Anaes · Applied cardiovascular & respiratory physiology
Hepatic physiology & drug metabolism
Also known as Liver blood flow · Hepatic drug metabolism · Phase I Phase II metabolism · Cytochrome P450 · First-pass metabolism · Hepatic extraction ratio
The liver receives a dual blood supply and is the body's principal organ of drug metabolism and plasma protein synthesis, so its function governs the duration of almost every anaesthetic drug. The framework rests on five exam-critical ideas: the liver receives about 25 percent of the cardiac output from a dual supply — the portal vein (about 75 percent, nutrient-rich, deoxygenated) and the hepatic artery (about 25 percent, oxygen-rich) — and their contributions to oxygen are roughly equal; drug metabolism proceeds in two phases — Phase I (cytochrome P450 oxidation, reduction, hydrolysis) and Phase II (conjugation, e.g. glucuronidation and sulfation) — producing a water-soluble metabolite for biliary and renal excretion; first-pass (presystemic) metabolism by the gut wall and liver reduces the oral bioavailability of many drugs; the hepatic extraction ratio (the fraction removed in one pass) classifies drugs as high-extraction (flow-limited, e.g. propranolol, morphine) or low-extraction (capacity-limited, e.g. diazepam); and hepatic dysfunction prolongs drug action (less metabolism), reduces clotting factors and albumin (coagulopathy and altered drug binding), and causes encephalopathy. Built on the itraconazole-CYP3A4 drug-interaction study (Scudamore 2026), the 7-hydroxymitragynine SH-SY5Y-cell study with CYP450-mediated mitragynine metabolism context (Chujan 2026), the dual-vessel hepatic infarction study (Madani 2026), the rat hepatic transport pathway study (Chan 2026), the MARS-piperacillin liver-failure study (Monet 2026), and the lumacaftor-advanced-liver-disease pilot study (Lim 2026).
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Red flags
- The hepatic extraction ratio classifies drugs: high-extraction drugs (propranolol, morphine, lignocaine) are FLOW-limited (liver blood flow determines clearance); low-extraction drugs (diazepam, warfarin) are CAPACITY-limited (enzyme capacity and protein binding determine clearance).
- First-pass metabolism by the gut wall and liver reduces oral bioavailability; a high-extraction drug given orally has low bioavailability, and the dose must exceed the IV dose (e.g. propranolol, morphine).
- Hepatic dysfunction prolongs the action of hepatically metabolised drugs (less Phase I and II metabolism) and reduces the synthesis of clotting factors (II, VII, IX, X — the INR rises) and albumin (more free drug) — the anaesthetist must titrate and check the coagulation profile.
- Cytochrome P450 enzymes are subject to induction (phenytoin, rifampicin, alcohol — faster metabolism, lower drug levels) and inhibition (itraconazole, erythromycin, grapefruit — slower metabolism, higher drug levels and toxicity).
- The dual hepatic blood supply means hepatic oxygen is shared roughly equally between the portal vein (low oxygen, high flow) and the hepatic artery (high oxygen, lower flow); hepatic arterial flow rises when portal flow falls (the hepatic arterial buffer response).
Meet the patient
A cirrhotic patient for a portacaval shunt wakes slowly after a routine propofol and fentanyl anaesthetic, and his recovery is further complicated by a rising INR. The same drug doses that send a healthy patient home in an hour leave him oversedated and bleeding-prone — because his hepatic blood flow, enzyme capacity and protein synthesis are all broken at once.[5]
That patient is the whole topic at the bedside: a dual supply that can fail, a clearance that is either flow- or capacity-limited, and a synthetic function that sets both the free drug fraction and the clotting profile. Name which of the three is broken, and you know how to dose. MARS liver-failure sessions cut piperacillin by about one-third per session, and advanced-liver-disease pharmacokinetics vary enough to need individualised profiling.[5][6]
References6ShowHide
- [1]Scudamore O, et al. The Effect of Multiple Doses of Itraconazole on the Pharmacokinetics of a Single Oral Dose of Zongertinib in Healthy Male Volunteers Pharmacotherapy, 2026.PMID 42324472
- [2]Chujan S, et al. Effects of 7-hydroxymitragynine on serotonin, amino acid profiles, and ion homeostasis in SH-SY5Y cells Mol Biol Rep, 2026.PMID 42334658
- [3]Madani R, et al. Case Report: Nonoperative management of traumatic dual-vessel hepatic infarction Front Surg, 2026.PMID 42305804
- [4]Chan A, et al. A Comprehensive Rat Model For Assessing Hepatic Transport Pathways Through Simultaneous Lymphatic And Blood Vascular Sampling J Vis Exp, 2026.PMID 42224152
- [5]Monet C, et al. Impact of Molecular Adsorbent Recirculating System Therapy on Piperacillin Concentration in Critically Ill Patients: An Observational Cohort Study Crit Care Med, 2026.PMID 42301229
- [6]Lim AYL, et al. Lumacaftor-Ivacaftor in Pediatric Patients With Cystic Fibrosis and Advanced Liver Disease: A Pilot Study Clin Ther, 2026.PMID 42031572