The obese patient: Difference between revisions
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= Definitions = | == Definitions == | ||
#Overweight = BMI 25-29 | #Overweight = BMI 25-29 | ||
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#Super obesity = BMI >50 | #Super obesity = BMI >50 | ||
= Dosing Scalars = | == Dosing Scalars == | ||
#Total body weight '''(TBW)'''= Actual weight | #Total body weight '''(TBW)'''= Actual weight | ||
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#Adjusted body weight (ABW) = Scalar derived from aminoglycoside pharmacokinetics (PK) in obesity | #Adjusted body weight (ABW) = Scalar derived from aminoglycoside pharmacokinetics (PK) in obesity | ||
== Estimating Patient Weight | === Estimating Patient Weight === | ||
Multiple formulas to estimate weight, below are most common: | Multiple formulas to estimate weight, below are most common: | ||
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##Http://www.medcalc.com/body.html | ##Http://www.medcalc.com/body.html | ||
= Medication Adjustments in Obesity = | == Medication Adjustments in Obesity == | ||
Obesity alters hepatic/renal clearance and volume of distribution (Vd) of many drugs. | Obesity alters hepatic/renal clearance and volume of distribution (Vd) of many drugs. | ||
{| width=" | {| width="400" border="1" cellpadding="1" cellspacing="2" | ||
|+ Medication Adjustment in Obesity | |+ Medication Adjustment in Obesity | ||
|- | |- | ||
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<br> | <br> | ||
= Airway Management = | == Airway Management == | ||
Obesity associated with difficult bag-mask ventilation (BMV) and intubation | Obesity associated with difficult bag-mask ventilation (BMV) and intubation | ||
# Consider prolonged pre-oxygenation period and two provider BMV | # Consider prolonged pre-oxygenation period and two provider BMV | ||
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### Both may decrease CO so use cautiously in unstable pts | ### Both may decrease CO so use cautiously in unstable pts | ||
= Lumbar Puncture = | == Lumbar Puncture == | ||
Obese pts have increased distance from skin to subarachnoid space | Obese pts have increased distance from skin to subarachnoid space | ||
# Upright pt positioning improves LP success | # Upright pt positioning improves LP success | ||
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# Ultrasound may improve the ID of landmarks | # Ultrasound may improve the ID of landmarks | ||
= Sources = | == 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. | 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. | ||
[[Category:Misc/General]] | [[Category:Misc/General]] |
Revision as of 18:04, 19 August 2013
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
- More accurate IBW estimate:
- 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
- More accurate IBW estimate:
- 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 | 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
- Methods to improve oxygenation/ventilation:
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.