Therapeutic Anticoagulation

Procedure Detail: Unfractionated Heparin (UFH) Infusion:

Loading Dose:

• Weight: TBW
• Dose: 80 units/kg (TBW), maximum 5,000 units
• Rationale: Higher blood volume requires higher dose

Infusion Rate:

• Weight: AdjBW or capped at 100-120 kg
• Starting rate: 18 units/kg/hr (AdjBW)
• Titrate to aPTT target

Obesity-Specific Considerations:

• Higher baseline antithrombin III activity in obesity
• May require higher infusion rates initially
• Monitor aPTT q6h until therapeutic

Low Molecular Weight Heparins (LMWH):

Enoxaparin (Therapeutic):

• Weight: TBW for loading, AdjBW may be considered for maintenance
• Dose: 1 mg/kg q12h (TBW), maximum 100 mg
• Monitoring: Anti-Xa levels in renal dysfunction, extreme obesity

Therapeutic Drug Monitoring (TDM)

TDM is essential in obesity due to unpredictable PK and narrow therapeutic index of many agents. [51,52]

Procedure Detail: TDM in Obese Critically Ill Patients:

Indications for TDM:

• Vancomycin (AUC-guided preferred)
• Aminoglycosides (peak and trough)
• Beta-lactams (increasingly available, valuable in ARC)
• Phenytoin (highly protein-bound, obesity affects free fraction)
• Voriconazole (lipophilic, variable PK)

Timing of Levels:

• Loading dose verification: 1-2 hours post-bolus (for vancomycin, aminoglycosides)
• Steady state: 3-5 doses after regimen change
• Critical illness: More frequent monitoring (q24-48h) due to rapid PK changes
• Renal function changes: Immediate remeasurement after significant change

Interpretation Challenges in Obesity:

• Protein binding: Hypoalbuminemia → higher free fraction
• Vd variability: Standard population models may not apply
• CL swings: ARC → AKI transitions require frequent adjustment
• Tissue penetration: Plasma levels may not reflect tissue concentrations

Bayesian Software:

• Advantages: Incorporates patient-specific covariates (weight, renal function)
• Recommended for: Vancomycin, aminoglycosides
• Reduces: Time to therapeutic levels
• Improves: Dosing accuracy in obesity

Special Populations

Super-Obesity (BMI greater than 50 or Weight greater than 200 kg)

Patients with extreme obesity present unique challenges beyond standard obesity. [53,54]

Clinical Pearl: Super-Obesity Dosing Modifications:

Weight Capping:

• Consider capping TBW at 150-180 kg for dosing calculations
• Rationale: Physiologic limits beyond certain mass
• Applies to: Loading doses, vasopressors, some maintenance regimens

Alternative Weight Metrics:

• LBW preferred over IBW for many calculations
• Consider capping at 120-130 kg for hydrophilic drugs

Physiologic Considerations:

• Reduced cardiac output reserve
• Higher baseline pulmonary artery pressures
• Delayed gastric emptying (affects oral medications)
• Venous access challenges

Drug-Specific Adjustments:

• Propofol: Maximum 200-250 mcg/kg/min (TBW)
• Vancomycin: Cap loading at 3,000 mg regardless of TBW
• LMWH: Consider fixed high dose rather than continued weight-based increase

Obesity with Renal Replacement Therapy (CRRT)

CRRT adds another layer of complexity to drug dosing in obesity. [55,56]

Procedure Detail: CRRT Dosing Principles in Obesity:

Weight Selection:

• Most equations: Use AdjBW or ideal weight
• Rationale: Drug removal from plasma compartment, not fat
• Exceptions: Some clinicians use 25th percentile of TBW

Clearance Considerations:

• Standard CRRT: CL ≈20-25 mL/min
• High-flow CRRT: CL ≈35-40 mL/min
• Effect: Significant drug removal, requires higher doses

Drug-Specific Adjustments:

Vancomycin:

• Dose: 15-20 mg/kg q12-24h (AdjBW)
• Timing: Post-filter (downstream from effluent)
• Monitoring: Daily levels required

Beta-Lactams:

• Dose: Standard doses (or higher due to high flow CRRT)
• Frequency: q6-8h typically adequate
• Continuous infusion: Excellent option

Aminoglycosides:

• Dose: 3-5 mg/kg q24-48h (AdjBW)
• Interval: Extended due to CRRT removal
• Levels: Essential

Anticoagulation:

• UFH: Higher infusion rates typical
• Monitoring: aPTT q4-6h
• Circuit clotting: More common in obesity

Practical Dosing Algorithm

Step-by-Step Approach to Obese ICU Patient

Procedure Detail: Step 1: Calculate All Weight Metrics

• TBW: Measure actual weight
• IBW: Use Devine formula based on height/sex
• AdjBW: IBW + 0.4 × (TBW - IBW)
• LBW: Janmahasatian formula (if available)
• BMI: Categorize obesity severity

Step 2: Determine Drug Properties

• Lipophilicity: High, moderate, or low
• Distribution: ECF, fat, or mixed
• Elimination: Renal, hepatic, or both
• Therapeutic index: Narrow (TDM essential) or wide
• Concentration dependence: Cmax/MIC or T > MIC

Step 3: Select Weight for Loading Dose

• Highly lipophilic (propofol, midazolam, fentanyl): TBW or LBW
• Moderately lipophilic: AdjBW (0.4-0.5)
• Hydrophilic (aminoglycosides, vancomycin): AdjBW (0.4) or TBW
• No fat distribution (digoxin, theophylline): IBW

Step 4: Select Weight for Maintenance Dose

• Renally eliminated, potential ARC: TBW or high AdjBW
• Hepatically eliminated: LBW for some, IBW for others
• Vasopressors: IBW (starting), titrate to effect
• Hydrophilic antibiotics: AdjBW, consider higher frequency

Step 5: Adjust for Critical Illness

• Capillary leak: Consider higher loading or repeat loading
• ARC: Higher maintenance dose or extended infusion
• Organ dysfunction: Reduce maintenance if hepatic/renal impairment
• Hypoalbuminemia: Monitor for increased free fraction

Step 6: Monitor and Reassess

• TDM: When available, guide dosing
• Clinical response: q24-48h reassessment
• Biomarkers: Trends in infection markers, organ function
• Side effects: Watch for toxicity signs

Step 7: Anticipate Transitions

• ARC to AKI: Rapid dose reduction when creatinine rises
• Fluid shifts: Reassess dosing after major fluid bolus
• Resolution of capillary leak: May reduce to standard doses

Clinical Case Studies

Case 1: Septic Shock in Morbid Obesity

Case Study: Patient Presentation:

• 52-year-old male, TBW 165 kg, IBW 72 kg, BMI 52 kg/m²
• Admitted with septic shock (urinary source)
• MAP 58 mmHg on norepinephrine 0.2 mcg/kg/min (IBW)
• Creatinine 78 μmol/L (CLcr estimated greater than 120 mL/min)

Initial Management:

• Fluid resuscitation: 4L crystalloids over 3 hours
• Vasopressors: Norepinephrine titrated to MAP 65
• Antibiotics: Piperacillin/tazobactam 4.5 g IV, vancomycin loading 3,000 mg

Clinical Challenge:

• Persistent hypotension requiring escalating vasopressors
• No improvement in lactate trend over 12 hours
• Repeat blood cultures positive despite initial antibiotics

Pharmacologic Issues Identified:

1. Antibiotic Subtherapeutic Levels:

   • Vancomycin trough at 12 hours: 7 mg/L (subtherapeutic)
   • Capillary leak + obesity → Vd significantly higher than anticipated
   • ARC (CLcr greater than 120) → rapid clearance

2. Vasopressor Resistance:

   • Endothelial dysfunction + catecholamine tolerance
   • Relative adrenal insufficiency possible

Management Adjustments:

Antibiotics:

• Vancomycin reload: 2,000 mg (Additional loading dose)
• Maintenance increased to 2,000 mg q12h (TBW)
• Piperacillin changed to continuous infusion: 16.2 g over 24h
• TDM ordered for both vancomycin and piperacillin

Vasopressors:

• Hydrocortisone 50 mg q6h started
• Vasopressin added: 0.03 U/min
• Phenylephrine: 0.3 mcg/kg/min (IBW)

Outcome:

• MAP improved to 72 mmHg within 4 hours
• Vancomycin trough at steady state: 18 mg/L (therapeutic)
• Lactate trend normalized over 24 hours
• Blood cultures negative at 48 hours

Key Learning Points:

• Loading doses based on TBW for vancomycin appropriate
• Maintenance required upward adjustment due to ARC
• Continuous infusion of beta-lactams improves T > MIC
• Adjunctive vasopressors improve responsiveness in obesity

Case 2: Prolonged Sedation in Obesity

Case Study: Patient Presentation:

• 48-year-old female, TBW 140 kg, IBW 58 kg, BMI 48 kg/m²
• ARDS on day 7 of ICU stay
• Sedation: Propofol 100 mcg/kg/min (TBW) infusion for 5 days
• Failed extubation attempt: Patient unarousable 6 hours after stopping propofol

Pharmacologic Assessment:

Propofol Accumulation:

• Total dose: ~14 mg/kg (TBW) over 5 days
• Lipophilicity: High (logP ~4)
• Expected Vd in obesity: 2-3x normal
• Expected accumulation in fat stores: Significant

Risk Factors for Delayed Emergence:

• Prolonged infusion greater than 48h
• Morbid obesity
• Renal function: Normal (no increased CL)
• Liver function: Normal

Management Strategy:

Immediate:

• Continued mechanical ventilation
• Supportive care while awaiting drug washout

Alternative Analgesia/Sedation:

• Opioid-sparing approach to minimize additional respiratory depression
• Dexmedetomidine infusion: 0.4 mcg/kg/hr (IBW)
• Consider benzodiazepine-sparing (already on midazolam from earlier)

Monitoring:

• Sedation score q2h (RASS)
• Expected time to awakening: 12-24 hours based on PK
• Plan for re-assessment at 18 hours

Second Extubation Attempt:

• Propofol stopped at 18 hours before planned attempt
• RASS -2 achieved at 14 hours post-propofol
• Spontaneous breathing trial successful
• Successfully extubated

Reflection and Learning:

• Propofol washout time significantly prolonged in obesity
• Consider sedation rotation after 48-72 hours in morbid obesity
• Dexmedetomidine valuable for facilitating extubation
• Early transition to weaning protocols reduces total sedative exposure

Assessment: Short Answer Questions

SAQ 1: Propofol Dosing in Morbid Obesity (15 marks)

Question:

A 155 kg, 42-year-old male (BMI 52 kg/m²) is admitted to ICU with community-acquired pneumonia requiring intubation and mechanical ventilation. The resident asks for advice on propofol dosing.

Tasks:

a) Calculate the patient's ideal body weight (IBW), adjusted body weight (AdjBW), and lean body weight (LBW) using the Janmahasatian formula. Height is 180 cm. [5 marks]

b) Recommend and justify the appropriate:

• Loading (induction) dose of propofol [3 marks]
• Starting maintenance infusion rate [3 marks]
• Maximum safe infusion rate to minimize PRIS risk [2 marks]

c) Explain the physiological rationale for using different weight metrics for loading versus maintenance dosing of propofol. [5 marks]

d) List three clinical manifestations of Propofol Infusion Syndrome (PRIS) and outline monitoring strategies to detect it early. [2 marks]

Model Answer:

a) Weight Calculations:

IBW (Devine formula):

• 180 cm = 5'11"
• IBW = 50 + 2.3 × 11 = 75.3 kg

AdjBW:

• AdjBW = 75.3 + 0.4 × (155 - 75.3)
• AdjBW = 75.3 + 0.4 × 79.7 = 75.3 + 31.9 = 107.2 kg

LBW (Janmahasatian formula):

• BMI = 155 / 1.80² = 155 / 3.24 = 47.8 kg/m²
• LBW = 9270 × 155 / (6680 + 216 × 47.8)
• LBW = 1,436,850 / (6680 + 10,324.8) = 1,436,850 / 17,004.8 = 84.5 kg

[5 marks - 2 for IBW, 1.5 for AdjBW, 1.5 for LBW]

b) Propofol Dosing Recommendations:

Loading (Induction) Dose:

• Use LBW = 84.5 kg
• Dose: 1.5-2.5 mg/kg (LBW)
• Calculation: 1.5 mg/kg × 84.5 kg = 127 mg
• Recommendation: 100-200 mg IV over 30 seconds
• Justification: Initial plasma concentration determined by Vd in vessel-rich organs (muscle, brain), not adipose tissue. LBW best represents this compartment.

[3 marks - 1.5 for weight selection, 1 for dose range, 0.5 for calculation]

Starting Maintenance Infusion:

• Use TBW for rate calculation
• Dose: 50-200 mcg/kg/min (TBW)
• Starting range: 50 mcg/kg/min × 155 kg = 7,750 mcg/min = 7.75 mg/min
• Recommendation: Start at 50-75 mcg/kg/min (TBW) = 8-12 mg/min
• Titrate to RASS -1 to 0

[3 marks - 1.5 for weight selection, 1 for dose range, 0.5 for calculation]

Maximum Safe Infusion Rate:

• Maximum: 4 mg/kg/hr (TBW) to minimize PRIS risk
• Calculation: 4 mg/kg/hr × 155 kg = 620 mg/hr = 10.3 mg/min
• Recommendation: Do not exceed 10 mg/min (600 mg/hr) without TDM
• Additional cap: Consider alternative sedative after 48 hours

[2 marks - 1 for rate, 1 for justification]

c) Physiological Rationale:

Loading Dose (LBW):

• Propofol is highly lipophilic but initial distribution is to well-perfused organs
• Vd in first few minutes reflects plasma and lean tissue volume
• Fat uptake occurs over hours (secondary distribution)
• Using LBW avoids excessive initial plasma concentration → severe hypotension

Maintenance Dose (TBW):

• Hepatic clearance of propofol correlates with total hepatic blood flow
• Cardiac output and hepatic perfusion increase with TBW
• Drug continuously partitions into fat during steady state
• Using TBW accounts for increased clearance and ongoing fat distribution

[5 marks - 2.5 for loading rationale, 2.5 for maintenance rationale]

d) PRIS Manifestations and Monitoring:

Clinical Manifestations:

1. Metabolic: Severe metabolic acidosis, hypertriglyceridaemia, rhabdomyolysis (elevated CK)
2. Cardiovascular: Refractory hypotension, bradycardia, right heart failure
3. Renal: Acute kidney injury, oliguria, myoglobinuria

Monitoring:

• Metabolic: ABG q12h, triglycerides q24h, CK q24h
• Cardiovascular: Continuous ECG, MAP monitoring, echocardiography if indicated
• Renal: Creatinine q12h, urine output hourly, urinalysis for myoglobin
• Sedation: Daily sedation holiday or switch to alternative agent after 48 hours

[2 marks - 1 for manifestations (0.5 each), 1 for monitoring]

Total: 15/15 marks

SAQ 2: Vancomycin Dosing and ARC in Obesity (15 marks)

Question:

A 45-year-old female (TBW 135 kg, IBW 60 kg, BMI 48 kg/m²) is admitted with MRSA bacteremia. Her creatinine is 65 μmol/L with urine output 120 mL/hr. Vancomycin therapy is initiated.

Tasks:

a) Estimate the patient's creatinine clearance using the Cockcroft-Gault equation with Adjusted Body Weight. [4 marks]

b) Recommend and justify:

• Appropriate loading dose of vancomycin [3 marks]
• Starting maintenance regimen based on your CLcr estimate [4 marks]

c) Explain the concept of Augmented Renal Clearance (ARC) and its implications for vancomycin dosing in obese critically ill patients. [4 marks]

Total: 15 marks

Model Answer:

a) Creatinine Clearance Estimation:

Cockcroft-Gault Formula:

• CLcr = [(140 - age) × weight] / (72 × Cr) × 0.85 (female)

Weight Selection:

• Use AdjBW for CLcr calculation
• AdjBW = 60 + 0.4 × (135 - 60) = 60 + 30 = 90 kg

Calculation:

• CLcr = [(140 - 45) × 90] / (72 × 65) × 0.85
• CLcr = 95 × 90 / 4680 × 0.85
• CLcr = 8,550 / 4680 × 0.85 = 1.826 × 0.85 = 1.55 mL/min
• Convert to mL/min: 1.55 mL/min × 1000 = 155 mL/min
• Adjust for body surface area (1.73 m²): 155 × 1.73 / (female BSA ≈1.8) ≈ 149 mL/min

ARC Definition:

• CLcr greater than 130 mL/min/1.73m²
• This patient meets ARC criteria (149 mL/min/1.73m²)

[4 marks - 1 for formula selection, 1 for AdjBW use, 1 for calculation, 1 for ARC identification]

b) Vancomycin Dosing Recommendations:

Loading Dose:

• Use TBW
• Dose: 20-25 mg/kg (TBW)
• Calculation: 20 mg/kg × 135 kg = 2,700 mg
• Recommendation: 2,500 mg IV over 90 minutes (rounded to nearest 500 mg)
• Justification: Vd of vancomycin correlates with TBW. Loading dose essential for rapid achievement of therapeutic concentrations in bacteremia. Cap at 3,000 mg to avoid toxicity.

[3 marks - 1.5 for TBW use, 1 for dose, 0.5 for justification]

Maintenance Regimen:

Considerations:

• ARC present (CLcr 149 mL/min/1.73m²) → higher than expected clearance
• Standard dosing (15 mg/kg q12h) likely subtherapeutic
• 2020 IDSA guidelines recommend AUC₀₋₂₄ target 400-600 mg·h/L

Options:

Option 1 (Higher Dose q12h):

• 20 mg/kg q12h (TBW) = 20 × 135 = 2,700 mg q12h
• Total daily: 5,400 mg (above 4,000 mg limit - higher V-AKI risk)

Option 2 (Standard Dose q8h) - Preferred:

• 15 mg/kg q8h (TBW) = 15 × 135 = 2,025 mg q8h
• Total daily: 6,075 mg (significantly above recommended)
• Consider capping at 1,750 mg q8h (5,250 mg/day)

Option 3 (Standard Dose q12h with TDM) - Recommended:

• 15-17 mg/kg q12h (TBW) = 2,000-2,300 mg q12h
• Total daily: 4,000-4,600 mg/day (within 4,000 mg cap with careful monitoring)
• TDM essential: Check trough and peak after 3-4 doses
• Use Bayesian software for individualized dosing

Recommendation:

• Start: 15 mg/kg q12h (TBW) = 2,000 mg q12h
• Monitor: First trough before 4th dose (36 hours), then weekly
• Target: AUC 400-600 (requires two levels and Bayesian calculation)
• Adjust: Based on TDM results

[4 marks - 1 for ARC recognition, 1 for option analysis, 1.5 for recommendation, 0.5 for monitoring]

c) ARC Explanation:

Definition:

• ARC = Creatinine clearance greater than 130 mL/min/1.73m²
• Incidence: 20-65% of critically ill patients, higher in younger obese patients
• Pathophysiology: Hyperdynamic circulation + obesity-related renal hyperfiltration

Impact on Vancomycin:

• Increased clearance: 30-50% higher than predicted by standard equations
• Consequence: Subtherapeutic troughs with standard q12h dosing
• Duration: May persist for days to weeks of critical illness

Clinical Implications:

1. Treatment failure: Subtherapeutic concentrations in bacteremia
2. Resistance emergence: Prolonged sub-MIC exposure
3. Need for TDM: Bayesian software essential for accurate dosing
4. Variable course: May transition to AKI, requiring rapid dose reduction

Obesity-Specific Issues:

• CLcr estimation error: Using TBW overestimates, using IBW underestimates
• AdjBW preferred: Best compromise for CLcr calculation
• Higher baseline GFR but higher AKI risk from shock

[4 marks - 1 for definition, 1 for pathophysiology, 1 for clinical implications, 1 for obesity issues]

Total: 15/15 marks

Assessment: Viva Voce

Viva 1: Comprehensive Obesity Drug Dosing (20 marks)

Examiner: "We have a 62-year-old male weighing 175 kg, admitted with septic shock. Height is 185 cm. Let's discuss drug dosing in this patient."

**Candidate: "I'll approach this systematically by calculating the various weight metrics and then discuss how they apply to different drug classes."

**Examiner: "Good. Start by calculating his ideal body weight, adjusted body weight, and BMI."

Candidate:

IBW Calculation (Devine formula):

• Height 185 cm = 6'1"
• IBW = 50 + 2.3 × 13 = 79.9 kg, approximately 80 kg

BMI:

• BMI = 175 / 1.85² = 175 / 3.4225 = 51.1 kg/m²

AdjBW Calculation:

• AdjBW = 80 + 0.4 × (175 - 80) = 80 + 0.4 × 95 = 80 + 38 = 118 kg

**Examiner: "Excellent. This patient has morbid obesity. Now, he's in septic shock. Let's start with vasopressors. What approach would you take for norepinephrine dosing?"

Candidate:

Norepinephrine Dosing Strategy:

Initial Approach:

• Start with IBW-based dosing
• Initial dose: 0.05-0.1 mcg/kg/min (IBW) = 4-8 mcg/min
• Titrate rapidly to MAP target 65-75 mmHg

Rationale for IBW:

• Vascular receptors (alpha-1) not present in adipose tissue
• Blood volume expansion less proportional to weight gain in obesity
• Avoid excessive doses that could cause limb ischemia

Titration Strategy:

• Rapid titration in first 30-60 minutes
• Goal: Achieve MAP target, not specific dose
• Monitor: MAP, urine output, lactate, distal perfusion

Alternative Vasopressors:

• If high doses required (greater than 1 mcg/kg/min IBW):
  • Consider vasopressin 0.03-0.04 U/min
  • Consider phenylephrine 0.1-1.0 mcg/kg/min (IBW)
• Rationale: Pure alpha-1 agonists may be more effective in obesity-related endothelial dysfunction

**Examiner: "Good. Now he needs intubation. What's your approach to propofol for induction?"

Candidate:

Propofol Induction Dosing:

Weight Selection:

• Use **Lean Body Weight (LBW) or IBW for induction
• NOT TBW - this would cause severe hypotension

LBW Calculation (Janmahasatian):

• LBW = 9270 × 175 / (6680 + 216 × 51.1)
• LBW = 1,622,250 / (6680 + 11,037.6) = 1,622,250 / 17,717.6 = 91.6 kg

Induction Dose:

• Range: 1.5-2.5 mg/kg (LBW)
• Dose: 1.5 mg/kg × 91.6 kg = 137 mg, approximately 120-150 mg
• Administration: Over 30 seconds, expect loss of consciousness in 15-30 seconds

Rationale:

• Initial Vd determined by plasma and well-perfused organs (muscle, brain)
• Fat uptake occurs over hours (secondary distribution phase)
• TBW dosing would cause excessive peak concentrations in plasma → myocardial depression, hypotension

**Examiner: "Excellent. After intubation, you need to maintain sedation. What maintenance propofol infusion rate would you use?"

Candidate:

Propofol Maintenance Strategy:

Weight Selection for Maintenance:

• Use TBW for maintenance infusion rate calculation

Starting Rate:

• Range: 50-200 mcg/kg/min (TBW)
• Starting point: 50-75 mcg/kg/min
• Dose: 50 mcg/kg/min × 175 kg = 8,750 mcg/min = 8.75 mg/min
• Recommendation: Start at 8-10 mg/min, titrate to RASS -1 to 0

Rationale for TBW:

• Hepatic clearance correlates with total hepatic blood flow
• Cardiac output and hepatic perfusion increase with TBW
• Continuous infusion allows drug to partition into fat stores over time

Maximum Dose Considerations:

• Maximum: 4 mg/kg/hr (TBW) to minimize PRIS risk
• For this patient: 4 mg/kg/hr × 175 kg = 700 mg/hr = 11.7 mg/min
• Recommend: Do not exceed 10-12 mg/min
• Cap duration: Consider alternative sedative (e.g., benzodiazepine) after 48 hours

**Examiner: "Good. Now for antibiotics. He has documented MRSA bacteremia. How would you dose vancomycin?"

Candidate:

Vancomycin Loading Dose:

Weight Selection:

• Use TBW for loading dose

Loading Dose:

• Dose: 20-25 mg/kg (TBW)
• Calculation: 20 mg/kg × 175 kg = 3,500 mg
• Recommendation: Cap at 3,000 mg (maximum recommended) IV over 90-120 minutes
• Rationale: Vd correlates with TBW; need rapid therapeutic concentrations in bacteremia

Vancomycin Maintenance Dosing:

Initial Approach:

• Start with TBW for calculation
• Dose: 15-20 mg/kg q12h
• Initial: 15 mg/kg × 175 kg = 2,625 mg q12h
• Cap at 4,000 mg/day → 2,000 mg q12h

Therapeutic Drug Monitoring:

• 2020 IDSA Guidelines recommend AUC-guided monitoring
• Target: AUC₀₋₂₄ 400-600 mg·h/L
• First levels: Trough before 4th dose (36h) + peak 2 hours post-dose
• Bayesian software preferred for individualized dosing

ARC Considerations:

• Septic shock initially: Potential for augmented renal clearance
• If ARC present: Higher doses may be required (up to q8h)
• Monitor: Creatinine q12h, urine output
• Transition: Rapid dose reduction if AKI develops

V-AKI Prevention:

• Obesity is independent risk factor for vancomycin nephrotoxicity
• AUC-guided dosing (target 400-600) reduces AKI risk vs trough-only
• Avoid troughs greater than 20 mg/L (historical practice)
• Monitor: Creatinine q24h, urine output

**Examiner: "Excellent discussion. What about his other antibiotics? You're starting piperacillin/tazobactam. How would you dose this?"

Candidate:

Piperacillin/Tazobactam Dosing in Obesity:

Loading Dose:

• Standard loading not typically used, but initial dose should be full dose
• Dose: 4.5 g IV

Maintenance - Standard vs Obesity-Adjusted:

Standard Dosing:

• 4.5 g q6h or q8h
• Issue: May be subtherapeutic in obesity + ARC

Obesity + Critical Illness Adjustments:

Option 1 (Extended Infusion - Recommended):

• Dose: 4.5 g over 3-4 hour infusion q8h
• Rationale: Increases T > MIC, compensates for increased Vd and CL
• For this patient: 4.5 g over 4h infusion q8h

Option 2 (Continuous Infusion):

• Dose: 16.2 g over 24h continuous infusion
• Rationale: Maximizes T > MIC (100% of time)
• Particularly valuable in sepsis with high inoculum

Rationale for Adjustments:

1. Increased Vd: Obesity + capillary leak → 1.3-1.5x normal Vd
2. Augmented Renal Clearance: Common in younger obese septic patients
3. T > MIC Target: Time-dependent killing, need T > MIC for 40-50% of dosing interval
4. Subtherapeutic Risk: 30-50% of obese critically ill patients have subtherapeutic levels

Therapeutic Drug Monitoring:

• Increasing availability for beta-lactams
• Consider random level 12-24h after initiation
• Target: Free drug concentration greater than 4×MIC (or 100% T > MIC)

**Examiner: "Excellent. Let's move to a different class. He develops acute kidney injury (creatinine now 250 μmol/L, oliguria). How does this affect your dosing?"

Candidate:

AKI Impact on Drug Dosing:

Vancomycin:

• Immediate dose reduction required
• If on maintenance 2,000 mg q12 h:
  • "CLcr now ~20 mL/min: reduce to q24-48h"
  • "Dose: 15 mg/kg, extend interval to q48h"
• TDM essential: Check trough after new regimen established

Piperacillin/Tazobactam:

• Dose reduction: 4.5 g q6h → q8h or q12h
• Consider: Reduced to 3.375 g q8h (standard renal adjustment)
• TDM if available

Other Considerations:

• Avoid nephrotoxins: Switch from aminoglycosides if used
• Contrast: Minimize IV contrast for imaging
• Hemofiltration/CRRT: May require for refractory acidosis/hyperkalaemia

NMBAs (if applicable):

• Rocuronium: Prolonged effect, reduced dosing frequency
• Monitor: Train-of-four before repeat dosing
• Avoid: Succinylcholine (pseudocholinesterase deficiency)

**Examiner: "Excellent. Before we conclude, let's discuss prophylactic anticoagulation. This patient is bedbound with sepsis. What VTE prophylaxis would you use?"

Candidate:

VTE Prophylaxis in Super-Obesity:

Risk Assessment:

• Sepsis: High VTE risk (Caprini score would be elevated)
• Immobilization: Additional risk factor
• Super-obesity (BMI 51): Independent risk factor
• Contraindication: Active bleeding (assess clinically)

Options:

Unfractionated Heparin (UFH) SC:

• Standard dose: 5,000 units q8h
• Obesity adjustment: 7,500 units q8h
• Rationale: Higher Vd in obesity, standard dose subtherapeutic
• Monitoring: Anti-Xa not routinely needed for prophylaxis

Low Molecular Weight Heparin (Enoxaparin):

• Standard dose: 40 mg q24h
• Obesity adjustment (BMI ≥40):
  • "Option 1: 40 mg q12h"
  • "Option 2: 0.5 mg/kg q24h (TBW) = 87.5 mg → round to 80-90 mg"
• Evidence: Reduced VTE incidence with q12h dosing in morbid obesity

Mechanical Prophylaxis:

• Sequential compression devices (SCDs)
• Contraindication if lower limb vascular compromise (assess in sepsis)

Recommendation:

• Enoxaparin 40-40 mg q12h (or 80 mg q24h)
• Monitor: Platelets q2-3d (heparin-induced thrombocytopenia)
• Reassess: Daily for bleeding complications

**Examiner: "Excellent comprehensive discussion. You've demonstrated understanding of weight-based dosing, obesity-specific PK changes, and how critical illness further complicates therapy. Thank you."

Total Marks: 20/20

• IBW/BMI calculations: 3 marks
• Norepinephrine strategy: 3 marks
• Propofol induction: 3 marks
• Propofol maintenance: 3 marks
• Vancomycin loading: 2 marks
• Vancomycin maintenance/TDM: 3 marks
• Beta-lactam dosing: 2 marks
• AKI adjustments: 2 marks
• VTE prophylaxis: 2 marks

Viva 2: Complex Clinical Scenarios in Obesity (20 marks)

Examiner: "Let's work through two clinical scenarios involving obesity and drug dosing. First scenario: 38-year-old female, 125 kg, with polytrauma, requires multiple surgeries. Current creatinine 45 μmol/L with urine output 200 mL/hr."

**Candidate: "I'll first characterize her physiology and then address specific drug dosing challenges."

**Examiner: "Good. Start by calculating her pharmacokinetic parameters."

Candidate:

Weight Calculations:

• AdjBW: IBW (for 165 cm, ~55 kg) + 0.4 × (125 - 55) = 55 + 28 = 83 kg
• She has morbid obesity

Renal Function Assessment:

Cockcroft-Gault with AdjBW:

• CLcr = [(140 - 38) × 83] / (72 × 45) × 0.85
• CLcr = 8,526 / 3060 × 0.85 = 2.79 mL/min = 279 mL/min
• Adjust for BSA ~1.9 m²: ~270 mL/min/1.73m²

Interpretation:

• Significant Augmented Renal Clearance (ARC)
• CLcr greater than 130 mL/min/1.73m² (270 is 2× threshold)
• Common in young, obese trauma patients

**Examiner: "Excellent. She needs surgery and will require rocuronium. How do you approach NMBA dosing?"

Candidate:

Rocuronium Dosing Strategy:

Intubating (Loading) Dose:

• Weight: IBW (not TBW)
• Dose: 0.6-1.2 mg/kg (IBW) = 33-66 mg
• Recommendation: 50 mg IV for rapid sequence
• Rationale: NMJ limited to neuromuscular junction, minimal fat distribution

Obesity-Specific Considerations:

• TBW dosing: Would be 75 mg (1.2 mg/kg) → prolonged paralysis
• No intubation benefit with higher doses
• Recovery time: Significantly prolonged with TBW dosing

Monitoring:

• Train-of-four (TOF) monitoring
• Aim for TOF ratio ≥0.9 before extubation
• Consider reduced maintenance dosing due to renal clearance

Sugammadex Reversal:

• Standard dosing effective in obesity
• Dose: 2-16 mg/kg (IBW)
• Rapid reversal of rocuronium at end of surgery

**Examiner: "Good. Post-operatively, she'll need analgesia. Discuss opioid selection and dosing."

Candidate:

Opioid Strategy in Obesity with ARC:

Drug Selection:

Remifentanil (Preferred):

• Rationale: Context-sensitive half-time unchanged in obesity and renal failure
• Dosing: TBW-based
• Advantage: Rapid offset when stopped (predictable extubation)
• Con: Higher cost, requires continuous infusion

Fentanyl (Alternative):

• Rationale: Potent, familiar, but accumulates in fat
• Loading: LBW-based (0.5-1 mcg/kg)
• Infusion: TBW-based (0.5-3 mcg/kg/hr)
• Challenge: Prolonged washout (12-24h after stopping infusion)
• Use: For cases where remifentanil not available or cost concerns

Oxycodone/Morphine (Oral - Later):

• Standard weight: IBW or AdjBW
• Active metabolites: Consider renal function (morphine M6G, oxymorphone)
• Monitor: Accumulation in ARC may occur (though less than expected)

Recommendation:

• Remifentanil infusion initially (predictable PK)
• Transition to oral oxycodone when appropriate
• Multimodal analgesia: Reduce opioid requirements

**Examiner: "Excellent. She develops post-op atrial fibrillation and needs amiodarone. How do you dose this highly lipophilic drug?"

Candidate:

Amiodarone Dosing Challenges:

Lipophilic Drug Properties:

• Extremely high Vd (60+ L in obesity)
• Accumulates in fat: Prolonged half-life (days to weeks)
• Loading dose critical for therapeutic effect

Loading Dose:

• Standard: 5-7 mg/kg over 1-3 days
• Weight selection: TBW typically recommended
• Dose: 5 mg/kg × 125 kg = 625 mg over 24 hours
• Rationale: Need to fill large Vd in adipose tissue

Maintenance Dose:

• Standard: 100-400 mg/day
• Weight adjustment: IBW or fixed low dose typical
• Recommendation: 200 mg/day initially (adjusted for response)
• Rationale: Accumulation occurs with TBW-based maintenance

Obesity-Specific Toxicity Risks:

Pulmonary Toxicity:

• Obesity-related restrictive lung disease → additive risk
• Higher cumulative dose → interstitial pneumonitis risk
• Monitor: PFTs, CXR q3-6 months, hypoxia symptoms

Thyroid Dysfunction:

• Amiodarone-induced: Hypo- or hyperthyroidism
• Obesity: Baseline subclinical thyroid disease more common
• Monitor: TSH, free T4 q3-6 months

Hepatic Toxicity:

• NAFLD common in obesity (60-80%)
• Amiodarone hepatotoxicity risk amplified
• Monitor: LFTs q1-3 months

QT Prolongation:

• Baseline ECG abnormalities more common in obesity
• Additional amiodarone effect → torsades risk
• Monitor: ECG, consider alternative agents

Drug Interactions:

• Warfarin: Amiodarone increases effect (major obesity consideration: VTE risk)
• Beta-blockers: Additive bradycardia, AV block
• Digoxin: Increased levels (if on renally-cleared version)

**Examiner: "Excellent. Now let's change scenarios. Second patient: 58-year-old male, 160 kg, with decompensated heart failure, requires intubation and mechanical ventilation. Current creatinine 180 μmol/L, oliguria."

**Candidate: "This patient has super-obesity with renal failure. Different challenges altogether."

**Examiner: "Good. Calculate his renal function and discuss propofol considerations."

Candidate:

Renal Function Assessment:

Cockcroft-Gault with AdjBW:

• IBW (180 cm, male): 50 + 2.3 × 12 = 77.6 kg
• AdjBW: 77.6 + 0.4 × (160 - 77.6) = 77.6 + 33 = 110.6 kg
• CLcr = [(140 - 58) × 110.6] / (72 × 180)
• CLcr = 9,082 / 12,960 = 0.70 mL/min

Interpretation:

• Severe AKI (CLcr below 10 mL/min)
• Need for RRT consideration
• Renal clearance of drugs markedly reduced

Propofol Dosing in Renal Failure:

Hepatic Clearance (Primary):

• Propofol metabolized by liver (glucuronidation)
• Renal failure minimal impact on clearance
• Maintenance: Can use TBW-based dosing

Accumulation Risk:

• Terminal half-life may be prolonged with severe hepatic congestion (cardiogenic shock)
• Prolonged infusion (greater than 48h) still significant risk

Propofol vs Renal Failure Considerations:

• Metabolites: Propofol glucuronide renally cleared
• Accumulation of inactive metabolite: Not clinically significant
• PRIS risk: Possibly higher with critical illness regardless of renal function

Dosing Strategy:

• Loading: LBW-based (91.6 kg × 1.5-2 mg/kg = 137-183 mg)
• Maintenance: TBW-based but reduced dose for critical illness
• Start: 30-50 mcg/kg/min (TBW) = 48-80 mg/min
• Titrate: To RASS -1 to 0 (may need higher due to agitation from pulmonary edema)
• Maximum: Lower threshold in severe critical illness (consider 2-3 mg/kg/hr)
• Alternative: Consider benzodiazepine-sparing approach

**Examiner: "Good. What about dexmedetomidine? Any special considerations?"

Candidate:

Dexmedetomidine in Obesity + Heart Failure:

Hemodynamic Effects:

• Alpha-2 agonist: Vasoconstriction (potential benefit in distributive shock)
• Reduced sympathetic outflow (desired in cardiogenic shock)
• Bradycardia risk: May be beneficial or detrimental depending on context

Dosing Strategy:

Loading Dose:

• Weight: IBW or AdjBW (not TBW)
• Dose: 0.5-1 mcg/kg over 10 minutes
• Slower administration in critical illness to mitigate hypotension

Maintenance Infusion:

• Weight: IBW or AdjBW
• Dose: 0.2-0.7 mcg/kg/hr (IBW)
• Start low: 0.2-0.3 mcg/kg/hr
• Titrate: To RASS 0 to -1 while monitoring HR greater than 50-55

Heart Failure-Specific Issues:

• Bradycardia: May exacerbate low cardiac output
• Hypotension: May worsen tissue perfusion
• Concomitant beta-blockers: Additive bradycardia risk

Renal Failure:

• No dose adjustment needed (hepatic metabolism)
• Consider prolonged accumulation in severe hepatic congestion

**Examiner: "Excellent. He requires vasopressor support. What's your approach?"

Candidate:

Vasopressor Selection and Dosing in Heart Failure + Obesity:

Clinical Context:

• Cardiogenic shock: Low cardiac output
• Obesity: Increased metabolic demand
• Goal: Maintain MAP 65-75 with minimal myocardial oxygen demand

Norepinephrine:

• First-line standard
• Dosing: IBW-based (0.05-0.5 mcg/kg/min) = 4-39 mcg/min
• Limit: High doses increase afterload (bad for failing heart)
• Monitoring: Cardiac output if available, lactate

Dobutamine:

• Consider in cardiogenic shock
• Dose: 2-20 mcg/kg/min (IBW)
• Benefit: Inotrope with mild vasodilation (decreases afterload)
• Risk: Tachyarrhythmias (monitor ECG)

Alternative/Adjunctive:

• Vasopressin: 0.03 U/min (may allow lower norepinephrine)
• Milrinone: Consider if refractory (inodilator, reduces afterload)
• Mechanical support: IABP if severe cardiogenic shock

**Examiner: "Excellent. Finally, he needs antibiotics for hospital-acquired pneumonia. How do you approach vancomycin dosing given his renal function?"

Candidate:

Vancomycin in Severe AKI:

Renal Function:

• CLcr 0.7 mL/min (essentially anuric)
• Will require RRT (discuss with treating team)

Pre-RRT Dosing:

Maintenance Dose:

• Significant reduction from standard
• Dose: 15 mg/kg q24-48h or q7-10 days
• Weight: AdjBW (110.6 kg) or IBW
• Calculation: 15 mg/kg × 110.6 kg = 1,659 mg q7-10 days

Therapeutic Drug Monitoring:

• Essential: Check trough before next scheduled dose
• Target: 10-15 mg/L (reduced from 15-20 due to toxicity risk)
• Frequency: q3-5 days initially

With CRRT:

Dose Adjustment:

• CRRT removes vancomycin: Increase dose frequency
• Standard: 15-20 mg/kg q24h (AdjBW)
• Monitoring: Trough before dialysis and post-dialysis

V-AKI Risk:

• Very high with high troughs + renal failure
• Aggressive TDM and AUC-guided dosing critical
• Consider alternative agents: Linezolid (not renally cleared)

Obesity Complications:

• Tissue penetration: May be impaired in obese tissue
• Vd: May be higher than expected despite renal failure
• Initial loading dose: May still be beneficial (TBW-based)

**Examiner: "Excellent discussion covering multiple complex scenarios. You've demonstrated understanding of obesity PK, renal failure impacts, and how to adapt therapy. Thank you."

Total Marks: 20/20

• Scenario 1 calculations: 3 marks
• Rocuronium dosing: 3 marks
• Opioid strategy: 3 marks
• Amiodarone: 4 marks
• Scenario 2 renal assessment: 2 marks
• Propofol in renal failure: 3 marks
• Dexmedetomidine: 2 marks
• Vasopressors: 2 marks

Key Learning Points

1. Weight selection is drug-specific: TBW for loading lipophilic drugs, IBW/AdjBW for hydrophilic, LBW for sedative induction
2. Critical illness amplifies PK changes: Capillary leak, ARC, organ dysfunction all require dosing adjustments
3. Propofol requires dual-weight strategy: LBW for induction, TBW for maintenance, monitor for PRIS
4. Vancomycin dosing evolved: AUC-guided preferred over trough-based, TBW loading, individualized maintenance
5. Beta-lactams often underdosed: Extended or continuous infusion recommended in obesity + ARC
6. **Vasopressors: Start IBW-based, titrate to hemodynamic response, avoid high absolute doses
7. TDM is essential: Bayesian software preferred, especially in obesity with variable PK
8. Super-obesity requires capping: Consider weight caps at 150-180 kg for many calculations
9. CRRT adds complexity: Use AdjBW for most equations, anticipate higher clearance with high-flow CRRT
10. Clinical response trumps equations: Monitor, reassess, adjust based on patient-specific factors

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