ECMO Pharmacology

Clinical Overview

Extracorporeal membrane oxygenation (ECMO) profoundly alters drug pharmacokinetics (PK) and pharmacodynamics (PD) through multiple mechanisms: circuit sequestration, increased volume of distribution (Vd), altered protein binding, and modified clearance pathways. These ECMO-specific changes compound the already complex pharmacokinetic alterations seen in critical illness (augmented renal clearance, capillary leak syndrome, hypoalbuminemia, organ dysfunction). The result is highly unpredictable drug exposure that frequently leads to therapeutic failure or toxicity.

Key ECMO Pharmacology Principles:

1. Circuit sequestration: Lipophilic and highly protein-bound drugs adsorb to polyvinyl chloride (PVC) tubing, oxygenator membranes, and blood-circuit interfaces, reducing bioavailable drug concentrations by 20-80% depending on the agent.[1,2,3]

2. Increased Vd: ECMO circuit prime volume (1,000-1,500 mL adults), systemic inflammation, capillary leak, and fluid resuscitation increase Vd by 30-200% for hydrophilic drugs, requiring higher loading doses.[4,5]

3. Altered clearance: ECMO mode affects drug clearance differently - veno-venous (VV) ECMO preserves hepatic and renal clearance, while veno-arterial (VA) ECMO may reduce organ perfusion and clearance due to altered hemodynamics.[6,7]

4. Protein binding changes: Hypoalbuminemia, systemic inflammation, and competitive displacement increase free drug fractions, altering therapeutic targets for highly protein-bound drugs.[8,9]

5. Therapeutic drug monitoring (TDM): Mandatory for antimicrobials (vancomycin, aminoglycosides, beta-lactams), antifungals (voriconazole, isavuconazole), sedatives (midazolam, propofol), and anticonvulsants on ECMO to ensure therapeutic efficacy and prevent toxicity.[10,11]

Australian/NZ Context:

ECMO is centralized in major Australian and New Zealand tertiary centers (The Alfred, St Vincent's Melbourne, Royal Prince Alfred Sydney, Prince of Wales Hospital Sydney, Auckland City Hospital, Christchurch Hospital). Australian ECMO data from the ANZICS CORE database and Extracorporeal Life Support Organization (ELSO) registry inform evidence-based pharmacotherapy practices. Australian Therapeutic Goods Administration (TGA) and New Zealand Medicines and Medical Devices Safety Authority (Medsafe) guidelines emphasize mandatory TDM for antimicrobials and sedatives during ECMO.

Incidence and Epidemiology:

• ECMO utilization increased 5-fold during COVID-19 pandemic: ANZICS CORE reported 800+ ECMO runs in 2020-2021 (predominantly VV-ECMO for severe ARDS).[12]
• Subtherapeutic antimicrobial concentrations occur in 40-70% of ECMO patients using standard dosing (particularly beta-lactams, vancomycin, lipophilic antifungals).[13,14]
• Drug sequestration is highest in first 24-72 hours of ECMO (up to 80% fentanyl, 50% midazolam, 40% propofol circuit loss), then plateaus as circuit saturates.[15,16]
• VV-ECMO patients have higher antimicrobial target attainment (60-70%) compared to VA-ECMO (40-50%) due to preserved renal/hepatic clearance.[17]

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Pathophysiology of ECMO-Induced Pharmacokinetic Changes

1. Circuit Sequestration Mechanisms

Drug Adsorption Sites:

The ECMO circuit comprises multiple surfaces with high drug sequestration potential: polyvinyl chloride (PVC) tubing (~6 m length, ~0.5 m² surface area), polymethylpentene (PMP) oxygenator membranes (~1.5-2.5 m² surface area), heat exchanger surfaces, and blood-circuit interfaces.[1,2]

Physicochemical Determinants of Sequestration:

• Lipophilicity (Log P): Lipophilic drugs (Log P >3) partition into PVC tubing and membrane lipid layers. Fentanyl (Log P 4.05), midazolam (Log P 3.89), propofol (Log P 3.79), and amiodarone (Log P 7.57) show extensive sequestration (40-80% circuit loss).[15,18,19]

• Protein binding: Drugs >80% protein-bound (fentanyl 84%, midazolam 95%, propofol 98%, voriconazole 58%) adsorb to circuit surfaces, reducing free drug concentrations. Circuit saturates over 24-72 hours as binding sites fill.[8,20]

• Molecular weight: Small molecules (below 500 Da) have higher membrane permeability and circuit distribution. Larger molecules (>1,000 Da) like amphotericin B liposomal formulation show minimal sequestration.[21]

Circuit Material Impact:

• Heparin-coated circuits: Reduce drug sequestration by 20-40% compared to non-coated circuits (particularly for cationic drugs like gentamicin, vancomycin).[22,23]
• Polymethylpentene (PMP) membranes: Lower sequestration than older silicone or polypropylene membranes for lipophilic drugs (fentanyl sequestration 60-70% PMP vs 80-90% older membranes).[24]

Temporal Dynamics:

Sequestration follows saturation kinetics: rapid drug loss in first 6-24 hours (up to 80% for fentanyl, 50% midazolam), followed by circuit saturation at 24-72 hours with stabilization of plasma concentrations.[15,25] Implication: higher loading doses required in first 24 hours, with potential dose reduction after circuit saturation.

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2. Volume of Distribution (Vd) Changes on ECMO

ECMO Circuit Prime Volume:

Adult ECMO circuits add 1,000-1,500 mL blood volume (circuit tubing + oxygenator + reservoir). This 15-25% increase in blood volume directly expands Vd for hydrophilic drugs distributed primarily in extracellular fluid (aminoglycosides, vancomycin, beta-lactams).[4,26]

Critical Illness Compounding Effects:

• Capillary leak syndrome: Systemic inflammation causes endothelial glycocalyx degradation and increased capillary permeability, expanding Vd by 50-200% for hydrophilic drugs (particularly in sepsis, ARDS - the most common ECMO indications).[27,28]

• Fluid resuscitation: ECMO patients receive mean 3-8 L positive fluid balance in first 48 hours, further diluting drug concentrations and expanding Vd.[29]

• Hypoalbuminemia: Albumin below 25 g/L occurs in 60-80% ECMO patients, reducing protein binding and increasing free drug fraction (Vd increases for drugs normally highly protein-bound).[30]

Vd Changes by Drug Class:

VV-ECMO vs VA-ECMO Vd Differences:

VV-ECMO patients have 20-30% lower Vd expansion compared to VA-ECMO, attributed to better cardiac output preservation, less fluid resuscitation requirement, and lower inflammatory burden (primarily respiratory failure vs cardiogenic shock + SIRS in VA-ECMO).[31]

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3. Clearance Alterations on ECMO

Renal Clearance:

• Augmented renal clearance (ARC): Occurs in 30-50% of VV-ECMO patients (younger age, preserved cardiac output, hyperdynamic resuscitation). CrCl >130 mL/min/1.73m² causes subtherapeutic beta-lactam, vancomycin, and aminoglycoside concentrations despite standard dosing.[32,33]

• Acute kidney injury (AKI): Occurs in 40-60% of VA-ECMO patients (reduced renal perfusion, cardiogenic shock, nephrotoxin exposure). CrCl below 50 mL/min requires dose reduction for renally cleared drugs and increased monitoring for accumulation.[34]

• Renal replacement therapy (RRT): 30-50% of ECMO patients receive concurrent CRRT, adding additional clearance (convective + diffusive) requiring dose adjustment - particularly beta-lactams, vancomycin (see RRT topic for dosing adjustments).[35]

Hepatic Clearance:

• VA-ECMO reduced hepatic clearance: Reduced hepatic blood flow (mean arterial pressure 60-70 mmHg, reduced pulsatility) decreases first-pass metabolism and hepatic extraction of drugs (propofol, fentanyl, midazolam, voriconazole). Plasma concentrations may be 20-50% higher than expected.[36,37]

• VV-ECMO preserved hepatic clearance: Cardiac output and hepatic perfusion preserved, maintaining normal hepatic drug metabolism.[38]

ECMO Circuit Clearance:

The ECMO circuit does NOT provide significant drug clearance (unlike hemodialysis). Drugs are sequestered but not removed from circulation. Exception: highly lipophilic drugs may accumulate in circuit then release during weaning.[39]

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4. Protein Binding and Free Drug Fraction Changes

Hypoalbuminemia on ECMO:

Albumin below 30 g/L occurs in 70% of ECMO patients (capillary leak, systemic inflammation, dilution). This increases free drug fraction for highly protein-bound drugs (phenytoin, valproate, propofol, voriconazole), potentially causing toxicity at "therapeutic" total drug levels.[40,41]

Competitive Protein Displacement:

Uremia (urea >20 mmol/L), hyperbilirubinemia (bilirubin >50 μmol/L), and free fatty acids (released during ECMO lipolysis) competitively displace drugs from albumin binding sites, increasing free fraction unpredictably.[42]

TDM Implications:

For highly protein-bound drugs (phenytoin, valproate, voriconazole), free drug level monitoring is superior to total level monitoring on ECMO. Example: phenytoin "therapeutic" total level 40-80 μmol/L equates to free level 4-8 μmol/L at albumin 40 g/L, but free level rises to 8-16 μmol/L (toxic) if albumin drops to 20 g/L.[43]

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Drug-Specific ECMO Pharmacology

Sedatives and Analgesics

Fentanyl

Sequestration Profile:

Fentanyl shows the highest circuit sequestration of all opioids: 60-80% loss in first 24 hours (Log P 4.05, 84% protein binding, high lipophilicity).[15,44] After 24-72 hours, circuit saturates and plasma concentrations stabilize, but remain 30-50% below non-ECMO patients at equivalent doses.

Dosing Strategy:

• Loading dose: 2-5 mcg/kg IV (vs 1-2 mcg/kg standard) to overcome Vd expansion + circuit sequestration
• Maintenance infusion: 2-10 mcg/kg/hr (vs 1-5 mcg/kg/hr standard), titrate to sedation scores
• First 24 hours: Expect 50-100% higher dose requirement, then reduce after circuit saturation
• Avoid prolonged infusions: Accumulation risk in VA-ECMO (reduced hepatic clearance) and obesity (large Vd)

Alternative Opioids:

• Morphine: Lower sequestration (hydrophilic, Log P 0.89), but active metabolite accumulation risk in renal dysfunction (morphine-6-glucuronide)
• Remifentanil: Minimal sequestration (esterase metabolism independent of organ function), preferred for VA-ECMO with hepatic dysfunction, but requires continuous infusion
• Hydromorphone: Moderate sequestration, suitable alternative to fentanyl on ECMO

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Midazolam

Sequestration Profile:

Midazolam shows 30-50% circuit sequestration in first 24 hours (Log P 3.89, 95% protein binding), stabilizing after 48-72 hours.[16,25] Plasma concentrations remain 20-40% lower than non-ECMO patients at equivalent doses.

Dosing Strategy:

• Loading dose: 0.05-0.2 mg/kg IV (vs 0.02-0.1 mg/kg standard)
• Maintenance infusion: 0.05-0.3 mg/kg/hr (vs 0.02-0.1 mg/kg/hr standard)
• VA-ECMO: Reduce infusion rate by 30-50% after 48 hours (hepatic clearance reduced, risk accumulation)
• VV-ECMO: Standard rates acceptable with preserved hepatic clearance

Alternative Sedatives:

• Propofol: Also sequestered (30-40% loss, Log P 3.79, 98% protein binding), but more predictable pharmacokinetics. Max 4 mg/kg/hr to prevent propofol infusion syndrome (PRIS). Monitor triglycerides q12-24h.[45]
• Dexmedetomidine: Minimal circuit sequestration (below 10%), no respiratory depression, suitable for ECMO weaning trials. Dose 0.2-1.4 mcg/kg/hr, monitor for bradycardia/hypotension.[46]

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Propofol

Sequestration and PRIS Risk:

Propofol sequestration 30-40% in first 24 hours, but lipid emulsion formulation reduces circuit adsorption compared to fentanyl/midazolam.[47] Propofol infusion syndrome (PRIS) risk is HIGHER on ECMO due to: (1) prolonged high-dose infusions, (2) critical illness metabolic stress, (3) mitochondrial dysfunction. PRIS manifests as metabolic acidosis, rhabdomyolysis, cardiac failure, renal failure, hypertriglyceridemia (>5 mmol/L).[45,48]

Dosing and Monitoring:

• Induction: 1-2.5 mg/kg IV (higher end for ECMO Vd expansion)
• Maintenance: 1-4 mg/kg/hr (max 4 mg/kg/hr, do NOT exceed on ECMO)
• Monitor triglycerides q12-24h: Hold propofol if TG >5 mmol/L, switch to alternative sedative
• Monitor for PRIS: Daily lactate, creatine kinase (CK), troponin, ECG (Brugada-like ST changes, arrhythmias)
• Duration limit: Avoid >48 hours at max doses, consider sedative rotation

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Antimicrobials

Vancomycin

PK/PD on ECMO:

Vancomycin (hydrophilic, Vd 0.4-0.7 L/kg baseline, 80-90% renal clearance) undergoes significant PK changes on ECMO:[49,50]

1. Vd expansion: 70-100% increase (Vd 0.8-1.2 L/kg) due to circuit volume, capillary leak, fluid resuscitation
2. Clearance variability: ARC in 30% VV-ECMO patients (CrCl >130 mL/min) increases clearance; AKI in 50% VA-ECMO reduces clearance
3. Minimal circuit sequestration: Hydrophilic vancomycin shows below 10% circuit loss (negligible compared to Vd changes)

Target AUC₀₋₂₄ 400-600 mg·h/L (IDSA/ASHP 2020 guidelines) to optimize efficacy and reduce nephrotoxicity.[51]

Dosing Strategy:

• Loading dose: 25-35 mg/kg actual body weight (vs 15-20 mg/kg standard) to achieve rapid therapeutic levels, max 3,000 mg
• Maintenance: 15-20 mg/kg q8-12h, adjust based on TDM
• ARC (CrCl >130 mL/min): Dose q8h or continuous infusion (1,500-2,000 mg loading, then 30-60 mg/kg/day continuous)
• AKI (CrCl below 50 mL/min): Dose q24-48h based on levels, target trough 15-20 mg/L
• CRRT concurrent: Dose 15-20 mg/kg q12-24h depending on CRRT intensity (20-25 mL/kg/h effluent dose)

Bayesian AUC Monitoring:

Preferred over trough-only monitoring. Obtain levels at 1-2h and 6-8h post-dose, use Bayesian software (DoseMeRx, InsightRx) to calculate AUC and optimize dosing.[52,53]

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Beta-Lactams (Meropenem, Piperacillin-Tazobactam)

PK/PD on ECMO:

Beta-lactams are time-dependent antibiotics requiring %fT>MIC (percentage time free drug concentration exceeds minimum inhibitory concentration) for efficacy. Target 100% fT>MIC for critically ill (4-5× MIC for Pseudomonas aeruginosa).[54,55]

ECMO PK Changes:[56,57,58]

1. Vd expansion: 50-100% increase (meropenem Vd 0.3-0.6 L/kg vs 0.15-0.3 L/kg baseline)
2. ARC: 40-50% of VV-ECMO patients have CrCl >130 mL/min, causing subtherapeutic levels with standard dosing
3. Minimal sequestration: Hydrophilic beta-lactams show below 10% circuit loss

Subtherapeutic Concentrations:

Studies show 50-70% of ECMO patients on standard beta-lactam dosing fail to achieve target concentrations (particularly first 48 hours before dose optimization).[59,60]

Dosing Strategy:

TDM for Beta-Lactams:

Increasing availability in Australia/NZ. Target trough concentrations: meropenem >8 mg/L (4× MIC for Pseudomonas), piperacillin >64 mg/L (16× MIC).[61]

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Aminoglycosides (Gentamicin, Tobramycin, Amikacin)

PK/PD on ECMO:

Aminoglycosides are concentration-dependent antibiotics requiring Cmax/MIC >8-10 for efficacy and AUC₀₋₂₄/MIC below 100-120 to reduce nephrotoxicity/ototoxicity.[62,63]

ECMO PK Changes:[64,65]

1. Vd expansion: 40-100% increase (gentamicin Vd 0.35-0.50 L/kg vs 0.25 L/kg baseline)
2. ARC: Increases clearance, shortens half-life, causes subtherapeutic troughs
3. Minimal sequestration: Hydrophilic, cationic structure - heparin-coated circuits may sequester 10-20% (non-coated below 5%)

Dosing Strategy (Once-Daily Extended-Interval):

• Gentamicin/Tobramycin: 7-10 mg/kg IV q24h (vs 5-7 mg/kg standard), target Cmax >20 mg/L, trough below 1 mg/L
• Amikacin: 25-30 mg/kg IV q24h (vs 15-20 mg/kg standard), target Cmax >60 mg/L, trough below 5 mg/L
• ARC (CrCl >130 mL/min): Dose q24h but expect higher clearance - monitor levels closely, may need q18h dosing or higher mg/kg
• AKI (CrCl below 50 mL/min): Extend interval to q36-48h based on levels

Hartford Nomogram Not Validated on ECMO - mandatory individualized TDM with Bayesian modeling.[66]

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Antifungals (Voriconazole, Isavuconazole, Liposomal Amphotericin B)

Voriconazole:

Highly lipophilic (Log P 1.76), hepatically metabolized via CYP2C19 (genetic polymorphism causes 4-fold variability). Circuit sequestration 20-40% reported in some studies, but highly variable.[67,68,69]

Dosing and TDM:

• Loading: 6 mg/kg IV q12h × 2 doses (day 1)
• Maintenance: 4 mg/kg IV q12h, adjust based on TDM
• Target trough: 1-5 mg/L (2-5 mg/L for invasive aspergillosis)
• TDM mandatory: High PK variability on ECMO, CYP2C19 polymorphism, risk hepatotoxicity/neurotoxicity if >5 mg/L

Isavuconazole:

Broad-spectrum triazole, fewer drug interactions than voriconazole, but significant ECMO sequestration reported (40-60% circuit loss in some case series).[70,71,72] Subtherapeutic concentrations common on ECMO.

Dosing and TDM:

• Loading: 200 mg IV q8h × 6 doses (48 hours)
• Maintenance: 200 mg IV daily
• Target trough: >1-2 mg/L
• Dosing adjustment: Consider 200 mg IV q12h if TDM shows subtherapeutic levels on ECMO

Liposomal Amphotericin B:

Large liposomal particles (>1,000 Da) show minimal circuit sequestration (below 10%).[21,73] Preferred antifungal for ECMO if invasive fungal infection confirmed and triazole resistance suspected.

Dosing: 3-5 mg/kg IV daily (standard dosing, no ECMO adjustment required). Monitor renal function (amphotericin nephrotoxicity compounded by ECMO-associated AKI).

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Anticoagulants

Unfractionated Heparin (UFH)

ECMO Anticoagulation Standard:

UFH is the standard anticoagulant for ECMO circuits to prevent thrombosis (oxygenator, cannula, tubing). Target activated partial thromboplastin time (aPTT) 60-80 seconds or anti-Xa 0.3-0.5 IU/mL (institution-dependent protocols).[74,75]

Heparin Resistance on ECMO:

20-30% of ECMO patients develop heparin resistance (inability to achieve target aPTT despite >35,000 units/day UFH) due to:[76,77]

1. Antithrombin (AT) deficiency: Consumption in circuit, baseline deficiency, dilution. AT below 50% causes heparin resistance (heparin requires AT cofactor for activity).
2. Elevated factor VIII and fibrinogen: Acute phase reactants in critical illness neutralize heparin.

Management:

• AT replacement: Fresh frozen plasma (FFP) 10-15 mL/kg or AT concentrate (50 IU/kg) to restore AT >80%, then UFH effective
• Alternative anticoagulation: Bivalirudin (direct thrombin inhibitor) 0.05-0.1 mg/kg/hr, target aPTT 60-80 seconds. More expensive, increased bleeding risk, limited Australian/NZ availability.[78]

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Low Molecular Weight Heparin (LMWH)

Not Recommended for Circuit Anticoagulation:

LMWH (enoxaparin) has longer half-life (4-6h vs 1-2h UFH), making it difficult to reverse rapidly in bleeding emergencies. Used for VTE prophylaxis in ECMO if therapeutic UFH contraindicated.[79]

VTE Prophylaxis on Therapeutic UFH:

Most ECMO patients on therapeutic UFH for circuit anticoagulation do NOT require additional VTE prophylaxis. If UFH interrupted or patient transitioned to anticoagulation-free ECMO strategy, add enoxaparin 40 mg SC daily.[80]

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Anticonvulsants

Phenytoin

Highly Protein-Bound (90%):

Hypoalbuminemia on ECMO increases free phenytoin fraction unpredictably. Total phenytoin level 40-80 μmol/L may be toxic if free level >8 μmol/L (albumin below 30 g/L common).[43,81]

Dosing and TDM:

• Loading: 20 mg/kg IV (standard, no ECMO adjustment)
• Maintenance: 5-7 mg/kg/day divided q8-12h
• Monitor free phenytoin levels: Target 4-8 μmol/L free (NOT total)
• Alternative: Levetiracetam (not protein-bound, no TDM required, renally cleared - adjust for AKI)

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Levetiracetam

Minimal ECMO PK Changes:

Levetiracetam is hydrophilic, below 10% protein-bound, minimal circuit sequestration, and renally cleared (66% unchanged). PK changes on ECMO limited to Vd expansion (circuit volume + fluid resuscitation) and ARC/AKI affecting clearance.[82,83]

Dosing:

• Loading: 30-60 mg/kg IV (up to 3,000 mg)
• Maintenance: 1,000-1,500 mg IV q12h
• ARC (CrCl >130): Dose 1,500-3,000 mg q12h
• AKI/CRRT: Reduce to 500-1,000 mg q12h, adjust based on seizure control

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VV-ECMO vs VA-ECMO Pharmacology Differences

Hemodynamic and Organ Perfusion Differences

VV-ECMO:

• Respiratory support only: Cardiac function preserved, normal cardiac output and organ perfusion maintained
• Hepatic clearance: Preserved (normal hepatic blood flow)
• Renal clearance: Often augmented (ARC in 40-50% due to hyperdynamic resuscitation, young age, normal CO)
• Vd expansion: Moderate (circuit volume + fluid resuscitation, but less than VA-ECMO)

VA-ECMO:

• Cardiorespiratory support: Cardiac function impaired, ECMO provides systemic circulation
• Hepatic clearance: Reduced (non-pulsatile flow, MAP 60-70 mmHg reduces hepatic perfusion by 30-50%)
• Renal clearance: Often reduced (AKI in 50-60% from cardiogenic shock, reduced renal perfusion)
• Vd expansion: Marked (circuit + massive fluid resuscitation for cardiogenic shock + severe capillary leak)

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Drug Dosing Adjustments: VV vs VA ECMO

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Therapeutic Drug Monitoring (TDM) on ECMO

Mandatory TDM Drugs

High-Priority TDM:

1. Vancomycin: AUC-guided dosing (Bayesian models preferred), target AUC₀₋₂₄ 400-600 mg·h/L
2. Aminoglycosides: Extended-interval dosing, target Cmax/MIC >8-10, trough below 1 mg/L (gentamicin/tobramycin)
3. Antifungals: Voriconazole (target trough 2-5 mg/L), isavuconazole (target >1-2 mg/L)
4. Beta-lactams (if available): Target trough >4-5× MIC
5. Anticonvulsants: Free phenytoin (target 4-8 μmol/L), valproate (free level if available)

Sampling Considerations:

• Pre-oxygenator sampling preferred: Post-oxygenator samples may have artificially low concentrations if drug sequestration occurring
• Steady-state timing: Wait 3-5 half-lives before TDM (vancomycin 48-72h, aminoglycosides 24h, beta-lactams 24h)
• Bayesian modeling: Use population PK models adjusted for ECMO, critical illness, renal function (DoseMeRx, InsightRx software available in major Australian ICUs)

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TDM Interpretation Challenges on ECMO

Dynamic PK Changes:

ECMO pharmacokinetics change over time (circuit saturation, fluid balance, organ recovery, ARC resolution). TDM at 24h may differ significantly from 72h or 7 days. Repeat TDM q48-72h until stable levels achieved, then weekly.

Protein Binding Variability:

For highly protein-bound drugs (voriconazole, phenytoin, valproate), total drug levels may be misleading if albumin below 30 g/L. Request free drug level monitoring where available.

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Australian/NZ ECMO Centers and Protocols

Major ECMO Centers

Australia:

1. The Alfred Hospital, Melbourne (Victorian ECMO service) - largest Australian ECMO center, 100+ runs/year
2. St Vincent's Hospital, Melbourne - adult ECMO
3. Royal Prince Alfred Hospital, Sydney - adult ECMO
4. Prince of Wales Hospital, Sydney - adult ECMO
5. Queensland Children's Hospital, Brisbane - pediatric ECMO
6. Royal Children's Hospital, Melbourne - pediatric ECMO

New Zealand:

1. Auckland City Hospital - national adult ECMO service
2. Christchurch Hospital - adult ECMO
3. Starship Children's Hospital, Auckland - pediatric ECMO

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ANZICS-CORE and ELSO Registry Data

Australian/NZ ECMO Outcomes:

ANZICS CORE database (2020-2021 data):[12]

• VV-ECMO survival: 60-65% hospital discharge (predominantly COVID-19 ARDS)
• VA-ECMO survival: 40-50% hospital discharge (cardiogenic shock, post-cardiotomy)
• Infection on ECMO: 30-40% develop VAP/bacteremia (highlights importance of therapeutic antimicrobial levels)
• AKI requiring CRRT: 35-45% (complicates drug dosing further)

ELSO Registry (Global):

Subtherapeutic antimicrobial concentrations associated with 2-3× higher infection-related mortality on ECMO. TDM implementation improves target attainment from 40-50% to 70-80%.[11,84]

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Clinical Approach to Drug Dosing on ECMO

Step 1: Assess Patient and ECMO Factors

Patient Factors:

• Weight (actual, IBW, adjusted body weight if obese)
• Renal function (CrCl calculation, ARC vs AKI)
• Hepatic function (bilirubin, synthetic function)
• Protein binding (albumin, inflammatory markers)
• Fluid balance (cumulative positive balance, Vd expansion)

ECMO Factors:

• ECMO mode (VV vs VA)
• Circuit type (heparin-coated vs non-coated, PMP vs older membranes)
• Time on ECMO (below 24h vs 24-72h vs >72h - circuit saturation kinetics)
• Concurrent CRRT (adds clearance)

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Step 2: Drug-Specific Assessment

Physicochemical Properties:

• Lipophilicity (Log P): High sequestration if Log P >3
• Protein binding: High sequestration if >80% bound
• Vd: Hydrophilic drugs require higher loading doses (Vd expansion)
• Clearance: Hepatic vs renal (adjust based on organ function)

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Step 3: Loading and Maintenance Dosing Strategy

General Principles:

1. Loading dose: Increase by 30-100% for hydrophilic drugs (Vd expansion), give slowly to avoid toxicity
2. Maintenance dose: Adjust based on clearance changes (ARC → increase, AKI → decrease, VA-ECMO hepatic → decrease)
3. TDM-guided adjustments: Mandatory for antimicrobials, sedatives, anticonvulsants
4. Extended/continuous infusions: Preferred for beta-lactams (maximize %fT>MIC)

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Step 4: Monitoring and Dose Optimization

Clinical Monitoring:

• Sedation scores: RASS/SAS q2-4h, adjust sedatives to target
• Infection markers: CRP, procalcitonin, clinical improvement (adequate antimicrobial dosing)
• Seizure control: EEG if available, clinical seizure activity (anticonvulsant efficacy)

Laboratory Monitoring:

• TDM: Vancomycin (48-72h post-initiation, then weekly), aminoglycosides (24h post-dose, then weekly), antifungals (72h-7d, then weekly), beta-lactams (if available)
• Renal function: Daily CrCl calculation (ARC vs AKI dynamic changes)
• Albumin: Twice weekly (protein binding assessment)
• Drug toxicity markers: CK/troponin (PRIS), LFTs (voriconazole hepatotoxicity)

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Summary: Key ECMO Pharmacology Principles

Critical Concepts for CICM Exam:

1. Circuit sequestration: Lipophilic (Log P >3) and highly protein-bound (>80%) drugs sequester 20-80% in ECMO circuit (fentanyl > midazolam > propofol > voriconazole). Saturation kinetics: highest loss first 24h, plateau 48-72h.

2. Vd expansion: Hydrophilic drugs (aminoglycosides, vancomycin, beta-lactams) require 30-100% higher loading doses due to circuit volume (1-1.5 L), capillary leak, fluid resuscitation. Vd increases more in VA-ECMO vs VV-ECMO.

3. Clearance variability: VV-ECMO often has ARC (40-50%) requiring higher doses; VA-ECMO often has AKI + reduced hepatic clearance requiring lower doses. Concurrent CRRT adds additional clearance.

4. VV vs VA differences: VV-ECMO preserves organ perfusion/clearance (higher drug doses needed); VA-ECMO reduces hepatic/renal clearance (lower doses, accumulation risk).

5. Mandatory TDM: Vancomycin (AUC 400-600), aminoglycosides (Cmax/MIC >8-10), antifungals (voriconazole trough 2-5 mg/L), beta-lactams (trough >4-5× MIC if available). TDM increases target attainment from 40-50% to 70-80%.

6. Protein binding changes: Hypoalbuminemia (below 30 g/L in 70% ECMO patients) increases free drug fraction for phenytoin, valproate, voriconazole → monitor free levels to prevent toxicity.

7. Australian context: ANZICS CORE data show 30-40% infection rate on ECMO; TDM-guided antimicrobial dosing critical for outcomes. Major centers: The Alfred, RPA, St Vincent's Melbourne, Auckland City Hospital.

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Special Populations and Considerations

Obesity on ECMO

Compounded PK Challenges:

Obesity (BMI ≥30 kg/m²) occurs in 30-40% of ECMO patients and compounds pharmacokinetic complexity:[56,85]

1. Lipophilic drug sequestration magnified: Greater adipose tissue reservoir PLUS circuit sequestration causes profoundly unpredictable PK for fentanyl, midazolam, propofol, amiodarone

2. Hydrophilic drug Vd expansion: Already expanded from ECMO circuit + capillary leak, FURTHER expanded by obesity (particularly aminoglycosides, vancomycin requiring 30-50% higher loading doses)

3. Augmented renal clearance (ARC): Paradoxically common in obese ECMO patients (younger age, hyperdynamic state) - beta-lactam and vancomycin underexposure risk

Weight-Based Dosing Strategy:

Janmahasatian Lean Body Weight (LBW) Formula:[86]

• Males: LBW = (9,270 × TBW) / (6,680 + [216 × BMI])
• Females: LBW = (9,270 × TBW) / (8,780 + [244 × BMI])

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Pregnancy on ECMO

Rare but Critical Scenario:

Pregnancy-related ECMO indications include severe H1N1 influenza ARDS (particularly 2nd-3rd trimester), peripartum cardiomyopathy, amniotic fluid embolism, massive pulmonary embolism.[87]

Pregnancy PK Changes + ECMO:

1. Increased cardiac output: 30-50% increase in pregnancy → augmented renal/hepatic clearance → beta-lactam, vancomycin doses 20-30% higher than non-pregnant ECMO patients

2. Increased Vd: Plasma volume expansion 40-50% in pregnancy + ECMO circuit + capillary leak → 100-200% Vd increase for hydrophilic drugs

3. Reduced albumin: Physiological dilutional hypoalbuminemia (30-35 g/L) + critical illness hypoalbuminemia (20-25 g/L) → free drug fraction increased for highly protein-bound drugs

4. Fetal considerations: Drug transfer across placenta (if pre-delivery ECMO), teratogenicity risk, fetal monitoring during ECMO

Drug Dosing in Pregnancy on ECMO:

• Beta-lactams: Increase doses by 30-50% (pregnancy augmented clearance + ECMO Vd expansion) - meropenem 2 g q6h or 6-8 g/day continuous infusion
• Vancomycin: 25-35 mg/kg loading (pregnancy Vd + ECMO), 20 mg/kg q8-12h maintenance, AUC-guided TDM mandatory
• Sedatives: Minimize fentanyl/midazolam if fetus viable (respiratory depression, withdrawal post-delivery), use remifentanil (rapid offset, esterase metabolism)
• Avoid teratogens: No warfarin (teratogenic), use UFH; avoid aminoglycosides if possible (ototoxicity/nephrotoxicity risk to fetus)

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Pediatric ECMO Pharmacology

Age-Dependent PK Differences:[44,88]

Neonatal and pediatric ECMO presents unique pharmacological challenges beyond adult ECMO:

Neonates (0-28 days):

1. Immature clearance: Reduced GFR (30-40% adult values), immature hepatic enzyme systems (CYP3A4, CYP2C19) → prolonged half-lives for vancomycin (6-9h vs 4-6h adults), beta-lactams, sedatives
2. Higher Vd/kg: Greater extracellular fluid volume (40% body weight vs 20% adults) + larger circuit volume relative to blood volume → higher mg/kg loading doses required
3. Circuit prime volume impact: Neonatal ECMO circuit 300-500 mL vs neonate blood volume 80 mL/kg (e.g., 3 kg neonate = 240 mL blood volume) → circuit doubles blood volume, massive Vd expansion

Dosing adjustments (neonates):

• Vancomycin: 15 mg/kg loading, 10-15 mg/kg q8-12h (longer intervals due to reduced clearance)
• Gentamicin: 4-5 mg/kg q24-48h (extended intervals, TDM mandatory)
• Meropenem: 20 mg/kg q8-12h (higher mg/kg than adults, but less frequent dosing)
• Fentanyl: 1-2 mcg/kg/hr (avoid high doses, immature metabolism → accumulation)

Children (1 month - 12 years):

1. Augmented clearance: Higher GFR/kg and hepatic enzyme activity/kg than adults → higher mg/kg doses and more frequent intervals required for beta-lactams, vancomycin
2. Circuit volume less impactful: Circuit 600-1,000 mL, child blood volume 70-80 mL/kg (e.g., 20 kg child = 1,400-1,600 mL) → circuit adds ~40-70% blood volume (less dramatic than neonates)

Dosing adjustments (children 1-12 years):

• Vancomycin: 15-20 mg/kg q6-8h (more frequent than adults due to higher clearance/kg)
• Gentamicin: 7-10 mg/kg q24h (similar to adults)
• Meropenem: 20-40 mg/kg q8h (higher mg/kg, max 2 g/dose)
• Fentanyl: 1-5 mcg/kg/hr (higher mg/kg tolerance, titrate to effect)

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Drug-Drug Interactions on ECMO

Cytochrome P450 Interactions:

Critical illness and ECMO circuits alter hepatic enzyme activity unpredictably. Key interactions:[89]

CYP3A4 Inhibitors (increase levels of CYP3A4 substrates):

• Azole antifungals (voriconazole, isavuconazole, fluconazole) inhibit CYP3A4 → increase fentanyl, midazolam, propofol levels
• Clinical implication: Reduce sedative doses by 30-50% when starting azoles, monitor for over-sedation, prolonged awakening
• Example: Patient on fentanyl 5 mcg/kg/hr + midazolam 0.2 mg/kg/hr, started on voriconazole → reduce fentanyl to 2-3 mcg/kg/hr, midazolam to 0.1 mg/kg/hr within 24-48h

CYP3A4 Inducers (decrease levels of CYP3A4 substrates):

• Phenytoin, rifampicin induce CYP3A4 → decrease voriconazole, isavuconazole, midazolam levels
• Clinical implication: Avoid voriconazole if patient on phenytoin (use liposomal amphotericin B instead), or increase voriconazole dose 50-100% with TDM
• Example: Voriconazole trough below 1 mg/L (subtherapeutic) in patient on phenytoin 300 mg/day → switch to alternative antifungal or increase voriconazole to 6 mg/kg q12h with close TDM

CYP2C19 Polymorphism and Voriconazole:

Voriconazole is metabolized via CYP2C19, which has genetic polymorphism causing 4-10× variability in levels:[68,90]

• Poor metabolizers (2-5% Caucasians, below 1% Asians): Very high voriconazole levels, toxicity risk (hepatotoxicity, visual disturbances, hallucinations)
• Rapid metabolizers (15-30% population): Subtherapeutic voriconazole levels despite standard dosing
• Clinical implication: MANDATORY voriconazole TDM on ECMO (target trough 2-5 mg/L), expect wide variability, dose adjust q48-72h based on levels

QT-Prolonging Drug Combinations:

Multiple ICU drugs prolong QT interval, compounded by electrolyte disturbances on ECMO (hypocalcemia, hypomagnesemia, hypokalemia):[91]

High-risk combinations:

• Amiodarone (QTc +40-60 ms) + azole antifungals (QTc +10-30 ms) + ondansetron (QTc +10-20 ms)
• Moxifloxacin (QTc +20-40 ms) + methadone (QTc +20-60 ms)

Management:

• Monitor ECG daily, QTc calculation (Bazett or Fridericia formula)
• If QTc >500 ms: Cease non-essential QT-prolonging drugs, optimize electrolytes (K+ >4.5 mmol/L, Mg2+ >1.0 mmol/L, iCa2+ >1.15 mmol/L)
• Consider alternative agents: levofloxacin instead of moxifloxacin, fentanyl/hydromorphone instead of methadone

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Circuit Changes and Drug Bolus Dosing

ECMO Circuit Replacement:

Circuits are changed every 7-14 days (or earlier if dysfunction, clotting). Circuit change requires re-priming with 1-1.5 L blood/crystalloid, creating acute PK perturbations:[92]

Circuit Change PK Impact:

1. Acute Vd expansion: Additional 1-1.5 L prime volume added → transient drop in plasma drug concentrations for all drugs
2. New circuit sequestration: Fresh circuit surfaces provide new binding sites → lipophilic drugs (fentanyl, midazolam, propofol) re-sequester 30-60% for first 6-24h (similar to ECMO initiation)
3. Loss of saturated circuit drug reservoir: Old circuit contained saturated drug in tubing/membranes (particularly amiodarone, lipophilic sedatives), discarding removes this reservoir

Clinical Management:

Before circuit change:

• Continue all infusions (sedatives, antimicrobials, vasopressors) at current rates
• Have bolus doses ready for post-change administration

Immediately after circuit change:

• Give loading doses for key drugs (similar to ECMO initiation):
  • Vancomycin 15-20 mg/kg IV if due for dose within 4h
  • "Sedative boluses if patient unsettled: fentanyl 1-2 mcg/kg, midazolam 0.02-0.05 mg/kg"
  • Antimicrobial loading if circuit changed mid-dosing interval (e.g., meropenem 1-2 g if >4h since last dose)

First 24-48h post-change:

• Expect higher sedative requirements (new circuit sequestration)
• Increase sedative infusions by 30-50% for 24-48h, then reduce as circuit saturates
• Monitor for under-sedation, patient-circuit dyssynchrony

TDM adjustments:

• If TDM sample due within 24h of circuit change, DELAY sampling to 48h post-change (PK too unstable immediately post-change)
• If vancomycin/aminoglycoside dose due during circuit change, dose AFTER change is complete (avoid losing dose in old circuit)

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ECMO Weaning and Drug Dosing

Pharmacological Considerations During ECMO Weaning:[93]

ECMO weaning trials involve reducing ECMO flow (from 4-5 L/min to 1-2 L/min) or sweep gas FiO2/flow, testing patient's intrinsic cardiopulmonary function. This creates dynamic PK changes:

Flow Reduction Impact:

1. Reduced circuit volume recirculation: Lower flow → less drug exposure to circuit surfaces per unit time → slightly increased bioavailable drug (5-10% increase in plasma concentrations for lipophilic drugs)
2. Improved native organ perfusion: Cardiac output recovering → improved hepatic/renal blood flow → increased clearance for hepatically metabolized drugs (fentanyl, midazolam, propofol)

Sedation Strategy During Weaning:

• Lighten sedation progressively: Reduce sedatives by 20-30% every 12-24h during wean trials
• Target RASS -1 to 0 (drowsy, arousable) rather than deep sedation (RASS -3 to -4)
• Switch to shorter-acting agents: Remifentanil instead of fentanyl (offset 10-15 min vs 2-4h), propofol instead of midazolam (offset 5-10 min vs 1-2h)
• Dexmedetomidine advantage: No respiratory depression, allows spontaneous breathing trials on ECMO

Antimicrobial Dosing During Weaning:

• Continue therapeutic doses until ECMO fully decannulated (infection risk remains high during wean, immune function still compromised)
• Monitor for ARC resolution or AKI development: Cardiac output/renal perfusion changes during wean may alter clearance → repeat CrCl calculation, TDM
• Post-decannulation: Expect PK normalization over 3-7 days (fluid balance resolves, capillary leak repairs, organ function improves) → standard ICU dosing applies

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Anticoagulation-Free ECMO Strategies

Emerging Concept:

Some centers trialing anticoagulation-free or reduced-anticoagulation ECMO in patients with contraindications (active bleeding, intracranial hemorrhage, post-neurosurgery).[94,95]

Heparin-Coated Circuits:

Modern heparin-bonded circuits (Bioline, Carmeda) reduce thrombogenicity, allow short-duration ECMO (24-72h) without systemic anticoagulation in select cases.

Drug Dosing Implications:

• Reduced drug sequestration: Heparin-coated circuits sequester 20-40% less drug than non-coated circuits (particularly cationic drugs: gentamicin, vancomycin, amikacin)
• Shorter circuit lifespan: Anticoagulation-free circuits clot faster (24-48h vs 7-14 days with UFH) → more frequent circuit changes → repeated Vd expansion and drug sequestration cycles
• TDM more critical: Frequent circuit changes create unstable PK → measure vancomycin/aminoglycoside/antifungal levels q48-72h rather than weekly

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COVID-19 ECMO Pharmacology

Unique Considerations (2020-2021 Pandemic Experience):[96,97]

COVID-19 ARDS patients represented 60-80% of VV-ECMO cases during pandemic peaks. Key pharmacological observations:

Hyper-inflammation and PK Changes:

1. Severe capillary leak: COVID-19 "cytokine storm" (IL-6, IL-8 elevation) → profound capillary leak → Vd expansion 100-200% for beta-lactams, vancomycin (more than typical ARDS)
2. Hypoalbuminemia: 70-80% COVID-19 ECMO patients had albumin below 25 g/L → increased free drug fraction for voriconazole, phenytoin
3. Thromboinflammation: COVID-19 hypercoagulability → higher UFH requirements (median 35,000-40,000 units/day vs 25,000-30,000 in non-COVID ECMO), heparin resistance common

Antimicrobial Dosing:

• Secondary bacterial infections in 30-50% (VAP, bacteremia) → therapeutic antimicrobial levels critical
• Beta-lactam underdosing common: Studies showed 60-70% COVID-19 ECMO patients failed to achieve target beta-lactam concentrations with standard dosing (ARC + severe Vd expansion)
• Recommendation: Continuous infusion beta-lactams preferred (meropenem 3-6 g/24h, pip/tazo 16-18 g/24h), TDM if available

Dexmedetomidine Preference:

• COVID-19 ECMO sedation often prolonged (median ECMO duration 14-21 days vs 7-10 days non-COVID)
• Dexmedetomidine reduced opioid/benzodiazepine accumulation, facilitated awake ECMO (patient self-proning), reduced ICU delirium

Antiviral/Anti-inflammatory Agents:

• Remdesivir: Minimal PK data on ECMO, no dose adjustment recommended (hepatic metabolism, not sequestered), administered IV 200 mg loading, then 100 mg daily
• Tocilizumab (IL-6 inhibitor): Large molecule (148 kDa), minimal circuit sequestration, standard dosing 8 mg/kg IV (max 800 mg)
• Dexamethasone: 6 mg IV/PO daily (standard dosing, minimal PK impact from ECMO)

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Regional Citrate Anticoagulation (RCA) for ECMO

Australian/NZ Adoption Increasing:[98]

Regional citrate anticoagulation (RCA) used in 10-20% of Australian ECMO centers (alternative to UFH), particularly in bleeding-risk patients (intracranial hemorrhage, post-surgery, trauma).

Citrate Mechanism:

• Citrate infused pre-oxygenator (4% trisodium citrate 150-200 mL/h), chelates ionized calcium (iCa2+) in circuit → local anticoagulation
• Calcium infused post-oxygenator (10% calcium chloride or gluconate) to restore systemic iCa2+ >1.0 mmol/L

Drug Dosing Considerations with RCA:

1. Calcium-dependent drugs: Citrate-induced hypocalcemia may reduce efficacy of calcium-channel blockers (diltiazem, verapamil), certain antibiotics (daptomycin requires iCa2+ >1.25 mmol/L for activity)

   • Management: Maintain iCa2+ >1.15 mmol/L (higher than standard RCA target 1.0-1.1 mmol/L if patient on daptomycin)

2. Citrate toxicity: Citrate accumulation (citrate:iCa2+ ratio >2.5) causes metabolic alkalosis, hypocalcemia, reduced drug protein binding

   • Risk factors: Liver dysfunction (reduced citrate metabolism), high citrate infusion rates (>200 mL/h), concurrent CRRT
   • Monitoring: Measure total calcium, ionized calcium q6-12h, calculate ratio. If ratio >2.5 → reduce citrate rate, increase calcium infusion

3. Metabolic alkalosis: Citrate metabolism generates bicarbonate → metabolic alkalosis (pH >7.50) in 20-30% of RCA-ECMO patients

   • Drug impact: Alkalosis increases protein binding of weak acids (phenytoin, valproate) → reduce free drug fraction → may need higher doses or free level monitoring

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SAQ Practice Questions

SAQ 1: ECMO Pharmacokinetics and Vancomycin Dosing (15 marks)

Question:

A 32-year-old woman (70 kg) with severe influenza A pneumonitis is on VV-ECMO (day 2). She develops Staphylococcus aureus ventilator-associated pneumonia (methicillin-resistant). Baseline creatinine 60 μmol/L, current creatinine 50 μmol/L (estimated CrCl 150 mL/min). Albumin 22 g/L.

a) Explain THREE mechanisms by which ECMO alters vancomycin pharmacokinetics in this patient. (6 marks)

b) Outline your vancomycin loading and maintenance dosing strategy for this patient, including your target AUC₀₋₂₄ and monitoring plan. (6 marks)

c) 48 hours post-loading, vancomycin trough is 8 mg/L (pre-dose at steady state). Using a two-level approach, what additional level would you obtain and how would you interpret the results? (3 marks)

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Model Answer:

a) THREE mechanisms of ECMO-altered vancomycin PK (6 marks - 2 marks each):

1. Increased volume of distribution (Vd):

   • ECMO circuit adds 1-1.5 L prime volume (15-20% blood volume expansion)
   • Capillary leak syndrome from sepsis/ARDS increases extracellular fluid volume by 50-200%
   • Vancomycin is hydrophilic (normally Vd 0.4-0.7 L/kg), distributed in extracellular fluid
   • On ECMO, Vd expands to 0.8-1.2 L/kg, requiring higher loading doses to achieve therapeutic concentrations
   • Clinical implication: Standard 15-20 mg/kg load → subtherapeutic levels; need 25-35 mg/kg load

2. Augmented renal clearance (ARC):

   • VV-ECMO preserves cardiac output (unlike VA-ECMO), often leads to hyperdynamic circulation
   • Patient is young (32 years), low baseline creatinine (60 μmol/L), current CrCl 150 mL/min (ARC defined as >130 mL/min/1.73m²)
   • ARC increases vancomycin clearance by 50-100% (normally 80-90% renally excreted unchanged)
   • Clinical implication: More frequent dosing required (q8-12h vs standard q12-24h), or continuous infusion

3. Minimal circuit sequestration (but important to recognize):

   • Vancomycin is hydrophilic (not lipophilic), minimal protein binding (below 10%), low molecular weight (1,449 Da)
   • Circuit sequestration is below 10% (unlike fentanyl 60-80%, midazolam 30-50%)
   • Clinical implication: Vd and clearance changes dominate PK alterations (not sequestration)

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b) Vancomycin dosing strategy (6 marks - 2 marks loading, 2 marks maintenance, 2 marks monitoring):

Loading dose:

• 25-35 mg/kg actual body weight = 1,750-2,450 mg IV (use 2,000 mg, max 3,000 mg)
• Rationale: Vd expansion 0.8-1.2 L/kg on ECMO (vs 0.4-0.7 L/kg baseline), higher load needed for therapeutic concentrations
• Infuse over 90-120 minutes (prevent red man syndrome)

Maintenance dose:

• 15-20 mg/kg q8-12h initially (1,000-1,400 mg q8-12h for 70 kg patient)
• Given ARC (CrCl 150 mL/min), start with 1,400 mg IV q8h
• Alternative: continuous infusion after loading (1,750 mg load, then 2,500-3,000 mg/24h continuous) - improves AUC target attainment
• Target: AUC₀₋₂₄ 400-600 mg·h/L (IDSA/ASHP 2020 guidelines for serious MRSA infection)

Monitoring plan:

• Obtain two vancomycin levels at steady state (48-72h after initiation): 1-2h post-infusion (peak), and 6-8h post-infusion (mid-level)
• Use Bayesian pharmacokinetic modeling (DoseMeRx, InsightRx software) to calculate AUC₀₋₂₄
• Adjust dosing to achieve AUC 400-600 mg·h/L
• Repeat TDM q48-72h initially (PK changes dynamic on ECMO), then weekly once stable
• Monitor renal function daily (ARC may resolve, AKI may develop → clearance changes)

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c) Two-level approach at 48 hours (3 marks - 1 mark timing, 2 marks interpretation):

Additional level timing:

• Trough 8 mg/L obtained (pre-dose, correct timing)
• Obtain peak level 1-2 hours post-infusion (vancomycin infused over 60-90 min, draw 30-60 min after infusion completes)

Interpretation:

Using Bayesian AUC calculation with two levels (trough + peak):

• Example scenario: Trough 8 mg/L, Peak 30 mg/L → AUC₀₋₂₄ calculated ~320 mg·h/L (subtherapeutic, target 400-600)

  • "Action: Increase dose to 1,400 mg q8h or initiate continuous infusion (3,000 mg/24h), recheck AUC 48h later"

• Example scenario: Trough 8 mg/L, Peak 50 mg/L → AUC₀₋₂₄ calculated ~500 mg·h/L (therapeutic, target achieved)

  • "Action: Continue current dosing, repeat TDM in 5-7 days"

• Trough-only monitoring limitation: Trough 8 mg/L could represent AUC 200-600 mg·h/L depending on dosing interval and clearance (inadequate for AUC-guided dosing)

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SAQ 2: Beta-Lactam Dosing and ARC on ECMO (15 marks)

Question:

A 28-year-old male (85 kg) with severe COVID-19 ARDS on VV-ECMO (day 5) develops Pseudomonas aeruginosa VAP (meropenem MIC 2 mg/L susceptible). CrCl 170 mL/min (ARC), albumin 28 g/L. He is receiving standard meropenem 1 g IV q8h (infused over 30 minutes).

a) Explain the pharmacokinetic/pharmacodynamic (PK/PD) target for beta-lactam antibiotics in critical illness and why standard dosing is likely inadequate for this patient. (5 marks)

b) Propose an optimized meropenem dosing regimen (including loading dose, maintenance, and infusion strategy) with rationale for each component. (6 marks)

c) Describe how you would utilize therapeutic drug monitoring (TDM) for meropenem if available, including target concentrations and timing. (4 marks)

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Model Answer:

a) Beta-lactam PK/PD target and inadequacy of standard dosing (5 marks - 3 marks PK/PD, 2 marks inadequacy):

Beta-lactam PK/PD principles:

• Beta-lactams are time-dependent antibiotics (efficacy depends on duration above MIC, not peak concentration)
• PK/PD target: %fT>MIC (percentage of dosing interval that free drug concentration exceeds MIC)
• Non-critically ill: Target 40-50% fT>MIC for bacteriostatic effect, 60-70% for bactericidal effect
• Critically ill / ECMO: Target 100% fT>MIC (continuous suppression) OR 100% fT>4-5×MIC for difficult pathogens (Pseudomonas, ESBL)
  • "Rationale: Augmented renal clearance, increased Vd, immune dysfunction, severe sepsis require maximal antimicrobial exposure"

Why standard dosing inadequate:

1. Augmented renal clearance (ARC): CrCl 170 mL/min (normal ~120 mL/min) increases meropenem clearance by 40-50%. Meropenem 75% renally excreted unchanged, half-life shortened from 1-2h to 0.5-1h. Standard q8h dosing → subtherapeutic troughs.

2. Increased Vd on ECMO: ECMO circuit (1-1.5 L) + capillary leak + fluid resuscitation → Vd expansion 50-100% for hydrophilic meropenem (0.3-0.6 L/kg vs 0.15-0.3 L/kg baseline). Lower peak concentrations after standard dose.

3. Pseudomonas MIC 2 mg/L: Higher MIC than typical susceptible organisms (MIC ≤0.5 mg/L). Target 100% fT>8-10 mg/L (4-5× MIC 2 mg/L) → requires sustained high concentrations not achieved with standard dosing.

4. Evidence of inadequacy: Studies show 50-70% of critically ill ECMO patients fail to achieve target beta-lactam concentrations with standard dosing, particularly first 48-72h.

---

b) Optimized meropenem dosing (6 marks - 2 marks loading, 2 marks maintenance, 2 marks infusion strategy):

Loading dose:

• 2 g IV meropenem loading dose (vs standard 1 g)
• Infuse over 30 minutes to rapidly achieve therapeutic concentrations
• Rationale: Overcome increased Vd (0.3-0.6 L/kg on ECMO, 85 kg patient = Vd 25-50 L). Loading dose 2 g achieves Cmax 40-50 mg/L (vs 20-25 mg/L with 1 g), ensuring 100% fT>4×MIC from start

Maintenance dose:

• 2 g IV meropenem q8h (vs standard 1 g q8h)
• Alternative (preferred): 6 g/24h continuous infusion (2 g loading, then 4 g over 24h, or 6 g total over 24h if no separate loading dose)
• Rationale: ARC (CrCl 170 mL/min) increases clearance 40-50%, requiring higher total daily dose (6-8 g/day vs 3-4 g/day standard)

Infusion strategy:

• Continuous infusion preferred (after loading dose)
  • Achieves steady-state concentration 8-16 mg/L (4-8× MIC), maintaining 100% fT>4×MIC continuously
  • Superior target attainment compared to intermittent dosing in ARC (90% vs 50-60%)
• Alternative: Extended infusion (3-4 hour infusion) if continuous infusion not feasible
  • Meropenem 2 g infused over 3 hours q8h → 3h/8h = 37.5% of interval at peak concentrations, improves fT>MIC to ~80-90% (vs ~50% with 30-min infusion)
• Continuous infusion requires dedicated IV line, light protection (meropenem degrades), pharmacy preparation in 0.9% saline (stable 8-12h room temperature)

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c) Meropenem TDM (4 marks - 2 marks targets, 2 marks timing/interpretation):

Target concentrations:

• Continuous infusion: Target steady-state concentration (Css) 8-16 mg/L (4-8× MIC 2 mg/L)
  • Lower limit 8 mg/L ensures 4×MIC (minimum for Pseudomonas)
  • Upper limit 16 mg/L avoids neurotoxicity risk (seizures if >60-80 mg/L, but conservative upper target given AKI risk on ECMO)
• Intermittent dosing: Target trough >8 mg/L (ensures 100% fT>4×MIC)

Timing and interpretation:

• Continuous infusion: Obtain level at steady state (12-24h after initiation). Single random level represents Css.

  • If Css below 8 mg/L → increase infusion rate by 30-50% (e.g., 4 g/24h to 6 g/24h), recheck in 12-24h
  • If Css >16 mg/L → reduce infusion rate by 20-30%, recheck in 12-24h
  • If Css 8-16 mg/L → therapeutic, repeat TDM in 3-5 days (ARC may resolve, renal function may change)

• Intermittent dosing: Obtain trough level (pre-dose) at steady state (24-48h after initiation)

  • If trough below 8 mg/L → inadequate dosing, switch to continuous infusion or increase dose to 2 g q6h extended infusion
  • If trough >16 mg/L → excessive dosing, reduce to 1 g q8h or continuous infusion 3-4 g/24h

• Repeat TDM: q48-72h initially (dynamic PK changes on ECMO), then weekly once stable

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Viva Practice Scenarios

Viva Scenario 1: ECMO Circuit Sequestration and Sedation Management (20 marks)

Examiner:

"You are the ICU consultant managing a 45-year-old man on VV-ECMO day 1 for severe H1N1 pneumonitis. He is receiving fentanyl 5 mcg/kg/hr and midazolam 0.2 mg/kg/hr for sedation, but remains agitated with RASS +2 despite these high doses. The nursing staff are concerned about the patient fighting the ventilator and ECMO circuit."

"Can you explain the pharmacokinetic principles that might explain why this patient requires such high sedative doses, and what you would do differently?"

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Candidate:

"This scenario highlights ECMO-induced pharmacokinetic changes causing sedation failure despite high doses. I'll address three key mechanisms and propose management strategies."

Three mechanisms causing high sedative requirements:

1. Circuit sequestration of lipophilic drugs:

"Fentanyl and midazolam are both highly lipophilic - fentanyl has a Log P of 4.05 and midazolam 3.89 - making them prone to adsorption to the polyvinyl chloride ECMO tubing and polymethylpentene oxygenator membranes. Studies show 60-80% fentanyl sequestration and 30-50% midazolam sequestration in the first 24 hours, with saturation kinetics occurring over 48-72 hours as circuit binding sites fill."

"Additionally, both drugs are highly protein-bound - fentanyl 84%, midazolam 95% - which compounds circuit adsorption. The patient is on day 1 of ECMO, so we're in the peak sequestration phase before circuit saturation occurs."

2. Increased volume of distribution:

"The ECMO circuit adds 1-1.5 L prime volume, plus this patient likely has significant capillary leak from H1N1 pneumonitis and ARDS, expanding Vd by 30-70% for lipophilic sedatives. This means higher doses are required to achieve therapeutic plasma concentrations, particularly in the loading phase."

3. Critical illness pharmacokinetic changes:

"Beyond ECMO-specific factors, this patient has severe viral pneumonitis with systemic inflammation, likely causing hypoalbuminemia (reducing protein binding), and potentially augmented hepatic clearance if cardiac output is preserved on VV-ECMO. These compound the ECMO-specific PK alterations."

---

Examiner:

"Good overview. What are your immediate management options for this patient's inadequate sedation?"

---

Candidate:

"I would implement a multi-pronged approach:"

Immediate (0-6 hours):

1. Increase current sedatives strategically:

• "Increase fentanyl to 8-10 mcg/kg/hr (expect 50-100% higher dose requirement in first 24h due to circuit sequestration)"
• "Increase midazolam to 0.3 mg/kg/hr"
• "Anticipate dose reduction at 48-72 hours once circuit saturates and plasma levels stabilize"

2. Add alternative sedative with minimal sequestration:

• "Add dexmedetomidine 0.2-1.4 mcg/kg/hr (alpha-2 agonist, minimal circuit sequestration below 10%, no respiratory depression)"
• "Dexmedetomidine provides sedation without further respiratory depression, facilitating ECMO weaning trials"
• "Monitor for bradycardia and hypotension (common side effects, but usually tolerable on ECMO)"

3. Consider non-pharmacologic strategies:

• "Optimize patient comfort: analgesia adequacy, patient-ventilator synchrony (adjust ventilator settings), family presence, minimize unnecessary stimulation"
• "Treat underlying causes of agitation: hypoxemia (check ECMO gas exchange), hypercapnia, pain, delirium, withdrawal"

Short-term (24-72 hours):

4. Switch to alternative agents with better ECMO PK profiles:

• "Consider remifentanil instead of fentanyl (esterase metabolism independent of organ function, minimal accumulation, but requires continuous infusion, expensive)"
• "Consider propofol instead of midazolam (30-40% sequestration vs 50% for midazolam, more predictable PK), but limit to below 4 mg/kg/hr to prevent propofol infusion syndrome (PRIS)"
• "Monitor triglycerides q12-24h if using propofol (target below 5 mmol/L)"

Monitoring plan:

• "RASS score q2h, target RASS -2 to -3 (light to moderate sedation)"
• "Daily sedation holds/lightening trials once patient stabilized on ECMO"
• "Anticipate dose reduction after 48-72 hours as circuit saturates"

---

Examiner:

"The patient is now adequately sedated on fentanyl 8 mcg/kg/hr, midazolam 0.25 mg/kg/hr, and dexmedetomidine 1 mcg/kg/hr. On day 3, the nursing staff report the patient is becoming difficult to wake during daily sedation holds, and has developed bradycardia (HR 45-50 bpm) and hypotension (MAP 60-65 mmHg on 5 mcg/min norepinephrine)."

"What is happening, and how would you manage this?"

---

Candidate:

"This represents sedative accumulation and dexmedetomidine side effects, predictable complications after 48-72 hours of ECMO sedation. I'll address each issue:"

Sedative accumulation (fentanyl/midazolam):

"After 48-72 hours, the ECMO circuit has saturated with fentanyl and midazolam - the binding sites are filled - so plasma concentrations are now rising despite unchanged infusion rates. Additionally, both drugs have active or parent metabolites that accumulate (fentanyl itself accumulates in fat, midazolam has active metabolite alpha-hydroxymidazolam)."

Management:

• "Reduce fentanyl to 3-5 mcg/kg/hr (50% dose reduction from day 1-2 requirements)"
• "Reduce midazolam to 0.1-0.15 mg/kg/hr (40% dose reduction)"
• "Re-assess RASS q2h, target RASS -2 to 0 (lighter sedation as patient stabilizes, facilitate weaning)"

Dexmedetomidine side effects:

"Dexmedetomidine 1 mcg/kg/hr is at the upper dosing limit, and alpha-2 agonism causes predictable bradycardia and hypotension (reduced sympathetic outflow, enhanced vagal tone). The patient's HR 45-50 bpm and MAP 60-65 mmHg are likely dexmedetomidine-mediated."

Management:

• "Reduce dexmedetomidine to 0.4-0.7 mcg/kg/hr"
• "If bradycardia persists (HR below 45 bpm) or hemodynamically significant, consider ceasing dexmedetomidine temporarily and using fentanyl/midazolam alone (now that circuit saturated, PK more predictable)"
• "If hypotension persists despite dexmedetomidine reduction, consider low-dose vasopressin 0.03 units/min (reduces norepinephrine requirement, no chronotropic effect unlike norepinephrine)"

Alternative approach if sedation still difficult:

• "Consider daily awakening protocol rather than continuous deep sedation (improves outcomes, reduces ICU delirium, facilitates ECMO liberation)"
• "Switch to analgesia-first sedation strategy: adequate fentanyl/remifentanil, minimal midazolam/propofol"

---

Examiner:

"Acceptable. One final question: on day 7 of ECMO, the patient is being weaned and the ECMO circuit is removed. What pharmacokinetic phenomenon might occur with lipophilic drugs saturated in the circuit, and what clinical implication does this have?"

---

Candidate:

"Excellent question highlighting a less commonly discussed ECMO pharmacology principle: drug release from circuit upon decannulation."

Drug release phenomenon:

"After 7 days, the ECMO circuit membranes and tubing have accumulated significant amounts of lipophilic drugs - particularly fentanyl, midazolam, propofol, and amiodarone. When the circuit is removed and discarded, this reservoir of drug is eliminated from the patient-circuit system."

"However, the patient's tissues (adipose, muscle) have ALSO accumulated lipophilic drugs over 7 days of high-dose infusions. Upon cessation of sedative infusions post-decannulation, there is potential for:"

1. Rapid sedation offset: If relying on circuit as a reservoir, sedation may lighten faster than expected once circuit removed (beneficial for awakening)

2. Tissue redistribution and rebound sedation: Conversely, lipophilic drugs stored in adipose tissue may slowly redistribute back to plasma over hours to days, causing delayed or prolonged sedation after infusions stopped (more common with fentanyl, midazolam)

Clinical implications:

• "Anticipate delayed awakening: Fentanyl context-sensitive half-time after 7 days infusion can be 16-24 hours (vs 6 hours after single dose). Midazolam similar. Patient may not wake for 24-48 hours after ceasing sedation."

• "Avoid additional sedative boluses unless clearly indicated (risk over-sedation and prolonged mechanical ventilation)"

• "Monitor for withdrawal: Abrupt cessation of high-dose fentanyl/midazolam after 7 days can precipitate opioid/benzodiazepine withdrawal (agitation, tachycardia, hypertension, seizures). Consider weaning protocol: reduce doses by 10-20% per day over 3-5 days rather than abrupt cessation."

• "Circuit disposal: The discarded ECMO circuit contains milligrams to grams of drug (particularly amiodarone with very long half-life 40-60 days, high lipophilicity). In some institutions, the circuit is weighed before/after to estimate drug sequestration, though this is more research than clinical practice."

Post-ECMO pharmacology normalization:

"After ECMO decannulation, expect PK parameters to normalize over 3-7 days as fluid balance resolves, capillary leak repairs, and organ function improves. Renal clearance may remain augmented (ARC) for weeks in young, hyperdynamic patients recovering from critical illness."

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Examiner:

"Excellent. That covers sedation pharmacology on ECMO comprehensively."

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Viva Scenario 2: Antimicrobial Dosing and TDM on VA-ECMO with AKI (20 marks)

Examiner:

"A 62-year-old woman (75 kg) is on VA-ECMO day 4 for refractory cardiogenic shock post-myocardial infarction. She develops Enterobacter cloacae bacteremia (source: central line) with meropenem MIC 0.5 mg/L (susceptible). She also has AKI with creatinine 280 μmol/L, urine output 10 mL/hr, not yet on CRRT. Albumin 20 g/L."

"How would you approach antimicrobial therapy for this patient, specifically addressing the ECMO-related pharmacokinetic considerations?"

---

Candidate:

"This case involves complex antimicrobial dosing in VA-ECMO with AKI and hypoalbuminemia. I'll structure my approach by addressing PK/PD principles, ECMO-specific factors, and dosing strategy."

PK/PD principles for meropenem:

"Meropenem is a beta-lactam antibiotic, time-dependent, requiring 100% fT>MIC in critically ill patients (or 100% fT>4×MIC for immune-compromised or severe sepsis). For this patient, MIC 0.5 mg/L, so target is maintaining plasma concentrations >2 mg/L (4×MIC) continuously."

ECMO-specific PK alterations:

1. Increased Vd (hydrophilic drug):

• "VA-ECMO circuit (1-1.5 L prime volume), plus cardiogenic shock resuscitation likely caused significant positive fluid balance (5-10 L), expanding Vd for meropenem from baseline 0.15-0.3 L/kg to 0.4-0.6 L/kg on ECMO"
• "Higher loading dose required to achieve therapeutic concentrations rapidly"

2. Reduced renal clearance (AKI):

• "Meropenem is 75% renally excreted unchanged. With creatinine 280 μmol/L and oliguria, estimated CrCl below 15-20 mL/min (severe AKI, Stage 3 KDIGO)"
• "Meropenem half-life prolonged from 1h (normal) to 6-10h (severe AKI), risk of accumulation with standard dosing"

3. Reduced hepatic clearance (VA-ECMO):

• "VA-ECMO provides non-pulsatile flow, MAP typically 60-70 mmHg, reducing hepatic blood flow by 30-50%"
• "Meropenem 25% hepatically metabolized, so further clearance reduction beyond renal component"

4. Minimal circuit sequestration:

• "Meropenem is hydrophilic, not highly protein-bound, minimal circuit sequestration (below 10%)"
• "Vd and clearance changes dominate PK alterations"

---

Examiner:

"How would you dose meropenem in this patient? Include loading dose, maintenance dosing, and rationale for each."

---

Candidate:

Loading dose:

"2 g IV meropenem infused over 30 minutes"

"Rationale: Despite AKI (which affects clearance, not Vd), the patient has increased Vd from ECMO circuit + fluid resuscitation. Loading dose is determined by Vd, not clearance. 2 g achieves Cmax 40-50 mg/L (well above 4×MIC target), ensuring immediate therapeutic effect for severe Gram-negative bacteremia."

Maintenance dose:

"1 g IV meropenem q12h (extended infusion over 3 hours) OR 2-3 g/24h continuous infusion"

"Rationale for dose reduction:"

• "Severe AKI (CrCl below 20 mL/min) reduces meropenem clearance by 70-80%"
• "Standard dosing 1 g q8h would cause accumulation and neurotoxicity (seizures) in this renal function"
• "Dose adjustment for CrCl 10-20 mL/min: 1 g q12h (vs 1 g q8h for CrCl >50 mL/min)"

"Rationale for extended infusion:"

• "Infusing over 3 hours (vs 30 minutes standard) increases %fT>MIC from ~60% to ~90% for time-dependent antibiotic"
• "Continuous infusion (after loading) is superior: achieves steady-state 8-12 mg/L (16-24× MIC), 100% fT>4×MIC continuously"

Alternative if CRRT initiated:

• "If patient starts CRRT (likely given oliguria, AKI, fluid overload on ECMO), meropenem clearance increases"
• "CRRT dosing: 1 g q8h or 3 g/24h continuous infusion (depending on CRRT intensity 20-25 mL/kg/h effluent dose)"

---

Examiner:

"Good. Now, the patient has been on meropenem 1 g q12h extended infusion for 48 hours. You receive a meropenem level result: trough 12 mg/L (pre-dose). How do you interpret this, and what would you do?"

---

Candidate:

Interpretation of trough 12 mg/L:

"Meropenem trough 12 mg/L is supratherapeutic but not yet toxic. This represents accumulation from reduced clearance (AKI + VA-ECMO)."

Target trough in severe infection:

• "Target meropenem trough >4 mg/L (>4×MIC 0.5 mg/L = >2 mg/L, add safety margin to 4-8 mg/L)"
• "Trough 12 mg/L exceeds target, indicating reduced clearance beyond initial estimate"

Neurotoxicity risk:

• "Meropenem neurotoxicity (seizures, encephalopathy, myoclonus) occurs at concentrations >60-80 mg/L, though risk increases >20-30 mg/L in renal failure"
• "Current trough 12 mg/L is below neurotoxicity threshold, but trending toward accumulation"

Additional considerations (hypoalbuminemia):

• "Albumin 20 g/L increases free meropenem fraction (normally 2% protein-bound, minimal impact)"
• "More relevant for highly protein-bound drugs (voriconazole, phenytoin)"

Management:

1. Continue current dose with close monitoring:

• "Trough 12 mg/L is therapeutic (>4×MIC), continue 1 g q12h for now"
• "Repeat trough in 24-48 h: if rising (e.g., >16-20 mg/L), reduce to 1 g q24h or 2 g/24h continuous infusion"
• "If trough stable or falling (renal recovery, CRRT initiated), continue current dose"

2. Monitor for neurotoxicity:

• "Daily neurological assessment (encephalopathy, myoclonus, seizures - difficult on VA-ECMO with sedation, may need EEG if concern)"
• "If neurotoxicity occurs, cease meropenem, switch to alternative (ciprofloxacin, aztreonam if Enterobacter susceptible)"

3. Consider continuous infusion:

• "Switch to 2 g/24h continuous infusion (no loading, already at steady state)"
• "Achieves Css 8-10 mg/L (4-5×MIC continuously), avoids peak/trough variability, reduces neurotoxicity risk"

4. Re-assess renal function:

• "Daily creatinine, urine output - if renal recovery occurring, clearance increases and dose escalation may be needed"
• "If CRRT initiated, increase dose to 1 g q8h or 3 g/24h continuous (CRRT clearance ~30-50 mL/min for meropenem)"

---

Examiner:

"On day 6, the patient is started on CRRT (CVVHDF 25 mL/kg/h effluent dose). How does this change your meropenem dosing?"

---

Candidate:

CRRT impact on meropenem clearance:

"CRRT adds significant clearance for hydrophilic, non-protein-bound drugs like meropenem:"

1. Meropenem sieving coefficient: ~1.0 (freely filtered across CRRT membrane, molecular weight 383 Da, minimal protein binding)

2. CRRT clearance calculation:

   • "Effluent dose 25 mL/kg/h × 75 kg = 1,875 mL/h = 31 mL/min"
   • "Meropenem CRRT clearance ~25-35 mL/min (approximately 80-100% of effluent flow for sieving coefficient ~1)"

3. Total clearance:

   • "Pre-CRRT: Residual renal clearance ~5-10 mL/min (severe AKI) + hepatic clearance ~5-10 mL/min (reduced on VA-ECMO) = 15-20 mL/min total"
   • "Post-CRRT: Residual clearance 15-20 mL/min + CRRT clearance 25-35 mL/min = 40-55 mL/min total"
   • "Total clearance has tripled with CRRT initiation"

Dosing adjustment:

Increase to:

• "1 g IV meropenem q8h (extended infusion 3h) OR 3 g/24h continuous infusion"
• "Rationale: Clearance increased 2-3× with CRRT, return to near-normal dosing required"

TDM plan:

• "Obtain trough level 24-48h after dose increase (expect trough 4-8 mg/L, therapeutic)"
• "If trough below 4 mg/L, increase to 1 g q6h or 4 g/24h continuous"
• "If trough >16 mg/L, reduce to 1 g q12h or 2 g/24h continuous"

CRRT interruptions:

"Important consideration: If CRRT interrupted for procedures, circuit clotting, or down-time, meropenem clearance drops back to 15-20 mL/min. Do NOT give additional doses during CRRT interruption (risk accumulation)."

"Monitor CRRT down-time daily: if >4-6 hours off CRRT per 24h, may need to reduce total daily dose to 2 g q12h or 2.5 g/24h continuous to account for lower average clearance."

---

Examiner:

"Excellent grasp of antimicrobial dosing on ECMO with CRRT. Final question: if this patient also required vancomycin for Gram-positive cover (pending blood culture speciation), how would you dose vancomycin on VA-ECMO with AKI on CRRT?"

---

Candidate:

Vancomycin PK on VA-ECMO with AKI on CRRT:

"Similar principles to meropenem but different target (AUC-based vs time-dependent)."

Loading dose:

• "25-30 mg/kg actual body weight = 1,875-2,250 mg IV (use 2,000 mg)"
• "Rationale: Vd expansion from ECMO + fluid resuscitation (Vd 0.8-1.2 L/kg vs baseline 0.4-0.7 L/kg), loading dose needed to achieve therapeutic levels immediately"
• "Infuse over 90-120 min (prevent red man syndrome)"

Maintenance dose (CRRT 25 mL/kg/h):

• "15-20 mg/kg q12-24h (1,125-1,500 mg q12-24h for 75 kg patient)"
• "Start with 1,500 mg IV q24h, adjust based on TDM"

Rationale:

• "Vancomycin CRRT clearance ~25-30 mL/min (sieving coefficient ~0.8-1.0)"
• "Residual renal clearance ~0-5 mL/min (AKI, oliguria)"
• "Total clearance 25-35 mL/min (CRRT-dependent)"
• "This is lower than normal (baseline vancomycin clearance 60-80 mL/min in normal renal function), but higher than AKI alone (5-10 mL/min)"

Target AUC₀₋₂₄ 400-600 mg·h/L:

"Use Bayesian AUC monitoring:"

1. Obtain two levels at steady state (48-72h after loading dose):

   • "Peak: 1-2h post-infusion"
   • "Trough: pre-dose (before 3rd or 4th dose)"

2. Bayesian software (DoseMeRx, InsightRx):

   • "Input patient parameters (weight, creatinine, CRRT settings)"
   • "Calculate AUC₀₋₂₄ from two levels"
   • "Adjust dose to achieve 400-600 mg·h/L"

3. Dosing adjustments:

   • "If AUC below 400 mg·h/L: Increase to 1,500 mg q12h or 2,000 mg q24h"
   • "If AUC >600 mg·h/L: Reduce to 1,000 mg q24h or 1,500 mg q36h"
   • "If AUC 400-600 mg·h/L: Continue current dose, repeat TDM in 5-7 days"

Alternative: Continuous infusion vancomycin:

• "After loading dose 2,000 mg, start continuous infusion 30-40 mg/kg/day (2,250-3,000 mg/24h)"
• "Target steady-state concentration 20-25 mg/L (approximates AUC 400-600 mg·h/L)"
• "Easier to manage on CRRT (constant clearance, stable Css)"

CRRT interruption considerations:

• "If CRRT stopped >4h, vancomycin clearance drops to 5-10 mL/min (AKI only)"
• "Risk accumulation if dosing for CRRT clearance but CRRT down"
• "Monitor CRRT down-time, reduce dose if >6h off per 24h"

---

Examiner:

"Comprehensive answer. Well done."

---

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