RSI Sedation Drug Dosing: Weight-Based Recommendations

Direct Answer

For RSI sedation drugs in obese patients, use ideal body weight (IBW) or adjusted body weight (ABW) rather than actual body weight for most induction agents, but use actual body weight for succinylcholine. 1

Drug-Specific Dosing Recommendations

Neuromuscular Blocking Agents

Succinylcholine:

Use actual body weight (TBW) for dosing 1
The 2010 American Heart Association guidelines explicitly state: "Actual body weight, rather than ideal body weight, should be used for some non-resuscitation medications (eg, succinylcholine)" 1
This is critical because succinylcholine distributes into the increased muscle mass present in obese patients 1

Rocuronium and other non-depolarizing NMBAs:

Use ideal body weight (IBW) or adjusted body weight (ABW) 1
The Critical Care Medicine guidelines recommend against using actual body weight for non-depolarizing neuromuscular blocking agents in obese patients (BMI ≥30 kg/m²) 1
Studies demonstrate that IBW-based dosing of atracurium resulted in significantly shorter recovery times and less variability compared to actual body weight dosing 1

Induction Agents (Sedatives)

Etomidate, Ketamine, Propofol, and Benzodiazepines:

Use lean body weight (LBW) or adjusted body weight (ABW) rather than total body weight 1
The Association of Anaesthetists guidelines state: "For most anaesthetic agents, dosing to total body weight is rarely appropriate and increases the risk of relative overdose" 1
The induction dose required to produce unconsciousness correlates well with lean body weight 1

Midazolam specifically:

In obese pediatric patients, calculate dose based on ideal body weight 2
The FDA label explicitly states: "In obese PEDIATRIC PATIENTS, the dose should be calculated based on ideal body weight" 2
For adult obese patients, use IBW or ABW based on expert consensus 3

Weight Calculation Formulas

Ideal Body Weight (IBW):

Males: IBW (kg) = height (cm) - 100 1
Females: IBW (kg) = height (cm) - 105 1

Adjusted Body Weight (ABW):

Formula: ABW (kg) = IBW (kg) + 0.4 × (TBW (kg) - IBW (kg)) 1
This accounts for the fact that obese individuals have increased lean body mass 1

Lean Body Weight (LBW):

Males: LBW (kg) = [9270 × TBW (kg)] / [6680 + (216 × BMI)] 1
Females: LBW (kg) = [9270 × TBW (kg)] / [8780 + (244 × BMI)] 1
LBW rarely exceeds 100 kg in men and 70 kg in women regardless of total body weight 1

Clinical Algorithm for RSI Drug Dosing

Step 1: Determine Patient's Weight Status

Calculate BMI to identify obesity (BMI ≥30 kg/m²) 1
Measure actual body weight and height 1

Step 2: Calculate Appropriate Weight Scalar

For non-obese patients: Use actual body weight for all RSI medications 1
For obese patients: Calculate both IBW and ABW using formulas above 1

Step 3: Select Weight-Based Dosing by Drug Class

Paralytic Selection:

If using succinylcholine: Dose based on actual body weight (1-1.5 mg/kg TBW) 1
If using rocuronium: Dose based on ideal body weight or adjusted body weight 1

Induction Agent Selection:

Etomidate: Dose based on lean body weight or adjusted body weight (0.2-0.4 mg/kg) 1
Ketamine: Dose based on lean body weight or adjusted body weight 1
Propofol: Dose based on lean body weight or adjusted body weight 1
Midazolam: Dose based on ideal body weight 3, 2

Critical Pitfalls and How to Avoid Them

Common Dosing Errors

Underdosing in obese patients:

Obese patients are significantly more likely to receive inadequate doses of etomidate and succinylcholine when providers fail to use appropriate weight scalars 4, 5
One study found 68% of patients ≥100 kg received underdosing of etomidate (<0.2 mg/kg) 5
Solution: Pre-calculate doses using appropriate weight scalars before the procedure 4

Overdosing in non-obese patients:

Non-obese patients are more likely to be overdosed when providers use fixed dosing or fail to calculate weight-based doses accurately 4
Solution: Always calculate weight-based doses rather than using empiric fixed doses 4

Inconsistent weight measurement:

Using different weight scalars for the same patient leads to dosing errors 1
Solution: Document which weight scalar is being used and maintain consistency throughout the resuscitation 1

Special Considerations

Awareness under anesthesia risk:

The NAP5 audit found a disproportionate number of obese patients experienced accidental awareness under anesthesia 1
Half of awareness incidents occurred during induction when neuromuscular blocking drugs were used 1
Critical point: Small doses of induction agents based on lean or adjusted body weight must be quickly followed by maintenance anesthesia to prevent awareness 1
More rapid redistribution of induction agents into larger fat mass means obese patients wake up more quickly after a single bolus dose 1

Hemodynamic considerations:

Using total body weight for induction agents in obese patients is likely to result in significant hypotension 1
Solution: Use IBW or ABW for induction agents and titrate to clinical effect 1

Pediatric-Specific Considerations

Non-Obese Children

Use actual body weight for initial resuscitation drug doses 1
Length-based tapes with precalculated doses are more accurate than age-based estimates and estimate the 50th percentile weight for length (ideal body weight) 1
For subsequent doses, expert providers may adjust based on desired therapeutic effect 1

Obese Children

Use actual body weight for calculating initial resuscitation drug doses despite theoretical concerns about toxic doses 1
There are no data regarding safety or efficacy of adjusting resuscitation medication doses in obese pediatric patients 1
For succinylcholine specifically: Use actual body weight 1
For midazolam: Calculate dose based on ideal body weight 2
The total dose should not exceed the standard adult dose 1

Evidence Quality and Nuances

Strength of Recommendations

High-quality guideline evidence:

The recommendation to use actual body weight for succinylcholine is explicitly stated in AHA guidelines 1
The recommendation to avoid total body weight for non-depolarizing NMBAs in obese patients is supported by randomized controlled trials 1
The recommendation to use lean or adjusted body weight for induction agents is based on expert consensus from multiple anesthesia societies 1

Limitations in evidence:

Most studies evaluating weight-based dosing were single-dose perioperative studies, not sustained ICU use 1
There is insufficient evidence to recommend one consistent weight measure (IBW vs ABW vs LBW) over another for most drugs 1
No prospective studies have examined whether choice of weight scalar influences clinical outcomes in RSI 1

Divergent Evidence

Pediatric resuscitation vs RSI:

Pediatric resuscitation guidelines recommend using actual body weight for obese children 1
However, RSI-specific guidelines recommend ideal body weight for succinylcholine dosing in pediatric patients 2
Resolution: The difference reflects the distinction between emergency resuscitation drugs (where underdosing is more dangerous) versus controlled RSI (where precise dosing is possible) 1, 2

Real-world practice patterns:

Research demonstrates that most patients (both obese and non-obese) receive inappropriate RSI medication doses in actual ED practice 4
This suggests a gap between guideline recommendations and clinical implementation 4
Practical solution: Use pre-calculated dosing charts or electronic decision support to improve accuracy 4

For RSI sedation, should actual, ideal or adjusted body weight be used for dosing calculations?

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