ANESTHESIA AND THE MORBIDLY OBESE
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.
- 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:
- Sux: some suggest–dose/total wt., others suggest– 120-140 mg absolute dose for all patients.
- Pancuronium: low lipid solubility, dose/total wt.
- Vecuronium: dose/lean body wt. — recovery is prolonged.
- Atracurium: dose/total wt–recovery time is unaffected.
- Fentanyl: dose/total wt.
- Alfentanyl: dose/lean body wt.
- Benzodiazipines: dose/total wt.
- Thiopental: highly lipophilic–use higher absolute dose–expect longer duration of action.
- Intravenous (IV) Lidocaine: dose/total wt.
- Epidural/spinal local anesthetics: dose/total body wt, but decrease dose by 20%-25%.
- 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.
- 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%).
- 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)
- 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:
- Keep pt. in semi-recumbent position (30 degrees – 45 degrees).
- Use humidified gases; Start chest physical therapy (P.T.) early.
- Nocturnal use of nasal continuous positive airway pressure (CPAP) at 10-15 cm H2O, if there is presence of Obstructive Sleep Apnea.
- 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:
- Slower onset: ability to titrate, less hypotension.
- Potential for less motor blockade.
- One study showed a decreased incidence of thromboembolic phenomenon after total hip surgery (unproved in obstetric patients).
- 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.