Is propofol (propofol) dosing based on ideal body weight or total body weight?

Propofol Dosing in Obese Patients

Propofol dosing should be based on lean body weight (LBW) rather than total body weight (TBW) in obese patients to minimize the risk of hypotension while ensuring adequate anesthesia. 1

Understanding Weight-Based Dosing for Propofol

The appropriate weight scalar for propofol dosing is critical for safe and effective anesthesia, particularly in obese patients. According to the Association of Anaesthetists of Great Britain and Ireland (2015), there are several important weight measurements to consider:

• Total Body Weight (TBW): The actual weight of the patient
• Ideal Body Weight (IBW): Approximated as height (cm) × 0.01 - 100 for men and height (cm) × 0.01 - 105 for women
• Lean Body Weight (LBW): The patient's weight excluding fat
• Adjusted Body Weight (ABW): IBW + 0.4 × (TBW - IBW)

Evidence Supporting LBW for Propofol Dosing

Induction Dosing

When administering propofol for induction:

• Using LBW results in appropriate dosing that achieves loss of consciousness without excessive hypotension 1, 2
• Dosing based on TBW leads to significantly shorter time to loss of consciousness but with greater risk of hypotension 1, 2
• A randomized controlled trial found that MO patients receiving propofol based on LBW required similar doses to non-obese controls, while those dosed on TBW received significantly larger doses 2

Maintenance Dosing

For continuous infusions:

• The Association of Anaesthetists recommends using lean or adjusted body weight for most anesthetic agents in obese patients 1
• Using TBW for propofol infusions can result in significant hypotension due to overdosing 1

Pharmacokinetic Considerations

The pharmacokinetics of propofol in obese patients differ from non-obese patients:

• After bolus administration, propofol undergoes rapid equilibration between plasma and brain, accounting for quick onset 3
• Distribution accounts for about half of the initial decline in plasma levels 3
• In obese patients, there is a larger volume of distribution but not proportionally increased to total body weight 1
• The increased fat mass in obese patients leads to more rapid redistribution of propofol from the central compartment, which can cause patients to wake up more quickly after a single bolus 1

Clinical Implications and Recommendations

1. For induction doses:

   • Calculate propofol dose using LBW rather than TBW 1, 2
   • This approach provides adequate anesthesia while minimizing cardiovascular depression

2. For maintenance infusions:

   • Use LBW or ABW for calculating infusion rates 1
   • Be aware that after very long infusions, reduction in the infusion rate by as much as half may be necessary to maintain constant plasma levels 3

3. For target-controlled infusions (TCI):

   • Standard TCI models (Marsh and Schnider) become unreliable for patients weighing over 140-150 kg or with BMI > 35-42 kg/m² 1
   • Consider depth of anesthesia monitoring in these situations 1

Important Caveats and Pitfalls

1. Risk of awareness:

   • The 5th National Audit into accidental awareness under anesthesia found a disproportionate number of obese patients who suffered awareness 1
   • Small doses of induction agent based on LBW must be quickly followed by maintenance anesthesia to prevent awareness 1
   • Consider depth of anesthesia monitoring in high-risk cases 1

2. Contradictory evidence:

   • While most evidence supports LBW-based dosing, one study suggested that propofol infusion should be calculated according to real body weight to prevent awareness 4
   • This highlights the importance of clinical vigilance and appropriate monitoring

3. Consistent approach:

   • Whichever weight scalar is chosen (LBW or ABW), it should be used consistently throughout anesthesia 1
   • Switching between different weight calculations during a case may lead to dosing errors

By using lean body weight for propofol dosing in obese patients, clinicians can achieve adequate anesthesia while minimizing the risks of hypotension and other adverse effects associated with overdosing.