---

TIVA vs TCI - MD Anaesthesiology Exam Notes

---

1. Definitions

Important exam point: TCI is NOT synonymous with TIVA. TIVA is a broader concept (all IV anaesthesia). TCI is a delivery method that can be used to achieve TIVA. You can have TIVA without TCI (manual infusion), but TCI in anaesthesia is almost always used as part of TIVA.

---

2. TIVA - Manual (Non-TCI)

Mechanism

• The anaesthetist manually calculates the drug doses based on body weight (mg/kg) and time
• No real-time knowledge of predicted plasma concentration
• Based on empirical "bolus + infusion" regimens

Classic Propofol Regimen (Roberts 10-8-6 / Bristol model)

• Induction: 1-2 mg/kg IV bolus
• Maintenance: 10 mg/kg/hr for first 10 min → 8 mg/kg/hr for next 10 min → 6 mg/kg/hr thereafter
• This stepwise reduction accounts for peripheral redistribution (tissues becoming saturated over time)

Classic Remifentanil regimen (TIVA)

• 0.25-0.5 mcg/kg/min infusion (constant, because remifentanil does NOT accumulate)

Drawback

• Does not account for individual PK variability
• Risk of drug accumulation with longer cases
• No real-time concentration estimate available

---

3. TCI - Target Controlled Infusion

Mechanism

1. Calculates the current estimated drug concentration
2. Adjusts infusion rate in real-time to achieve and maintain that target
3. Accounts for elimination, metabolism, redistribution, and accumulation

The Three-Compartment Model (basis of TCI)

• Compartment 1 (Central): Blood/plasma - where drug is injected and measured
• Compartment 2 (Rapid peripheral): Well-perfused tissues (muscle, gut)
• Compartment 3 (Slow peripheral): Poorly-perfused tissues (fat)
• Effect compartment (Ce): Biophase - the actual site of drug action (brain). Has a rate constant keo

Targeting: Plasma (Cp) vs Effect-site (Ce)

---

4. TCI Models for Propofol

Marsh Model

• Based on a 3-compartment model in healthy adults
• Covariates: weight only (does NOT factor age)
• Targets plasma concentration (Cp) by default
• Has a variable, larger V1 (central volume) → gives a larger initial bolus → faster induction
• Modified Marsh uses keo = 1.2 min⁻¹ (vs original 0.26 min⁻¹) for faster Ce equilibration

Schnider Model

• Based on a 3-compartment model with fixed V1 = 4.27 L (fixed central compartment)
• Covariates: weight, age, height, and lean body mass (LBM)
• Targets effect-site concentration (Ce) by default
• V2 and keo are variable (age-dependent)
• Overall uses less propofol and has fewer adverse events
• Limits BMI: < 42 (males), < 35 (females) to prevent absurd LBM calculations

Bristol (Manual) Model

• The original first PK model
• Assumes premedication, fentanyl 3 mcg/kg, N₂O
• Target Cp = 3 mcg/mL
• Not a TCI pump model - it's the basis for the manual 10-8-6 regimen

---

5. TCI Models for Opioids

Remifentanil - Minto Model

• Covariates: age, weight, height, LBM
• Works well in both adults and children (very predictable PK, ester hydrolysis clearance)
• Context-sensitive half-time (CSHT) is constant ~3-4 min regardless of infusion duration

Alfentanil - Maitre & Scott Models

• Available on commercial TCI pumps
• Covariates: weight, age, sex

---

6. Key Pharmacokinetic Concepts (Exam Must-Knows)

Context-Sensitive Half-Time (CSHT)

• Time for plasma concentration to fall by 50% after stopping an infusion of specified duration
• NOT the same as elimination half-life
• Remifentanil: CSHT ~3-4 min (constant, regardless of duration) - ideal for TIVA
• Fentanyl: CSHT rises dramatically with infusion duration (up to 300 min after 8 hr infusion) - NOT ideal for TIVA
• Propofol: Intermediate - CSHT rises modestly

Biophase / Effect Compartment

• The theoretical compartment representing the site of drug action (brain/CNS)
• Ke0 = rate constant for drug equilibration between plasma and effect site
• High Ke0 = faster equilibration (e.g. remifentanil Ke0 ~1.0 min⁻¹)
• Lower Ke0 = slower equilibration (e.g. propofol Ke0 ~0.26-1.2 min⁻¹)

---

7. Head-to-Head Comparison Table

---

8. Sources of Error in TCI (Exam Favourite)

1. Incorrect patient data entry - wrong weight, age, height → wrong model output
2. Wrong PK model selected for a given patient population
3. Individual PK variability - population models ≠ individual patient
4. Failure to account for co-medications (opioids reduce propofol requirement by ~30-50%)
5. Hardware errors - syringe size mismatch, infusion line disconnection
6. IV line issues - leaks, partial occlusion, extravasation
7. No real-time blood concentration measurement - all values are estimated/predicted

---

9. Indications / When to Use Which

Prefer TCI when:

• Long cases (benefit of accumulation management)
• High-risk patients needing precise titration
• Neuromuscular blocking agents used (pEEG + TCI)
• Trained operator with TCI pump available

Prefer manual TIVA when:

• Paediatrics (no validated TCI model <16 yr / <30 kg for propofol)
• Resource-limited settings
• Short procedures
• No TCI pump available

---

10. TIVA vs Volatile Anaesthesia (Bonus - Often Asked Together)

---

Key Exam Summary (Mnemonics)

• TIVA = Route (all IV); TCI = Method (PK-controlled infusion)
• TCI needs: 3-compartment model + microprocessor pump + target setting (Cp or Ce)
• Marsh = Marsh = Mass (weight only); Schnider = Schnider = Six covariates (weight, age, height, LBM + fixed V1)
• Best TIVA drug pair: Propofol + Remifentanil (shortest/most predictable CSHT)
• pEEG monitoring is mandatory when NMBDs are used with TIVA

---