ANESTHESIA AND THE MORBIDLY OBESE

Equations:

• Estimated Ideal Body Weight (wt) in kilograms (Kg) = Height (ht) in cm - [[100 for men] or [105 for women]] {1}.
• Body Mass Index (BMI) = Wt in Kg/([Ht in Meters (M)]squared)--(normal is around 24) {2}

Definitions:

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

Incidence: {1}

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

Pathophysiology:

Cardiovascular:

• 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.

Respiratory:

• 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.

GI:

• 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}
  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. {2} Incidence of "halothane hepatitis" is allegedly higher in obese patients.{2}

Anesthetic Management:

Pre-Op:

• 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.

Intra-op:

• 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%). {3}
• 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.

Induction:

• 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.

Maintenance:

• 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.

Post-op:

• 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: {3}

Problems:

• 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.

Suggestions:

• 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 handeled 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