Hepatic physiology & drug metabolism

Why this matters to the anaesthetist

Primary: dual blood supply, hepatic functions list, Phase I vs II, extraction ratio and bioavailability, enzyme induction/inhibition. Final: dose adjustments in cirrhosis, volatile choice myths vs reality, coagulopathy of liver disease, and portal hypertension physiology.[1]

One-liner: Portal vein ~75% flow / low O2 + hepatic artery ~25% / high O2; clearance of high-extraction drugs is flow-limited, low-extraction is enzyme/protein-binding-limited; Phase I (CYP) then Phase II (conjugate) make drugs excretable. [1]

Dual blood supply and microanatomy

• Portal vein: ~75% of hepatic blood flow, nutrient-rich, relatively deoxygenated — from splanchnic bed.
• Hepatic artery: ~25% of flow, oxygen-rich — buffers oxygen delivery.
• Mix in sinusoids → hepatic veins → IVC.
• Acinus zones: zone 1 periportal (highest O2) to zone 3 centrilobular (lowest O2, CYP-rich, vulnerable to ischaemia and paracetamol NAPQI injury). [1]

Hepatic blood flow falls with volatile anaesthesia, surgical retraction, hypovolaemia, and α-agonists; rises with hypercapnia and some β-agonists. Portal hypertension shunts portal blood away from hepatocytes → reduced first-pass for oral drugs and variceal risk. [1]

Liver function exam list

1. Synthetic: albumin, clotting factors (I, II, V, VII, IX, X, protein C/S, AT), thrombopoietin contribution.
2. Metabolic: carbohydrate (glycogen, gluconeogenesis), lipid, protein (urea cycle).
3. Detoxification/drug metabolism: Phase I/II, ammonia handling.
4. Bile formation: bilirubin excretion, fat/vitamin absorption.
5. Immunologic: Kupffer cells.
6. Storage: glycogen, vitamins, iron/copper (pathology when overloaded). [1]

Drug metabolism — Phase I and Phase II

Phase I (functionalisation)

Oxidation, reduction, hydrolysis — predominantly cytochrome P450 isoforms (CYP3A4 is the workhorse for many anaesthetics/opioids/benzodiazepines; CYP2D6, 2C9, 2C19, 1A2 also high-yield for variability). May activate prodrugs or create reactive intermediates (paracetamol → NAPQI). [1]

Phase II (conjugation)

Glucuronidation (UGT), sulfation, acetylation (NAT2 — slow/fast acetylators), glutathione conjugation (detox NAPQI), methylation. Generally increases water solubility for excretion. [1]

Neonates: immature Phase I/II — prolonged drug effect (link neonatal leaf). Elderly: reduced hepatic flow and mass. [1]

Hepatic clearance, extraction ratio, bioavailability

Hepatic clearance (CL_H) conceptually depends on liver blood flow (Q), fraction unbound (fu), and intrinsic clearance (CLint): [1]

For teaching, extraction ratio E = (C_in − C_out)/C_in. [1]

Oral bioavailability F reduced by incomplete absorption and first-pass hepatic/gut metabolism. High first-pass → large PO vs IV dose difference (e.g. propranolol, morphine oral vs IV). [1]

Restrictively cleared low-E drugs: only unbound drug is metabolised — hypoalbuminaemia raises free fraction transiently; steady-state free concentration often re-equilibrates via clearance (advanced viva — avoid oversimplified “low albumin always toxicity”). [1]

Biliary excretion and enterohepatic recirculation

Some conjugates are excreted in bile, deconjugated by gut flora, and reabsorbed (enterohepatic recirculation) — prolongs effect (e.g. some NSAIDs, digoxin partially, oestrogens). Cholestasis impairs elimination of biliary-dependent drugs. [1]

Enzyme induction and inhibition

Induction takes days (protein synthesis); inhibition can be immediate. Exam favourite pairs: CYP3A4 inhibitors + midazolam/fentanyl; inducers + failure of drug effect. [1]

Hepatic dysfunction patterns

• Hepatocellular injury: high ALT/AST; synthetic failure late.
• Cholestasis: high ALP/GGT/bilirubin; pruritus; fat-soluble vitamin issues.
• Synthetic failure: high INR, low albumin — true function tests for anaesthesia risk.
• Portal HTN: ascites, varices, encephalopathy, hypersplenism (low platelets).
• PK changes: reduced CLint, reduced Q, altered binding, volume changes from ascites/oedema. [1]

Anaesthetic relevance

• Prefer drugs with organ-independent routes when possible in severe liver disease (cisatracurium Hofmann/ester; remifentanil esterases).
• Reduce dose/titrate high-extraction IV agents when cardiac output/hepatic flow low.
• Coagulopathy: factors + fibrinogen + platelets; vitamin K if deficient; viscoelastic guidance useful.
• Avoid sedatives that precipitate encephalopathy; careful opioids (morphine-6-glucuronide renal issues also).
• Paracetamol: safe at standard doses in stable chronic liver disease usually, but toxic in overdose via NAPQI when glutathione depleted (alcohol, malnutrition).
• Volatiles: modern agents undergo minimal metabolism (desflurane/sevoflurane/isoflurane low %); historical halothane hepatitis is the classic immune/metabolic teaching story. [1]

Numbers board

• Total hepatic blood flow ~1.5 L/min (~25% CO).
• Portal : arterial flow ≈ 3 : 1.
• CYP3A4 metabolises a large fraction of common drugs.
• Factor VII short half-life → INR early marker of synthetic acute change. [1]

[1] [1] [1]

Viva scripts

Draw dual supply and state percentages. [1]

Define extraction ratio and give one high and one low E drug with dosing implication. [1]

Phase I vs II with paracetamol as the worked toxin example. [1]

Why oral morphine dose ≫ IV dose — first-pass. [1]

Extended viva dialogue

Examiner: Propofol clearance is high — what does that mean clinically? [1]

Candidate: Clearance depends heavily on hepatic blood flow (and extrahepatic sites contribute). In low-output states clearance falls and accumulation risk rises; titration and context-sensitive half-time still matter for long infusions. [1]

Examiner: How does enzyme inhibition present? [1]

Candidate: A perpetrator drug immediately reduces metabolism of a victim drug — levels rise, sedation or toxicity appears within dosing intervals, especially for narrow therapeutic index agents. [1]

Clinical synthesis: Hepatic physiology for anaesthesia is blood flow + enzymes + synthesis. Name which of the three is broken in the patient in front of you. [1]

Well-stirred model equation (exam form)

CL_H = Q × (f_u × CL_int) / (Q + f_u × CL_int) [1]

• If f_u·CL_int ≫ Q → CL_H ≈ Q (flow-limited, high E).
• If f_u·CL_int ≪ Q → CL_H ≈ f_u·CL_int (capacity-limited, low E). [1]

This single equation unifies extraction ratio teaching. [1]

Paracetamol pathway (toxin map)

Most dose: glucuronidation/sulfation (safe). Minor CYP2E1 path → NAPQI → glutathione conjugation. Overdose: glutathione depletes → NAPQI binds hepatocytes (zone 3). NAC restores glutathione pathways. [1]

Worked SAQ

SAQ: Explain hepatic extraction ratio and give anaesthetic examples (8 marks)

Extraction ratio is the fraction of drug removed from blood in one pass through the liver. High-extraction drugs (propofol, lidocaine, ketamine teaching set) have clearance dominated by hepatic blood flow and large first-pass after oral dosing. Low-extraction drugs have clearance sensitive to enzyme activity and unbound fraction, so induction/inhibition and cirrhosis enzyme loss dominate. Shock that halves liver blood flow halves clearance of high-E drugs. [1]

Primary exam expansion — dense examiner pack

Gross hepatic functions for anaesthesia

1. Drug metabolism and first-pass. 2. Synthesis: albumin, clotting factors (except VIII partly extrahepatic), thrombopoietin nuances, bile. 3. Bilirubin handling. 4. Glucose (glycogen/gluconeogenesis). 5. Lactate clearance. 6. Immune (Kupffer). 7. Hormone inactivation. [1]

Dual blood supply and extraction

Portal vein ~75% flow; hepatic artery ~25%; O2 delivery more balanced. Total hepatic blood flow ~25% CO. Hepatic arterial buffer response defends flow when portal falls. Under anaesthesia/surgery: volatiles, hypocapnia, surgical traction, pneumoperitoneum, haemorrhage reduce flow. [1]

Extraction ratio and clearance (must write)

E = (C_in − C_out)/C_in. CL_H = Q × E. Well-stirred: CL_H = Q (fu CLint)/(Q + fu CLint). [1]

Phase I and Phase II metabolism

Phase I: oxidation/reduction/hydrolysis — CYP450 dominant (CYP3A4 biggest share). Phase II: conjugation (glucuronidation, sulfation, acetylation, glutathione) — usually makes more polar renally excreted metabolites. Some metabolites active (morphine-6-G) or toxic (NAPQI). [1]

Enzyme induction and inhibition

Induction (days): rifampicin, carbamazepine, phenytoin, chronic alcohol, St John’s wort → ↑CLint → treatment failure. Inhibition (hours): azoles, macrolides, cimetidine, grapefruit → ↑ victim drug levels → toxicity (midazolam, fentanyl prolonged). Competitive vs mechanism-based inhibition concepts. [1]

First-pass and bioavailability

Oral drugs with high E have low F. Cirrhosis + portosystemic shunts raise F of high-extraction oral drugs → exaggerated effects (oral morphine, midazolam). IV bypasses first-pass. [1]

Liver disease anaesthetic implications

Child–Pugh / MELD risk stratify — state as scoring systems without inventing operative cut-offs. [1]

Paracetamol NAPQI map

Therapeutic: glucuronidation/sulfation. Overdose: CYP2E1 → NAPQI → glutathione depletes → hepatocyte necrosis. Antidote N-acetylcysteine restores glutathione pathways. Chronic alcohol/enzyme inducers increase risk teaching. [1]

Hofmann elimination / esterases contrast

Atracurium/cisatracurium: Hofmann + ester hydrolysis — organ-independent component useful in hepatic/renal disease (laudanosine historical atracurium concern high doses). Remifentanil tissue esterases. Not hepatic CYP stories — compare explicitly in viva. [1]

SAQ: hepatic extraction ratio with examples (8 marks)

Define E and CL_H (2). High vs low extraction drugs examples (3). Well-stirred limiting cases (2). One clinical CO or cirrhosis implication (1). [1]

Viva

Q: Why does shock raise propofol plasma for same infusion? A: High-E drug clearance falls when hepatic blood flow falls. Q: Why is INR high in liver failure? A: Reduced synthesis of clotting factors (VII short half-life early). Q: Does atracurium need liver? A: Predominantly organ-independent Hofmann/ester — preferred narrative in hepatic failure vs aminosteroid accumulation risk. [1]

High-yield viva battery and numbers lock-in

High versus low extraction drug lists (commit)

High E: propofol, lignocaine, morphine, ketamine, metoprolol, pethidine teaching. Low E: warfarin, phenytoin, diazepam, theophylline, many highly bound capacity-limited drugs. Intermediate: midazolam. [1]

Cirrhosis anaesthetic preference logic

Use drugs with organ-independent elimination when possible (cisatracurium, remifentanil); reduce doses of sedatives; avoid long-acting benzos; careful with opioids (encephalopathy); expect reduced plasma cholinesterase sometimes; correct coagulopathy based on clinical bleeding/viscoelastic not INR alone; manage ascites respiratory effects; prepare for variceal and haemodynamic instability. [1]

Enzyme history questions that matter

"What enzyme-inducing anticonvulsants or rifampicin? Any azole or macrolide? Chronic alcohol? St John's wort? Grapefruit?" — maps to midazolam/fentanyl/volatile minor paths and transplant drug interactions. [1]

Full viva dialogue (additional)

Examiner: Write the well-stirred model and take high-E and low-E limits. [1]

Candidate: CL_H = Q × (fu × CLint) / (Q + fu × CLint). If fu CLint is much greater than Q, CL_H approximates Q — flow-limited high extraction. If fu CLint is much less than Q, CL_H approximates fu × CLint — capacity-limited low extraction. [1]

Examiner: Why might oral bioavailability of morphine rise in cirrhosis? [1]

Candidate: Portosystemic shunting and reduced first-pass extraction allow more drug into the systemic circulation, so the same oral dose produces higher exposure and more sedation or respiratory depression. [1]

Exam traps

• Saying all drugs need dose cuts the same way in liver disease.
• Ignoring flow-limited clearance in shock.
• Assuming atracurium is heavily CYP-dependent.
• Treating INR as a full factor-by-factor map. [1]

References

1. [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. [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. [3]Madani R, et al. Case Report: Nonoperative management of traumatic dual-vessel hepatic infarction Front Surg, 2026.PMID 42305804
4. [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. [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. [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