Anaes · Anaesthetic adjuncts

Fentanyl

Also known as Synthetic phenylpiperidine opioid · Workhorse intraoperative opioid · Cardiac-anaesthesia opioid · Rising context-sensitive half-time opioid

Fentanyl is a synthetic phenylpiperidine opioid and a full agonist at the mu-opioid receptor, roughly 100 times more potent than morphine, and the workhorse intraoperative analgesic (Lewis 2026, Kurowska 2026). Two pharmacological features make it exam-critical. First, it releases NO histamine, so it preserves cardiovascular stability, is the opioid of choice for cardiac anaesthesia and the haemodynamically unstable, and is safe in asthma where morphine is not (Lewis 2026, de Souza 2026). Second, it is highly lipid-soluble with a rapid onset (1 to 2 minutes intravenously) and a rapid apparent offset after a single bolus (about 20 to 30 minutes), but this early offset is due to REDISTRIBUTION from brain to muscle and fat, NOT elimination; with repeated boluses or a prolonged infusion the peripheral compartments saturate, the CONTEXT-SENSITIVE HALF-TIME RISES, and the drug ACCUMULATES, producing delayed and prolonged respiratory depression and slow emergence (Sheridan 2026, Kurowska 2026). It is metabolised by hepatic CYP3A4 to INACTIVE norfentanyl with no active metabolites, so it is preferred over morphine in renal failure, and it is excreted renally. At high doses or with rapid intravenous bolus it can cause chest-wall rigidity (the wooden chest) impairing ventilation, and its respiratory depression and toxicity are reversed by the competitive mu antagonist naloxone (Lewis 2026, Voronkov 2026).