthetics: Overview

Parenteral anesthetics rapidly induce general anesthesia when infused intravenously. Most agents target GABAA receptors to produce sedation and hypnotic effects. Ketamine is an exception, inducing dissociative anesthesia and analgesia by inhibiting NMDA receptors instead. These drugs have short elimination half-lives, making them ideal for anesthesia induction and maintenance during procedures.

Propofol is a widely used parenteral anesthetic formulated as a 1% emulsion in soybean oil, glycerol, and egg phosphatide. It induces rapid anesthesia due to quick distribution from the bloodstream to target tissues and hepatic metabolism. Fospropofol, a water-based prodrug of propofol, is used for sedation during diagnostics and lacks propofol's adverse effects like injection pain and hypertriglyceridemia.

Parenteral anesthetics can cause hypotension due to vasodilation and myocardial contractility depression. Propofol may trigger respiratory depression, requiring oxygenation and ventilation monitoring. Prolonged propofol use can cause propofol infusion syndrome (PRIS), a rare but potentially fatal condition marked by metabolic acidosis and hyperlipidemia. Other effects include myoclonic movements, nausea, vomiting, and adrenal steroid synthesis inhibition.

Benzodiazepines like midazolam enhance GABA effects in the central nervous system, providing sedation and amnesia during anesthesia. Dexmedetomidine, a selective α2 adrenergic agonist, is employed for short-term sedation in ICUs, offering sedation and analgesia without respiratory depression, making it valuable for adjunct anesthetic support.

Ketamine offers profound analgesia and dissociation, making it beneficial for patients at risk of hypotension and bronchospasm. Unlike most parenteral anesthetics that depress cardiovascular function, ketamine maintains hemodynamic stability. However, it can induce hallucinations and vivid dreams, which may limit its use in some clinical settings despite its protective cardiovascular profile.

Opioid analgesics like fentanyl provide analgesia during anesthesia by acting on opioid receptors in the central nervous system. They are used to manage postoperative analgesia and intraoperatively as part of a balanced anesthesia regimen. Opioids complement parenteral anesthetics by addressing pain while other agents handle sedation and hypnosis.

Etomidate is used for anesthetic induction in patients at risk for hypotension due to its minimal cardiovascular effects. It has a rapid onset and is metabolized in the liver. Unlike propofol and barbiturates, etomidate preserves hemodynamic stability, making it suitable for hemodynamically unstable patients requiring safe anesthetic induction.