The obese patient

Definitions

• Overweight = BMI 25-29
• Obesity = BMI 30-39
• Morbid obesity = BMI 40-49
• Super obesity = BMI >50

Dosing Scalars

• Total body weight (TBW)= Actual weight
  • May result in over dosing
• Ideal body weight (IBW)= Calc. based on height & weight
  • May result in under dosing
• Lean body weight (LBW)= Difference between TBW and fat mass
  
  • Technically best weight scalar for drug administration but difficult to accurately measure
• Adjusted body weight (ABW) = Scalar derived from aminoglycoside pharmacokinetics (PK) in obesity

Estimating Patient Weight

Multiple formulas to estimate weight, below are most common:

• Estimated male TBW = 93.2 + 3.29[arm circumference (cm)] + 0.43[height (cm)]
• Estimated female TBW = 64.6 + 2.15[arm circumference (cm)] + 0.54[height (cm)]
• Estimated male IBW in kg = height in cm - 100
  • More accurate IBW estimate:
    • IBW (male) in kg = 50kg + 2.3kg for each in. over 5ft
• Estimated female IBW in kg = height in cm - 105
  • More accurate IBW estimate:
    • IBW (female) in kg = 45kg + 2.3kg for each in. over 5ft
• Estimated LBW = 1.3 x IBW
• Estimated ABW = IBW + [C x(TBW-IBW)]
  • C = correction factor ususally 0.2 to 0.4
• Apps/Online resources:
  • Epocrates has IBW calculator
  • Http://www.medcalc.com/body.html

Medication Adjustments in Obesity

Obesity alters hepatic/renal clearance and volume of distribution (Vd) of many drugs.

Medication Adjustment in Obesity

Medication	Dose Adjustment
Morphine	Dose by IBW
Fentanyl	Dose by LBW; Alternative: Initial dose TBW, subsequent dose by IBW (risk over dose when use TBW)
Midazolam	Initial dose TBW, subsequent dose by IBW
Propofol	Dose by LBW
Etomidate	Dose by LBW
Succinylcholine	Dose by TBW @ 1mg/kg
Rocuronium	Dose by IBW
Heparin	Same as non-obese pt (80U/kg followed by 18U/kg/h using TBW)
Vancomycin	30 mg/kg TBW (then follow serum levels)
Aminoglycosides	Dose by ABW (C= 0.4; then follow serum levels)
Beta-lactams	No good data, may consider doubling dose
Carbapenems	No empiric change recommended
Antifungals	No empiric change recommended; use LBW when dosing weight-based agents

Airway Management

Obesity associated with difficult bag-mask ventilation (BMV) and intubation

• Consider prolonged pre-oxygenation period and two provider BMV
• May improve lung function through use of pre-intubation period of NIPPV
  • Obesity hypoventilation syndrome may cause chronic hypercapnia
• Place pt in "ramp" position
  • Stack blankets behind pt's back to horizontally align external auditory meatus with sternal notch
• Consider Airtraq laryngoscope (shortened mean intubation time & desat episodes vs Mac DL in obese pts)
• Increased BMI associated with higher rate of developing ARDS
  • Methods to improve oxygenation/ventilation:
    • Use PEEP 10 to improve oxygenation if needed
    • Place in reverse Trendelenburg ("beach chair" position) to improve ventilation
    • Both may decrease CO so use cautiously in unstable pts

Lumbar Puncture

Obese pts have increased distance from skin to subarachnoid space

• Upright pt positioning improves LP success
• Estimated lumbar puncture depth (cm) = 1 + 17[weight(kg)/height(cm)]
• Ultrasound may improve the ID of landmarks

Sources

Tintinalli, Challenging & Emerging Conditions in EM by A. Venkat, Ingrande, J., and H. J. M. Lemmens. "Dose adjustment of anaesthetics in the morbidly obese." British journal of anaesthesia 105.suppl 1 (2010): i16-i23.