Author: ResoluteSupportMedia
CC BY 2.0

Obesity is a relatively common condition, that can have a profound impact on morbidity/mortality and anesthesia. Physiological derangements, difficult airway management, and alterations in pharmacokinetics & dose/response relationships can all be part of the picture.


  • Estimated Ideal Body Weight (wt) in kilograms (Kg) = Height (ht) in cm – [[100 for men] or [105 for women]]
  • Body Mass Index (BMI) = Wt in Kg/([Ht in Meters (M)]squared)–(normal is around 24)


  • Obese = 20% > Ideal Body Weight (IBW)—-(or BMI > 28)
  • Morbidly Obese ==> Wt >or= 2 x IBW—-(or BMI > 35)


  • 33% of North America is obese; 5% are morbidly obese
  • In the morbidly obese, mortality is 3.9 times that in non-obese.



  • Excess body mass –> increased metabolic demand –> increased cardiac output (C.O.)
  • Increased absolute blood volume (although it is actually a lower % of total body weight [fat has little water]. Blood volume can be as low as 45 cc/kg).
  • Stroke volume index (SVI) & stroke work index (SWI) equals that in non-obese patients (pts.), therefore SV & SW must increase in proportion to body wt; increased SV & SW can lead to left ventricular (LV) dilatation/hypertrophy.
  • Hypoxia/hypercapnia –> pulmonary vasoconstriction –> chronic pulmonary hypertension (htn) –> right heart failure.
  • Increased risk of arrhythmias, secondary to: hypertrophy, hypoxemia, fatty infiltration of the cardiac conduction system, diuretics (which can lead to hypokalemia), increased incidence coronary artery disease (CAD), increased catecholamines, sleep apnea.


  • Excess metabolically active adipose + increased workload on supportive muscle –> increased oxygen consumption & increased carbon dioxide production.
  • Decreased myocardial compliance (35% of normal), increased work of breathing & decreased efficiency (more work spent on lung inflation–lifting extra weight on chest).
  • Decreased resting functional residual capacity (FRC); decreases further with induction (contrasts with FRC in non-obese pts, which does not change with induction).
    • post-induction decline in FRC may be predicted by: (post induction) FRC (as % pre-anesthesia value) = 137.7 – 164.4 x (wt/ht).
    • FRC may decline to less than closing volume –> ventilation/perfusion (V/Q) mismatch may lead to hypoxemia post induction.


  • Increased incidence gastroesophageal (GE) reflux & hiatus hernia, increased abdominal pressure –> severe risk of aspiration.
  • Fatty changes in liver –> may be present, but not be reflected in liver function tests.

Pharmacological Considerations:

  • Increased volume distribution –> increased elimination half life.
  • Increased glomerular filtration rate –> increased clearance of untransformed drugs.
  • Increased fat stores may increase requirements for and clearance time of fat soluble anesthetics.
  • Dosing guidelines for certain anesthetics:
    1. Sux: some suggest–dose/total wt., others suggest– 120-140 mg absolute dose for all patients.
    2. Pancuronium: low lipid solubility, dose/total wt.
    3. Vecuronium: dose/lean body wt. — recovery is prolonged.
    4. Atracurium: dose/total wt–recovery time is unaffected.
    5. Fentanyl: dose/total wt.
    6. Alfentanyl: dose/lean body wt.
    7. Benzodiazipines: dose/total wt.
    8. Thiopental: highly lipophilic–use higher absolute dose–expect longer duration of action.
    9. Intravenous (IV) Lidocaine: dose/total wt.
    10. Epidural/spinal local anesthetics: dose/total body wt, but decrease dose by 20%-25%.
    11. Inhalational agents: metabolism of inhalational agents is increased over non-obese pts. Higher fluoride concentrations with enflurane & methoxyflurane are seen when compared to non-obese patients. Incidence of “halothane hepatitis” is allegedly higher in obese patients.

Anesthetic Management:


  • Avoid opioids & sedation.
  • H2 blocker, metoclopromide are appropriate
  • Avoid intramuscular (IM) injections due to unpredictable absorption.
  • Electrocardiogram (EKG): look for ischemia, arrhythmias, strain pattern, & hypertrophy.
  • Chest X-ray (CXR): examine heart size & pulmonary vasculature (for evidence of pulmonary htn).
  • Consider cardiology consult, if indicated.


  • Consider regional, if possible & not contra-indicated.
  • Appropriate size non-invasive blood pressure (b/p) cuff is important for accurate b/p measurement. If too short, b/p will be over-estimated (length should exceed arm circumference by 20%).
  • Positioning:
    • 2 O.R. tables (side by side, with a board accross the lower half–so the back can still flex to a sitting position) should be used if the pt’s wt. > 350 lbs.
    • Pt. must be able to sit upright –> if supine, increased abdominal pressure on chest will lead to a decreased FRC.
    • Prone position is poorly tolerated; lateral decubitus is better because it keeps abdominal weight off chest.


  • Be prepared for a difficulty intubation AND a difficult mask ventilation.
  • Induction may cause airway collapse, leading to upper airway obstruction.
  • Consider awake intubation (with minimal to no sedation): avoids airway collapse with induction.
  • Consider having a tracheostomy kit & surgeon standing by in case emergent airway management is needed.


  • Combined epidural/general (GA) may be beneficial to decrease GA requirements.
  • Consider a “balanced” GA –> decreases required dose of each agent, so less will be around post-op.
  • Consider using short acting agents (e.g. alfentanyl, propofol, versed, atracurium), and avoid using long acting agents (e.g. morphine, valium, pancuronium)
  • Ventilator:
    • Use large tidal volumes — 15-20 ml/kg ideal body wt.
    • Titrate PEEP to maintain oxygen saturation.


  • Increased mortality — 6.6% vs. 2.7 % in non-obese.
  • Patient controlled analgesia (PCA) can provide good pain relief — dose should be based on IBW .
    • Epidural route is preferred because it allows administration of a smaller dose than the IV route.
  • Decreased lung capacities are expected for at least 5 days postoperatively.
  • Acute airway obstruction is more likely in obese pts. who also have sleep apnea.
  • Increased incidence of wound infection.
  • Increased incidence of deep vein thrombosis and pulmonary embolus (almost 2 times that in non-obese).
  • Measures to avoid pulmonary complications:
    1. Keep pt. in semi-recumbent position (30 degrees – 45 degrees).
    2. Use humidified gases; Start chest physical therapy (P.T.) early.
    3. Nocturnal use of nasal continuous positive airway pressure (CPAP) at 10-15 cm H2O, if there is presence of Obstructive Sleep Apnea.
    4. Extubate only when fully awake. Consider having a surgeon standing by for emergency tracheostomy (especially if the patient was a difficult intubation).

Considerations in Obstetrics:


  • Increased risk of chronic htn, pregnancy induced htn (preeclampsia) and diabetes (2 to 8 fold increase in incidence).
  • Greater likelyhood of difficulty in labor, or abnormal labor. A higher incidence of induced labor, and an increased incidence of cesarean section (c/s).
  • Weight gain & maternal diabetes may increase the incidence of fetal macrosomia, with attendant risks and difficulty in delivery.
  • Possibility of greater blood loss during c/s, the surgery tends to be longer, and the incidence of post-op erative complications tends to be higher.
  • Increased risk of anesthesia related maternal morbidity/mortality during c/s, when compared with non-obese pts.
  • Increased risk of fetal morbidity/mortality. Some studies show a higher incidence of fetal distress.
  • Cephalad retraction of panniculus in morbidly obese during c/s may lead to hypotension & fetal compromise, as well as maternal difficulty in breathing (secondary to extra weight on the chest).
  • Loss of intercostal muscle function during spinal anesthesia may create greater breathing problems in the obese parturient, when compared with the non-obese pt.
  • Supine and trendelenburg positions may further decrease FRC, increasing the likelyhood of hypoxemia.
  • Use of PEEP to increase oxygenation may decrease cardiac output, and possibly compromise uterine blood flow.


  • Some studies show a greater cephalad spread of local anesthetics during spinal anesthesia, although some do not.
    • The consequences of excessive blockade dictate caution when selecting a single dose spinal in an obese patient with a difficult airway.
  • Increased surgical time of c/s should be a consideration when selecting an anesthetic.
  • Higher incidence of failed epidurals in the morbidly obese has been noted in some studies.
    • Make sure the catheter is reliable, BEFORE depending on it in an emergency!
  • Advantages of selecting an epidural:
    1. Slower onset: ability to titrate, less hypotension.
    2. Potential for less motor blockade.
    3. One study showed a decreased incidence of thromboembolic phenomenon after total hip surgery (unproved in obstetric patients).
    4. Facilitates post-operative analgesia.
  • Anticipate a difficult laryngoscopy secondary to large breasts, poor neck range of motion & a decreased chin to chest distance.
    • The mother’s well being should be considered first: a rapid sequence induction should not be attempted if the intubation is anticipated to be difficult.
    • Consider use of a short handled laryngoscope, or a fiberoptic intubation.
  • Anticipate a more rapid oxygen desaturation than that in non-obese (and non-pregnant) pts.
    • Pre-oxygenate with 100% oxygen for 3 minutes of tidal ventilatio

n (better), or 4 full vital capacity breaths (acceptable in emergencies).

  • Anticipate difficulty in securing emergent jet ventilation via cricothyrotomy, secondary to poorly defined landmarks.
  • Postoperative hypoxemia is more severe in obese pts, and the incidence is increased with a vertical incision.
    • This can be reduced with supplemental oxygen administration and a semi-recumbent position.