Anesthesia and Obesity
body mass index, BMI
-
BMI = weight/height² in kg/m²
extreme obesity (old "morbid obesity"): BMI > 40
Health risk increases with
- BMI
- abdominal distribution of weight
- men: waist > 40 inches
- women: waist > 35 inches
Clinical Manifestations
- Associated diseases
- type II diabetes
- hypertension
- coronary artery disease
- cholelithiasis
- Physiologic consequences
- metabolic rate is proportional to body weight
- increased O2 demand
- increased CO2 production and alveolar ventilation
- restrictive lung disease
- decreased chest wall compliance
- diaphragm forced cephalad
- decreased lung volumes
- accentuated by supine and Trendelenberg postions
- functional residual capacity (FRC) may fall below closing capacity leading to
- alveolar collapse with ventilation/perfusion mismatch
- often relatively hypoxemic
- occasionally hypercapnic (obesity-hypoventilation or Pickwickian syndrome)
- obesity usually extreme
- hypercapnia
- cyanotic
- polycythemia
- right-sided heart failure (cor pulmonale)
- somnolence
- often have obstructive sleep apnea syndrome (OSAS)
- obstructive sleep apnea syndrome (OSAS)
- snoring
- dry mouths and short arousal during sleep reported
- partners report apnea pauses during sleep
- associated with perioperative
- hypertension
- hypoxia
- dysrhythmias
- myocardial infarction
- pulmonary edema
- stroke
- difficult airway management during induction
- perioperative airway obstruction
- more vulnerable to airway obstruction after opioids or sedatives
- more vulnerable in supine or Trendelenberg position
- consider trial of postoperative coninuous positive airway pressue (CPAP)
- heart
- increased workload
- hypertension
- left ventricular hypertrophy (LVH)
- increased pulmonary blood flow and hypoxic pulmonary vasoconstriction leads to
- pulmonary hypertension and
- cor pulmonale
- gastrointestinal
- hiatal hernia
- gastroesophageal reflux
- poor gastric emptying
- hyperacidic gastrc fluid
- increased risk of gastric cancer
- fatty infiltration of the liver
- elevated liver function tests
- metabolic rate is proportional to body weight
Anesthetic Considerations
Preoperative
- increased risk for aspiration pneumonitis
- consider H2 antagonist (e.g. ranitidine, Zantac) and/or
- metoclopramide (Reglan)
- avoid unnecessary respiratory depressants
- assess
- cardiopulmonary reserve
- chest X-ray
- ECG
- arterial blood gases
- pulmonary function tests
- blood pressure with appropriate size cuff
- plan/examine for venous and arterial access, possible regional anesthesia
- airway
- limited TM joint mobility
- limited atlanto-occipital mobility
- narrow upper airway
- small space between mandible and sternal fat pads
- cardiopulmonary reserve
- awake fiberoptic intubation good choice if difficult direct laryngoscopy expected
- breath sounds distant, ETCO2 more important
- relatively high FIO2 may be needed
- lithotomy
- Trendelenberg
- prone
- more extensive metabolism of volatile anesthetics
- increased volume of distribution (and delayed clearance) of lipid-soluble drugs
- suggests larger loading (and less frequent maintenance) doses
- rationale to dose based on actual body weight
- opioids
- benzodiazepines
- water-soluble drugs
- limited volume of distribution, uninfluenced by fat stores
- rational to base dose on ideal body weight
- neuromuscular blocking agents
- regional anesthesia
- technically more difficult
- usually need 20-25% LESS local anesthetic for spinal or epidural anesthesia because of epidural fat and distended epidural veins
- epidural anesthesia may lessen postoperative respiratory complications
- respiratory failure risk increased by
- preoperative hypoxia
- thoracic or upper abdominal (especially with vertical incision) surgery
- delay extubation until
- complete revesal of muscle relaxation
- patient awake, following commands
- provide supplemental O2 after extubation (including during transport from OR to recovery room)
- 45-degree head up position helps
- unload diaphragm
- improve oxygenation and ventilation
- wound infection risk increased
- deep venous thrombosis risk increased
- pulmonary embolism risk increased
See also: Morbid Obesity and Gastric Bypass