Obesity is defined in terms of body mass index (BMI). Body mass index = Body weight (kg) divided by Height2 (meters) Ideal weight is estimated as height (cm) -100 =weight (kg) e.g. height of 5 feet is 150 cm -100 = 50 kg. (for females use height -105) Normal BMI : 18.5-22.9. Overweight: 23-29.9. Obesity Class : I: 30-34.9, II: 35-39.9, III >40 (moderate, severe, high health risk) Morbid obesity : > 35 or body weight > ideal weight + 45 kg Visceral fat : Waist circumference > 102cm in men and >88cm in women increase the risk of CVS disease and type II diabetes. PATHOPHYSIOLOGY INCREASED TISSUE MASS Respiratory System I\ r~~r·~ airway closure 1 iwork of breathing Metabolic Rate . I \ . . iventilatory i Circulatmg requirement blood volume ioxygen/ 1 consump~on i Pulmonary i Cardiac blood output isv volume Alveolar Hypoxic \ hypoxia pulmonary i Pulmonary Cardiovascular i HypT""" i L.V. work l L.V. dysfunction l vasoconstriction blood flow ~ / R.V. Arterial hypoxaemia PHARMOCOKINETICS Pulmonar dysfunction hypertension Fat which has a limited blood supply does not play a part in the acute phase of drug distribution and elimination. Water soluble drugs e.g. muscle relaxants : no difference in volume of distribution (VD), clearance, or half life Fat soluble drugs have increased VD and half life, with normal clearance 8.10
Carefully titrate drug against effect. • Drugs to be given in dose I total body weight : benzodiazepines, opioids, lignocaine. Preferably do not exceed 30% above ideal body weight for the initial dose. • Drugs to be given in dose I ideal body weight : muscle relaxants, propofol, remifentanyl. PRE-OPERATIVE ASSESSMENT PROBLEMS Problem Signs and symptoms Test 1. Obstructed airway Sleep disturbance Sp02 during sleep Snoring 2. Difficult intubation Assess for difficulty 3. Respiratory Cyanosis Chest radiograph disease Dyspnoea LFfandABG 4. Cardiovascular Dyspnoea Blood pressure disease Orthopnoea ECG and stress test Angina Telechest Effort intolerance Echocardiogram 5. Gastrointestinal Hiatus hernia Relevant disease Incompetent LOS investigations Gastric pH < 2.5 High gastric volume 6. Metabolic Diabetes mellitus History disease Glucose intolerance Blood glucose Identify patients with sleep apnoea. (see page 4.32) Additional problems : Venous access, sepsis, moving patient and positioning, difficult intubation, reflux, aspiration, reduced FRC, prolonged surgery and difficulty in performing regional blocks, impaired immune function and healing, DVT, arthritis PRE-OPERATIVE PREPARATION Check availability of staff, larger sizes of tables, trolleys, equipment, BP cuffs etc. Control of medical diseases Chest physiotherapy Assessment for difficult intubation (see page 4.1)
PRE-MEDICATION Anxiolysis and antacid prophylaxis (see page 4.29) Thromboprophylaxis (see page 1. 7) Anticholinergics if awake intubation is planned INTRA-OPERATIVE PERIOD Equipment Large sized table, appropriate size of blood pressure cuff (see page 3.3) Difficult intubation trolley with large size equipment (see page 4.16) Venous access with infusion even for minor surgery REGIONAL ANAESTHESIA • Difficult identification of landmarks and positioning of patient • Dose and volumes should be scaled down as there is a greater tendency to higher levels of block both in sub-arachnoid block due to compression of the space and epidural block due to the increase in epidural fat. • CSE is best if titrated to effect. • A void high regional blocks which may embarrass respiration. • Regional blocks are excellent for lower limb and lower abdominal surgery. • Always be ready to convert to GA with IPPV. • Management of combining regionals with thromboprophylaxis (see page 5.11) GENERAL ANAESTHESIA Possible expected complications during anaesthesia • Difficult intubation especially in those with sleep apnoea (see page 4.32) • Difficult positioning (see page 4.5) • Hypoxaemia and respiratory inadequacy • Peri-operative hypertension, myocardial ischaemia and left ventricular failure Monitoring Pulse, BP (intra-arterial maybe needed), ECG with ST analysis, Sp02, ETC02 Pre oxygenation for 5 minutes in 25° ramped position if BMI > 45 Positioning Avoid Trendelenburg position except for the most minor procedures. Avoid compression of nerves and blood vessels by padding. Lumbar support to prevent backache DVT stockings I intermittent pneumatic compression for DVT
Airway ETI for all types of surgery. Ensure ramped position (see page 4.5) LMA is best avoided for IPPV due to risk of aspiration and high pressures required. Awake intubation is recommended if weight is > 1. 75 x ideal body weight. For intubation under anaesthesia use suxamethonium (difficult intubation) and cricoid pressure (risk of regurgitation). Ventilation IPPV with relaxant drugs (dose I ideal body weight titrated to effect) is required to prevent hypoventilation. Give 50% oxygen to prevent hypoxaemia. Drugs Establish adequate depth of anaesthesia with narcotic and inhalational agent to avoid increase in cardiac output due to light anaesthesia. Narcotics must be titrated but may cause hypoventilation postoperatively. Propofol and isoflurane allow rapid recovery (halothane has a large fat depot) Reversal and recovery Ensure recovery from relaxant and narcotics. Do not reverse muscle relaxant until respiratory movements begin or TOF >3. This ensures that post operative hypoventilation is not due to inadequate reversal. Do not reverse the patient in the Trendelenburg or lithotomy position. Prop up after reversal and recover in the head up or semi-recumbent position. Keep ETI in trachea or naso-pharynx or use a LMA till fully awake, since maintenance of the airway is difficult in the semiconscious patient. If ventilation is inadequate after reversal (with no lung signs) consider: • Residual neuromuscular block (unlikely if the limbs move freely). • Frusemide 80mg as there maybe pulmonary interstitial oedema due to early L VF. • Narcotic overdose (breathing will be deep and slow) POST-OPERATIVE PERIOD 1. Maintain the airway (oro I nasopharyngeal airway I LMA). 2. Supplemental oxygen should be continued in the ward and CPAP in HDU. Post-operative hypoxemia is common lasting up to 6 days for abdominal surgery. 3. Keep the patient in the semi recumbent position throughout. 4. Analgesia is best with regional techniques to avoid hypoventilation. Give opiates as i.v. infusion or PCA as absorption is inadequate with s.c. or i.m. 5. Physiotherapy for chest and limbs and early mobilization.