In a 125‑kg obese patient undergoing transforaminal lumbar interbody fusion (TLIF) spine surgery, can a target‑controlled infusion (TCI) of remifentanil (Minto model) be combined with sevoflurane, and should the remifentanil dose be calculated using ideal body weight rather than total body weight? What is the appropriate remifentanil infusion dosing?

Yes, you can combine TCI remifentanil (Minto model) with sevoflurane in this 125 kg obese patient undergoing TLIF spine surgery, and you must use lean body weight (LBW) or ideal body weight (IBW), NOT total body weight, for remifentanil dosing.

Weight Scalar for Remifentanil Dosing

The FDA label explicitly states that remifentanil should be dosed based on ideal body weight (IBW) in obese patients (>30% over IBW) 1. This is strongly supported by pharmacokinetic research demonstrating that remifentanil's clearance and distribution volumes correlate with lean body mass rather than total body weight 2. Using total body weight in obese patients results in excessively high remifentanil concentrations and increased risk of overdose 2.

The 2015 AAGBI/SOBA guidelines reinforce this principle, stating that for most anesthetic agents, dosing to total body weight is rarely appropriate and increases the risk of relative overdose 3. The guidelines specifically recommend using lean or adjusted body weight as scalars for anesthetic drug dosing in obese patients 3.

Calculate the Appropriate Weight:

For your 125 kg patient, you need to calculate:

• Ideal Body Weight (IBW): Use the formula from the guidelines

  • Male: IBW (kg) = height (cm) ÷ 100
  • Female: IBW (kg) = height (cm) ÷ 105

• Lean Body Weight (LBW) (Janmahasatian formula from guidelines 3):

  • Male: LBW = [9270 × TBW] ÷ [6680 + (216 × BMI)]
  • Female: LBW = [9270 × TBW] ÷ [8780 + (244 × BMI)]
  • Note: LBW rarely exceeds 100 kg in men and 70 kg in women 3

Use whichever weight you calculate (IBW or LBW) for all remifentanil dosing calculations.

Remifentanil Infusion Dosing for TLIF Spine Surgery

Induction Phase:

• Bolus: 1 mcg/kg (based on IBW/LBW) over 30-60 seconds 1
• Initial infusion: 0.5-1 mcg/kg/min (based on IBW/LBW) 1

Maintenance Phase:

• Continuous infusion: 0.05-2 mcg/kg/min (based on IBW/LBW) 1
• Typical range for major surgery: 0.1-0.5 mcg/kg/min 1
• Titrate to clinical effect while monitoring hemodynamics and depth of anesthesia

Practical Dosing Example:

If your patient's IBW is approximately 70 kg (assuming average height male):

• Induction bolus: 70 mcg over 30-60 seconds
• Initial infusion: 35-70 mcg/min (2.1-4.2 mg/hour)
• Maintenance infusion: 7-35 mcg/min (0.42-2.1 mg/hour), typically 7-21 mcg/min for balanced anesthesia with sevoflurane

Preparation:

• Dilute to 25 mcg/mL or 50 mcg/mL concentration 1
• Use infusion pump with TCI capability if available
• Refer to FDA tables for mL/hour delivery rates based on your chosen concentration 1

Combination with Sevoflurane

This combination is safe and effective 4, 5. Research demonstrates that sevoflurane combined with remifentanil in obese patients provides:

• Superior hemodynamic stability compared to propofol-based techniques 4
• Faster recovery from anesthesia 4
• Satisfactory outcomes in morbidly obese patients 5

Important Considerations for Obese Patients:

Critical Safety Points:

• Awareness risk: Obese patients are disproportionately represented in awareness cases (NAP5 audit) 3. Ensure adequate maintenance anesthesia immediately after induction—don't rely solely on small IBW-based induction doses without quickly establishing volatile or IV maintenance
• Respiratory depression: Monitor closely for hypoxemia, though remifentanil-based techniques show acceptable respiratory profiles 6
• Hemodynamic monitoring: Expect more stable blood pressure with sevoflurane-remifentanil compared to propofol-based techniques 4

Avoid These Pitfalls:

1. Never dose remifentanil on total body weight (125 kg) - this will cause dangerous overdose 1, 2
2. Don't use TCI models beyond their validated range - most TCI pumps have weight limits (Marsh model caps at 150 kg) 3
3. Don't delay maintenance anesthesia after IBW-based induction - rapid redistribution in obese patients increases awareness risk 3

For this 125 kg patient undergoing TLIF, calculate IBW or LBW (likely 60-80 kg depending on height/sex), use that weight for all remifentanil calculations, combine with sevoflurane for maintenance, and ensure adequate depth of anesthesia throughout the case.