7.4 Dosage Adjustments in Renal and Hepatic Impairment

Renal Impairment: Assessment and Dosing Principles

The kidneys represent the primary organ for the elimination of hydrophilic parent drugs and polar metabolites. Declines in glomerular filtration rate (GFR), tubular secretion, and tubular reabsorption alter drug clearance, prolonged elimination half-lives, and generate systemic drug accumulation.

Renal Function Estimation: Cockcroft-Gault Equation

Although nephrologists use the CKD-EPI equation (indexed to $1.73\text{ m}^2$ BSA) for staging chronic kidney disease, Health Canada and FDA drug monographs and manufacturer dosing guidelines are standardized to non-indexed Creatinine Clearance (CrCl in mL/min) calculated via the Cockcroft-Gault equation.

$\text{CrCl (mL/min)} = \frac{(140 - \text{Age [years]}) \times \text{Weight [kg]}}{72 \times \text{Serum Creatinine [mg/dL]}} \times (0.85\text{ if female})$

When Serum Creatinine is reported in SI units ($\mu\text{mol/L}$): $\text{CrCl (mL/min)} = \frac{(140 - \text{Age [years]}) \times \text{Weight [kg]} \times 1.2}{\text{Serum Creatinine [}\mu\text{mol/L]}} \times (0.85\text{ if female})$

Body Weight Selection Rules for Cockcroft-Gault

1. Underweight ($\text{Actual Body Weight [ABW]} < \text{Ideal Body Weight [IBW]}$): Use Actual Body Weight (ABW).
2. Normal Weight ($\text{ABW within } 100-120% \text{ of IBW}$): Use IBW (or ABW depending on institutional policy).
3. Obese ($\text{ABW} > 120% \text{ of IBW}$): Use Adjusted Body Weight ($A_{\text{adj}}$) to avoid drastically overestimating CrCl due to excess metabolically inactive adipose mass: $A_{\text{adj}} = \text{IBW} + 0.4 \times (\text{ABW} - \text{IBW})$

Ideal Body Weight (Devine Formula):

• $\text{IBW}_{\text{Male}} = 50\text{ kg} + 2.3\text{ kg} \times (\text{Height in inches} - 60)$
• $\text{IBW}_{\text{Female}} = 45.5\text{ kg} + 2.3\text{ kg} \times (\text{Height in inches} - 60)$

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Pharmacokinetic Dosage Adjustment Strategies in Renal Disease

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|               RENAL DOSAGE ADJUSTMENT STRATEGIES                        |
+-------------------------------------------------------------------------+
|  1. LOADING DOSE PRINCIPLE:                                             |
|     - Loading Dose = Target Concentration x Volume of Distribution      |
|     - UNCHANGED in renal impairment! (Clearance does NOT affect Vd)     |
|                                                                         |
|  2. MAINTENANCE DOSE STRATEGIES:                                        |
|     - Maintenance Dose Rate = Target Steady-State Conc. x Clearance     |
|                                                                         |
|     A. DOSE REDUCTION METHOD:                                           |
|        * Administer lower dose at standard dosing interval              |
|        * Keeps constant steady-state with small peak-trough swings      |
|        * Preferred for TIME-DEPENDENT antibiotics (e.g., beta-lactams)  |
|                                                                         |
|     B. INTERVAL EXTENSION METHOD:                                       |
|        * Administer standard full dose at longer time intervals         |
|        * Preserves high peak-to-MIC ratios                              |
|        * Preferred for CONCENTRATION-DEPENDENT drugs (aminoglycosides)  |
|                                                                         |
|     C. COMBINED METHOD:                                                 |
|        * Decrease dose AND extend interval (e.g., Vancomycin, LMWH)     |
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High-Risk Renally Eliminated Medications

The "Triple Whammy" Acute Kidney Injury Mechanism

Concurrent use of an ACE inhibitor or ARB, an NSAID, and a Diuretic creates a catastrophic hemodynamic collapse of intraglomerular hydrostatic pressure:

1. Diuretic: Depletes intravascular volume and reduces renal blood flow.
2. NSAID: Inhibits cyclooxygenase (COX-1/2), blocking synthesis of vasodilatory prostaglandins ($PGE_2, PGI_2$) at the afferent arteriole $\rightarrow$ severe afferent arteriolar vasoconstriction.
3. ACE inhibitor / ARB: Blocks angiotensin II-mediated vasoconstriction at the efferent arteriole $\rightarrow$ efferent arteriolar vasodilation.
4. Result: Afferent inflow is choked while efferent outflow remains wide open $\rightarrow$ collapse of glomerular filtration pressure $\rightarrow$ precipitous prerenal Acute Kidney Injury (AKI) and life-threatening hyperkalemia.

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Dialysis Clearance Principles (Hemodialysis vs Peritoneal Dialysis)

Factors governing whether a medication is removed by hemodialysis:

• Molecular Weight: Small molecules ($< 500\text{ Da}$) cross dialyzer pores easily. Large macromolecules ($> 1000-2000\text{ Da}$) are not cleared by standard low-flux filters.
• Plasma Protein Binding: Only unbound (free) drug traverses the dialysis membrane. Drugs with high protein binding ($> 80-90%$, e.g., warfarin, ceftriaxone, phenytoin) are poorly dialyzed.
• Volume of Distribution ($V_d$): Drugs with large $V_d$ ($> 1-2\text{ L/kg}$, e.g., digoxin, TCAs, amiodarone) reside predominantly in peripheral tissue compartments and are not effectively cleared by hemodialysis. Drugs with small $V_d$ ($< 0.5\text{ L/kg}$, e.g., aminoglycosides, beta-lactams) are substantially dialyzed.
• Water Solubility: Hydrophilic drugs dialyze readily; highly lipophilic drugs do not.
• Clinical Rule: Administer dialyzable medications (e.g., ampicillin, cefazolin, aminoglycosides, gabapentin) after the hemodialysis session (or give a post-dialysis supplemental replacement dose) to prevent therapeutic failure.

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Hepatic Impairment and Cirrhosis Pharmacokinetics

Child-Pugh Staging System

The Child-Pugh score grades the severity of chronic liver disease using 5 clinical and laboratory parameters:

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|                      CHILD-PUGH CLASSIFICATION                          |
+-------------------------------------------------------------------------+
|  PARAMETERS ASSESSED (1 to 3 points each):                              |
|  1. Total Bilirubin (< 34 umol/L = 1; 34-50 = 2; > 50 = 3)              |
|  2. Serum Albumin (> 35 g/L = 1; 28-35 = 2; < 28 = 3)                   |
|  3. INR (< 1.7 = 1; 1.7-2.2 = 2; > 2.2 = 3)                             |
|  4. Ascites (None = 1; Mild/Controlled = 2; Moderate/Severe = 3)        |
|  5. Hepatic Encephalopathy (None = 1; Grade I-II = 2; Grade III-IV = 3) |
|                                                                         |
|  SEVERITY STAGING:                                                      |
|  - Class A (5 to 6 points): Mild impairment (100% 1-year survival)      |
|  - Class B (7 to 9 points): Moderate impairment (80% 1-year survival;   |
|    typically requires 25-50% dose reduction for hepatically cleared drug|
|  - Class C (10 to 15 points): Severe impairment (45% 1-year survival;  |
|    avoid hepatically cleared drugs with narrow therapeutic index)       |
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High vs. Low Extraction Ratio Drugs in Cirrhosis

1. High Extraction Ratio Drugs ($E_H > 0.7$, Flow-Limited Clearance):
   • Examples: Morphine, Propranolol, Metoprolol, Verapamil, Lidocaine, Labetalol.
   • In healthy livers, these drugs undergo extensive first-pass metabolism ($60-90%$ extraction). In cirrhosis, fibrous tissue architecture and portosystemic shunting divert portal blood directly into systemic circulation, bypassing hepatocytes.
   • Clinical Impact: Oral bioavailability surges by $200%\text{ to }400%$, causing massive, potentially toxic systemic exposure at standard oral doses. Oral doses must be reduced by $50-75%$.
2. Low Extraction Ratio Drugs ($E_H < 0.3$, Capacity-Limited / Enzyme-Limited Clearance):
   • Examples: Warfarin, Phenytoin, Theophylline, Diazepam, Valproic Acid.
   • Clearance is independent of blood flow but strictly dependent on intrinsic CYP450 enzyme capacity ($CL_{\text{int}}$) and unbound fraction ($f_u$). Cirrhosis destroys functional hepatocytes, reducing enzyme synthesis and albumin production, prolonging half-lives.

Critical Medication Safety in Cirrhosis

• Acetaminophen: Contrary to widespread myth, acetaminophen is the safest first-line analgesic in cirrhosis when used at a reduced maximum dose of $\le 2000\text{ mg/day}$ in divided doses ($500\text{ mg}$ PO QID or $650\text{ mg}$ PO TID). It lacks renal, vascular, and gastrointestinal toxicity.
• NSAIDs: Strictly contraindicated in cirrhosis and portal hypertension. NSAIDs inhibit renal prostaglandins, precipitating Hepatorenal Syndrome (rapidly fatal functional renal failure), trigger catastrophic gastrointestinal bleeding from esophageal/gastric varices, and exacerbate portal hypertension-induced ascites through profound sodium retention.
• Sedatives and Opioids: Benzodiazepines, opioids, and z-drugs can precipitate or exacerbate Hepatic Encephalopathy due to impaired drug clearance and heightened baseline neuro-GABAergic sensitivity.

Which constant does the exam expect? The PEBC reference sheet supplied on screen during the Evaluating Examination gives the male form as (140 minus age) x actual body weight (kg) x 1.2 divided by serum creatinine in micromol/L, with the female value obtained by multiplying the male result by 0.85 (an effective factor of about 1.02). Many textbooks print 1.23 and 1.04 instead; the two forms differ by under 3% and rarely change a dosing band, but answer with the sheet you are given.