Physiologic and Pharmacologic Consequences of Obesity

Physiologic and Pharmacologic Consequences of Obesity and its Anesthetic Implications Andrew R. Biegner CDR, NC, USN Naval Medical Center, San Diego Chief Anesthetist Things I’ve Learned From My Children • Garbage bags do not make good parachutes. • When you hear the toilet flush and the words “Uh-oh,” it’s already too late. • Super glue is forever. • VCR’s do not eject PB&J sandwiches even though TV commercials show they do. • A king size waterbed holds enough water to fill a 2000 sq. foot house 4 inches deep. Objectives • Describe the causes of obesity and its social implications. • Discuss the cardiovascular and respiratory changes that occur in morbidly obese patients. • Describe what changes in gastrointestinal and hepatic function can be expected in obese patients. • Review how obese patients metabolize anesthetic drugs differently from non-obese patients. Obesity: Definition • A condition in which excess body fat may put a person at health risk. • A metabolic disorder that is primarily induced and sustained by an over consumption or underutilization of caloric substrate Equations • Ideal body weight in Kg (IBW) – Height in centimeters - 100 for men – Height in centimeters - 105 for women • Body mass index (BMI) – weight in Kg / height (m) 2 Definitions • Obese – 20% > IBW – BMI > 28 – 35 • Morbidly Obese – 2 x IBW – BMI > 35 Height (ft'in") Height (in) 5'0" 60 5'2" 62 5'4" 64 5'6" 66 5'8" 68 5'10" 70 6'0" 72 6'2" 74 6'4" 76 6'6" 78 BM I (k g/m2) 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 370 380 390 400 410 29.4 31.3 33.3 35.2 37.2 39.1 41.1 43.1 45.0 47.0 48.9 50.9 52.8 54.8 56.8 58.7 60.7 62.6 64.6 66.5 68.5 70.5 72.4 74.4 76.3 78.3 80.2 27.5 29.3 31.2 33.0 34.8 36.7 38.5 40.3 42.2 44.0 45.8 47.7 49.5 51.3 53.2 55.0 56.8 58.7 60.5 62.3 64.1 66.0 67.8 69.6 71.5 73.3 75.1 25.8 27.5 29.2 31.0 32.7 34.4 36.1 37.8 39.6 41.3 43.0 44.7 46.4 48.2 49.9 51.6 53.3 55.0 56.8 58.5 60.2 61.9 63.6 65.4 67.1 68.8 70.5 24.3 25.9 27.5 29.1 30.7 32.3 34.0 35.6 37.2 38.8 40.4 42.1 43.7 45.3 46.9 48.5 50.1 51.8 53.4 55.0 56.6 58.2 59.8 61.5 63.1 64.7 66.3 22.9 24.4 25.9 27.4 28.9 30.5 32.0 33.5 35.0 36.6 38.1 39.6 41.1 42.7 44.2 45.7 47.2 48.8 50.3 51.8 53.3 54.9 56.4 57.9 59.4 60.9 62.5 21.6 23.0 24.4 25.9 27.3 28.8 30.2 31.6 33.1 34.5 35.9 37.4 38.8 40.3 41.7 43.1 44.6 46.0 47.4 48.9 50.3 51.8 53.2 54.6 56.1 57.5 59.0 20.4 21.7 23.1 24.5 25.8 27.2 28.5 29.9 31.3 32.6 34.0 35.3 36.7 38.1 39.4 40.8 42.1 43.5 44.8 46.2 47.6 48.9 50.3 51.6 53.0 54.4 55.7 19.3 20.6 21.9 23.2 24.4 25.7 27.0 28.3 29.6 30.9 32.2 33.5 34.7 36.0 37.3 38.6 39.9 41.2 42.5 43.7 45.0 46.3 47.6 48.9 50.2 51.5 52.8 18.3 19.5 20.7 22.0 23.2 24.4 25.6 26.8 28.1 29.3 30.5 31.7 32.9 34.2 35.4 36.6 37.8 39.0 40.3 41.5 42.7 43.9 45.1 46.4 47.6 48.8 50.0 17.4 18.5 19.7 20.8 22.0 23.2 24.3 25.5 26.6 27.8 29.0 30.1 31.3 32.4 33.6 34.7 35.9 37.1 38.2 39.4 40.5 41.7 42.8 44.0 45.2 46.3 47.5 Overweight Obese Morbidly Obese Incidence of Obesity in North America • 33% are obese • 5% are morbidly obese – Mortality is 3.9 times that in non-obese Twenty Years of Increasing Obesity 40% 30.5% 22.9% 13.4% 14.5% 15% % Obesity 30% 20% 10% 0% 1960 1974 1980 1994 2000 Source NCHS -- JAMA 2002:14:1723-27. Causes of Obesity • Complex and multifactorial – – – – – – – – – – – Genetic predisposition Socialization Age Sex Race Economic status Psychological Cultural Emotional Environmental factors Cessation of smoking Diseases Linked to Obesity • • • • • • • • • Diabetes Coronary Heart Disease High Blood Pressure Stroke Arthritis Gastroesophageal reflux Cancer High cholesterol Endocrine disease Diseases Linked to Obesity • • • • • • • • • Hypertrophic Cardiomyopathy Infertility Depression Obstructive sleep apnea Gallstones Fatty liver Stress incontinence Venous ulcers Sudden death Physical Complications of Obesity • • • • • • • • • Heart disease Type II diabetes mellitus Hypertension Stroke Cancer (endometrial, breast, prostrate, colon) Gallbladder disease Sleep apnea Osteoarthritis Reduced fertility Psychological Complications of Obesity • Emotional distress • Discrimination • Social stigmatization Obesity Related Diseases Treated Surgically • • • • • • • • Cholelithiasis Thromboembolism Urolithiasis Osteoarthritis Varicose veins Esophagitis Hiatus hernia Abdominal wall hernia • Cancer – – – – – Obesity Related Diseases Treated Surgically Endometrial Breast Prostate Colorectal Renal • • • • • Fibroadenoma of the breast Uterine fibroma Ovarian cysts Cesarean section Stress urinary incontinence Cardiovascular Pathophysiology • Excess body mass –  metabolic demand   CO •  workload • LVH •  pulmonary blood flow and HPV – Pulmonary HTN  cor pulmonale  right heart failure Cardiovascular Pathophysiology • Stroke volume index and stroke work index are the same as non-obese • SV and SW must  – Proportion to body weight •  SV and SW – LVH dilatation Cardiovascular Pathophysiology •  risk of arrhythmias – Hypertrophy – Hypoxemia – Fatty infiltration of cardiac conduction system – Diuretics –  catecholamines – Sleep apnea Cardiovascular Pathophysiology • For every 13.5 kg of fat gained: – 25 miles of neovascularization occurs – Increased CO of 0.1 L/min for each kg of fat. Cardiac Evaluation: Assess For • • • • Prior MI HTN Angina PVD Cardiac Evaluation: EKG • Determination of – resting rate – Rhythm – Ventricular hypertrophy or strain Cardiac Evaluation: EKG • Investigate ischemic changes or evidence of coronary artery disease • Low voltage EKG – Excess overlying tissue • Underestimate LVH Cardiac Evaluation: EKG • Axis deviation and atrial tachyarrhythmias • Sudden cardiac death is more prevalent with – LVH – Ventricular ectopy Cardiac Evaluation • Indications of LV dysfunction – Limitations in exercise tolerance – History of orthopnea – Paroxysmal nocturnal dyspnea Cardiac Evaluation • Testing of exercise tolerance is likely to be impossible if CAD is suspected. – Echocardiography – Dipyridamole-thallium – TEE • Consult cardiologist – Control of BP – Treatment of heart failure – Coronary angioplasty Vascular Access • Challenging at best – Excessive fat obscures blood vessels • Central line placement – Vessels impeded by distortions of the underlying anatomy by adipose. Volume Replacement • Adult total body water percentage is 60% to 65%. • Severely obese total body water is 40%. • Estimated blood volume in obese patient is 45 to 55 mL/kg actual body weight – 70 mL/kg for the non-obese Volume Replacement • Avoid rapid rehydration – Lessen cardiopulmonary compromise. • Administer Hetastarch at recommended volumes per kilogram of IBW – 20 mL/kg • Albumin 5% and 25% used as indicated – Support circulatory volume and oncotic pressure. • Replace blood loss with crystalloid – 3:1 ratio Respiratory Pathophysiology • Excess metabolically active adipose +  workload on supportive muscle –  CO2 production • Hypercarbia –  O2 consumption • Hypoxia Respiratory Pathophysiology • Restrictive lung disease – Decreased chest wall compliance – Diaphragm forced cephalad – Decreased lung volumes – Accentuated by supine and Trendelenberg positions – FRC may fall below closing capacity • Alveolar collapse – Ventilation / perfusion mismatch Changes in Pulmonary Volumes and Function Tests • Tidal volume – Normal or decreased • Inspiratory reserve volume – Decreased • Expiratory reserve volume – Greatly decreased Changes in Pulmonary Volumes and Function Tests • FRC – Greatly decreased – Direct inverse relationship between BMI and FRC • FEV1 – Normal or slightly decreased Respiratory Pathophysiology • Relatively hypoxemic • Occasionally hypercapnic – Obesity-hypoventilation (Pickwickian syndrome) • • • • • • • • Obesity usually extreme Hypercapnia Cyanotic / hypoxemia Polycythemia Pulmonary HTN Biventicular failure Somnolence Obstructive sleep apnea syndrome (OSAS) OSAS • Definition – 10 seconds or more of total cessation of airflow despite respiratory efforts • Clinically relevant – 5 episodes per hour – 30 episodes per night OSAS • Snoring • Dry mouth and short arousal during sleep • Partners report apnea pauses during sleep OSAS • More vulnerable to airway obstruction – Opioids – Sedatives • More vulnerable in supine or Trendelenberg position OSAS and Difficult Intubation • 15% of obese patients are a difficult intubation • Short thick neck • Obesity and short thick neck – Related to OSAS and to each other • Fat in lateral pharyngeal walls are difficult to exam awake Detecting OSAS • Nocturnal polysomnography GI Pathophysiology •  incidence – Gastroesophageal reflux – Hiatal hernia –  abdominal pressure • Severe risk of aspiration GI Pathophysiology • After 8 hour fast – 85 – 90% of morbidly obese patients have • Gastric volumes > 25 ml • Gastric pH < 2.5 Pharmacological Considerations •  volume distribution –  elimination half life •  GFR –  clearance of untransformed drugs •  fat stores – May  requirements for and clearance of fat soluble anesthetics Pharmacological Considerations • More extensive metabolism of volatile anesthetics • Obesity  biotransformation rate – Methoxyflurane – Enflurane – Halothane •  serum fluoride ions – Associated with renal toxicity Pharmacological Considerations • Sevoflurane increases serum inorganic fluorides levels. – Metabolized 100% faster in obese patients • Renal physiology can be negatively affected by elevated fluorides. – Sevoflurane is inappropriate in patients with questionable kidney function. Pharmacological Considerations • Desflurane is the most resistant to hepatic degradation – < 0.02% with Desflurane – 0.2% with Sevoflurane • Desflurane preferred inhalational agent – – – – Low solubility profile Rapid washout Absence of hepatic and renal toxicity Support of blood pressure Pharmacological Considerations •  volume of distribution – Delayed clearance of lipid-soluble drugs – Suggests larger loading doses • Less frequent maintenance doses • Dose based on actual body weight – Opioids – Benzodiazepines Pharmacological Considerations • Water-soluble drugs – Limited volume of distribution • Uninfluenced by fat – Base dose on IBW • Neuromuscular blocking agents • Intravenous anesthetics Anesthetic Considerations: Preoperative •  risk for aspiration pneumonitis – Consider H2 antagonist – Metoclopramide • Avoid unnecessary respiratory depressants • Assess – – – – Cardiopulmonary reserve EKG ABG PFT’s Anesthetic Considerations: Preoperative • BP with appropriate size cuff • Plan / examine for venous / arterial access – Possible regional anesthesia Anesthetic Considerations: Preoperative Airway Assessment • Limited TM joint mobility • Limited atlanto-occipital mobility • Narrow upper airway • Small space between mandible and sternal fat pads Anesthetic Considerations: Induction • Prepare for difficult intubation • Prepare for difficult mask ventilation • Induction may cause airway collapse – Leading to upper airway obstruction Induction Airway Equipment • Light wand • Gum elastic bougie • Oral airway • LMA’s • ETT with stylet Anesthetic Considerations: Induction • Consider awake intubation – Avoids airway collapse – Minimal to no sedation • Consider tracheotomy kit and surgeon standing by Anesthetic Considerations: Intraoperative • Awake fiberoptic intubation if difficult airway suspected • Breath sounds distant – ETCO2 more important • Relatively high FIO2 may be needed in: – Lithotomy – Trendelenberg – Prone Anesthetic Considerations: Intraoperative • Positioning – 2 OR tables side by side • If > 350 lbs – Prone position is poorly tolerated • Lateral decubitus is keeps abdominal weight off chest Anesthetic Considerations: Intraoperative • Morbidly obese patient should never lie flat – Semi-Fowler’s position • Upper body elevated 30 – 40 • Semi-recumbent position – Best position during post-operative period Reverse Trendelenburg Position • RTP is best intraoperative position – Can ameliorate deleterious effects of supine position • RTP –  pulmonary compliance –  FRC – Returned P(A-a)O2 to baseline • RTP may be a better solution than – Large TV and PEEP • Perilli et al. Obes Surg. 2003 Aug;13(4):605-9 Anesthetic Considerations: Intraoperative • Pulmonary compliance and FRC  – Worsened by GETA and high intraabdominal pressure • Opening the abdomen or lifting the panniculus –  FRC – Improves oxygenation Anesthetic Considerations: Intraoperative • Regional anesthesia – Technically more difficult – Require 20 – 25% less LA for SAB or epidural anesthesia • Epidural fat and distended epidural veins • Epidural anesthesia may  postoperative respiratory complications Goals for Maintenance of Anesthesia • • • • • Strict maintenance of airway Adequate skeletal muscle relaxation Optimum oxygenation Avoid residual effects of muscle relaxants Appropriate intraoperative and postoperative tidal volume • Effective postoperative analgesia. Anesthetic Considerations: Postoperative • Respiratory failure risk increased by – Preoperative hypoxia – Thoracic or upper abdominal surgery • Vertical incision • Delayed extubation until – Complete reversal of muscle relaxation – Patient fully awake • Follows commands Anesthetic Considerations: Postoperative • Supplemental O2 after extubation – Transport from OR to PACU • 45 degree head up position – Unload diaphragm – Improves oxygenation – Improves ventilation Anesthetic Considerations: Postoperative • Increased mortality – 6.6% vs. 2.7% in non-obese • Increased risk – Wound infection – DVT – PE Anesthetic Considerations: Postoperative • PCA – Can provide good pain relief – Dose based on IBW • Epidural route is preferred – Administration of smaller dose than IV route