A patient with hepatic cirrhosis (Child-Pugh B) requires an analgesic. Which pharmacokinetic parameter is most critically affected, and what adjustment principle should guide dosing?
- A First-pass extraction is reduced for high-extraction ratio drugs (e.g., morphine, lidocaine), dramatically increasing bioavailability and risk of toxicity — dose should be reduced and extended dosing intervals considered ✓
- B Volume of distribution is reduced for lipophilic drugs due to ascites reducing fat compartment — dose should be increased to achieve adequate distribution
- C Renal clearance is primarily affected by hepatic disease through altered tubular secretion — dose based on creatinine clearance
- D Protein binding increases due to elevated acute-phase reactants in cirrhosis, requiring higher doses to achieve free drug concentrations
Explanation
In hepatic cirrhosis, two major pharmacokinetic alterations affect high-extraction-ratio drugs: (1) Reduced hepatic blood flow through portosystemic shunting bypasses hepatocytes, reducing presystemic (first-pass) metabolism. For drugs with high hepatic extraction ratios (e.g., morphine ER ~60%, lidocaine ~70%, propranolol ~60%), oral bioavailability can increase 2–5-fold compared to normal subjects. (2) Reduced hepatocellular mass impairs intrinsic clearance. The net effect for high-extraction drugs: both oral bioavailability increases AND systemic clearance decreases, dramatically elevating plasma concentrations. Additionally, in cirrhosis, serum albumin is reduced (hypoalbuminemia), decreasing protein binding of acidic drugs (e.g., NSAIDs, warfarin, phenytoin), increasing free drug fractions. CNS sensitivity to opioids also increases due to blood-brain barrier changes and relative hyperammonemia. Dose reduction, avoiding hepatotoxic drugs, and prolonged dosing intervals are standard principles.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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