Obesity

1/3 of the American population is overweight

Morbid obesity is when 45 kg (100 lbs) above ideal body weight - 3-5% of the population

BMI (weight in kg/(height m)²) >28 is associated with an increased risk of stroke, CAD, DM

For every 13.5 kg of fat gained, 25 miles of neovascularization to provide blood flow

CO increases 1 L for every 10 kg of fat

Ideal Body Weight (IBW)

Males

Height in cm minus 100 = ideal weight in kg

Females

Height in cm minus 105 = ideal weight in kg

Physiologic Changes

Cardiac

Systemic HTN, cardiomegaly, CHF, CAD, pulmonary hypertension

Respiratory

Decreased lung volumes and capacities, arterial hypoxemia

• May cause polycythemia

Obesity hypoventilation syndrome

• Pickwickian syndrome
• Differentiates from obstructive sleep apnea by CHF

Liver

Abnormal liver function tests, fatty liver infiltration

Metabolic

Insulin resistance (DM) and hypercholesterolemia lead to cardiac disease/gallstones

Liver function is often abnormal, and drug metabolism may be altered

GI

Increased intraabdominal pressure, gastric volume, and acidity with increased incidence of hiatal hernia

Increased risk of aspiration

• Possible pre-medicate with H₂ blocker (ranitidine, cimetidine), promotability drug (metoclopramide), and antacids (Alka-Seltzer Gold)

Anesthetic Considerations

Respiratory

Extremely sensitive to respiratory depressant effects of sedatives

Anticipate difficulty with masking and intubation due to more soft tissues, decreased neck mobility, and possible short neck

May have a history of obstructive sleep apnea; ask about any home treatments

Low FRC due to weight pressing down on chest (restrictive disease)

Increased elastic recoil but low compliance

• Increased work-of-breathing for O₂/CO₂ exchange
• Preoxygenation
• Monitor O₂ saturation closely as obese patients have less reserve

Increased risk of airway obstruction

Before extubation

• NIF (negative inspiratory force) of negative 25-30, RR and VT as before surgery, head lift for 5 seconds minimal, awake, following commands

Induction

Elevate head and shoulders to facilitate sniffing position and prevent reflux.

HELP (Head Elevated Laryngoscopy Position)

Cricoid pressure may decrease lower esophageal sphincter pressure, but abdominal pressure is less

Insert an NG tube to decompress the stomach

Warmer to prevent cooling, possible blood warmer for fluids

Fluids

Fluid replacement is based on 45-55 mL/kg due to a lower total water percentage

• 40% compared to 60-65%

Drugs

Lipid-soluble drugs (benzodiazepines and opioids) should be dosed based on actual body weight b/c of larger fat stores providing an increased volume of distribution, but clearance is expected to be slower due to fat stores

Water soluble drugs (eg, MRs) should be dosed based on IBW b/c of a more limited volume of distribution - maintenance based on IBW to avoid overdosing

• Elimination of drugs normal or increased in phase 1 (oxidation, reduction, hydrolysis)
  • Increased in phase II reactions (metabolism)
• Renal clearance increased due to augmented renal blood flow and glomerular filtration rate

Positioning

Pad pressure points well, at risk for brachial plexus injury, sciatic nerve injury, and ulnae nerve injury

Arms either tucked by the side with palms facing body or hands supinated <90 degrees. Pillows under knees to facilitate normal lordotic curve and diminish stretch on the sciatic nerve. ROM might be less