610 – Anesthesia and Obesity

– Also called Quetelet’s index
– Can be used to estimate degree of obesity
– BMI = weight in kg/(height in m)²

BMI Measurements
– < 18.5 = Underweight - 18.5-29.9 =Normal - Overweight - 30-39.9 = Obesity (I & II) - ? 40 = Morbid Obesity (III) - ? 50 = Super Obesity - ? 60 = Super-super obesity

Ideal Body Weight (IBW)
– Concept of life insurance companies
– Weight associated with lowest mortality rate for given height and gender
– Estimated using Broca’s index
– IBW (kg) = height (cm) – x;
– x = 100 for adult males & 105 for adult females

Predicted Body Weight (PBW)
– MEN: 50 + 0.91 X (CM HT – 152.4)
– WOMEN: 45.5 + 0.91 X (CM HT – 152.4)

Lean Body Weight (LBW)
– Total body weight (TBW) minus adipose tissue
– Approximately 80% TBW for males & and 75%
TBW for females
– Increase IBW by 20% to 30% gives estimate of
LBW – in morbidly obese patients (more muscle from being so fat)

Weight from lbs to kg
– 1 kg = 2.2 lbs
– 150 lbs/2.2 = 68.18 kg

Height from feet to cm
– 1 m = 3.26 feet
– 6’2″/3.26 = 1.88 m or 188 cm

Height from inches to cm
1 in = 2.54 cm
– 5’4″ x 2.54 = 64 x 2.54
= 162.5 cm
– 170 cm/2.54 = 66.9 in

Types of Obesity
Android (central)
Gynecoid (peripheral)

Android (central) obesity
Android (central) obesity
– Adipose located in upper body (truncal)
– Associated with increased oxygen consumption & cardiovascular disease
– Visceral fat associated with cardiovascular disease & left ventricular dysfunction

Gynecoid (peripheral) obesity
– Adipose located predominantly in the hips,
buttocks, & thighs
– Less metabolically active

Medical Tx for Obesity
– Intended to reduce energy intake, increase energy use, or decrease absorption of nutrients
– Indicatations: BMI ?30 kg/m² or BMI 27 – 29.9 kg/m² with obesity-related medical complication

FDA Approved Obesity Meds
– Phentermine
– Orlistat

Phentermine (Adipex-P)
– A sympathomimetic that decreases appetite
– Approved for 3 month use
– Causes tachycardia, palpitations, & hypertension;
dependence, abuse, & withdrawal symptoms
– Used in combination with fenfluramine (Combo known as Phen-Fen): Caused pulmonary hypertension and valvular heart disease and Topamax: May cause dry mouth, paresthesias,
constipation, insomnia, and dizziness

Orlistat (tetrahydrolipstatin, OTC: Alli, prescribed:
– Inhibits lipases in the GI tract, decreasing fat
– Leads to weight loss & improved BP, fasting BS and lipid profile
– Fat malabsorption causes common complaints of oily spotting, liquid stools, fecal urgency, flatulence,& abdominal cramping
– Chronic use may result in fat-soluble vitamin
deficiency (NAKED without the N)

Surgical Treatment: Bariatric Surgery
– Most effective treatment for morbid (class III)
– Guidelines for patient eligibility: BMI >40 kg/m² or BMI >35 kg/m² and/or obesity-related comorbidities not controlled with medical tx

Types of Bariatric Surgery
– Malabsorptive procedures: jejunoileal bypass
& biliopancreatic diversion
– Restrictive/Partitioning procedures: verticalbanded gastroplasty and adjustable gastric banding (LAP BAND)
– Combined procedures: Combined gastric restriction with a minimal degree of malabsorption (RYGP)

Respiratory Pathophys
– Decreased chest wall compliance due to fat accumulation on thorax & abdomen
– Decreased lung compliance due to increased pulmonary blood volume
– Decreased functional residual capacity, vital capacity & total lung capacity
– Increased elastic resistance and decreased compliance further impaired by recumbent positions
– Shallow and rapid breathing; increased work of
– Decreased maximum ventilatory capacity
Increased oxygen consumption & CO2 production – due to metabolic activity of excess fat &
increased workload on supportive tissues – both cardiac output & alveolar ventilation increase
– Basal metabolic activity WNL – normocapnia maintained
– Decreased arterial oxygen tension – leading to polycythemia, pulmonary hypertension, & cor pulmonale

Cardiovascular Pathophysiology
– Total blood volume increased – Renal and splanchnic blood flow increased
– Cardiac output increased – ventricular dilation & increased stroke volume
– Increased left ventricular wall stress –> Leading to LV hypertrophy, reduced compliance, & impaired LV filling
– Eventually, biventricular heart failure occurs
– Many patients have mild to moderate HTN due to renin-angiotensin system –> ncreased levels of angiotensinogen, aldosterone and angiotensin-converting enzyme
– Normal to increased SNS activity –> leading to insulin resistance, HTN and dislipidemia
– Adipose releases a large number of bioactive
mediators – abnormal lipids, insulin resistance, inflammation & coagulopathies(hypercoagulability)

EKG changes with Obesity
– Low QRS voltage – cuz of fat
– L atrial enlargement
– T-wave flattening in inferior & lateral leads

Lean Body Weight Used for What Drugs?
– Drugs mainly distributed to lean tissues
– For maintenance – drugs with similar clearance values in both obese and nonobese individuals
– Propofol (induction), NDMRs, fentanyl, sufentanil, remifentanil

Total Body Weight used for What drugs?
– Drugs equally distributed between adipose and lean tissues
– For maintenance – drugs whose clearance increase with obesity
– Propofol (maintenance), succinylcholine, dexmedetomidine, neostigmine

Obesity Preop eval
– Anatomic changes associated with obesity
– Upper thoracic and low cervical fat pads
– Excessive tissue folds in mouth and pharynx
– Short, thick neck
– Thick submental fat pad
– Suprasternal, presternal, and posterior cervical
– Large breasts in females

Obesity and Airway Issues
– Obesity is risk factor for difficult mask ventilation and airway management
– BMI by itself does not have much predictive value for difficulty

Difficulty Correlates With
– Increased age, male gender, temporomandibular
joint pathology, Mallampati classes 3 and 4, OSA,
and abnormal upper teeth
– Neck circumference – single biggest predictor
– Male gender, a higher Mallampati score, laryngoscopy grade 3 views, and OSA

Preop Cardiac Evaluation for Obesity
– Evaluate for systemic hypertension, pulmonary
hypertension, right and/or left ventricular failure &
ischemic heart disease
– Pulmonary hypertension d/t chronic pulmonary
impairment – S/s exertional dyspnea, fatigue, & syncope
– Echo: tricuspid regurgitation
– ECG: RVH = tall precordial R waves, right axis deviation, & right ventricular strain
– CXR: Underlying lung disease & prominent pulmonary arteries

OSA & Obstructive Hypoventilation Syndrome (OHS)
– Recognize early d/t airway management issues &
associated increased perioperative pulmonary
– Hx of HTN &/or neck circumference >40 cm
– OSA is a legitimate reason to delay surgery for a proper workup & should be treated as inpatients
– Home CPAP devices should be brought for use pre, peri, or post-operatively
– Routine pulmonary function tests are not cost effective in asymptomatic patients

Metabolic Issues with Obesity
– High prevalence of insulin resistance & diabetes
– Consider glucose checks & correct
– Assess therapies, last time & dose, usual BS
– Check electrolytes
– Elevated liver function tests (particularly alanine aminotransferase) – No clear correlation between abnormalities & the capacity to metabolize drugs

Postop Metabolic Issues With Obesity
– Nutritional deficiencies include vitamin B12, iron,
calcium, folate, & chronic vitamin K deficiency
– Myelopathy associated with vitamin B12 and copper deficiencies

– APGARS: Acute postgastric reduction surgery
– Postop polyneuropathy characterized by protracted postoperative vomiting, hyporeflexia, and muscular weakness
– Differential diagnoses include thiamine deficiency (Wernicke encephalopathy, beriberi), vitamin B12
deficiency, & Guillain-Barré syndrome
– Requires judicial dosing & monitoring of NMBs

Hematologic Issues
– Risk of perioperative thromboembolic events; plan for thromboprophylaxis
– Recommended: intermittent pneumatic
compression devices with heparin (unfractionated
or low molecular weight heparin) for patients
undergoing bariatric surgery
– Prolonged postoperative thromboembolic
prophylactic regimen (1 to 3 weeks) may be

Assessing DVT RISK
1) Venous stasis disease
2) BMI ? 60
3) Central obesity
4) OHS and/or OSA
– Preoperative IVC filter should be considered in
addition to anti-thrombolitic therapy

– Compression boots during surgery
– Heparin
– Sometimes 1-3 weeks

– Regular OR tables have max weight limit of 200 kg
– Bariatric tables capable of 455 kg
– Have greater width/side accessories to accommodate the extra girth
– Bean bags and straps keep patients positioned and stable
– Ensure adequate padding between positioning
devices and patient skin

Monitoring in Obesity
– BP falsely elevated if a cuff is too small
– Choose cuffs with bladders** that encircle a
minimum of 75% of the upper arm circumference
– Forearm BP overestimate both systolic and diastolic BPs
– ABP indicated for the super morbidly obese patient, presence of cardiopulmonary disease, & improperly fitting NIBP cuffs
– Central venous catheterization for patients with
inadequate peripheral access

Airway Management in Obesity
– Preoxygenation is vital
– Rapid desaturation occurs after loss of spontaneous respiration
– Four vital capacity breaths with 100% oxygen,
within 30 seconds of induction
– Head-up position, or semi-sitting position – Delays hypoxia w/ apnea
– Prevent pulmonary aspiration of gastric
– Preoperative use NIPPV and/or PEEP

Airway Management Continued
– Awake FOI for anticipated difficult intubation
– Avoid hypoxia and aspiration
– Induce with experienced colleague
– ENT available for surgical airway
– “Ramped” or “stacked” position elevates the upper body and improves laryngoscopy
– Ear to sternal notch positioning

Other Techniques for Intubating A Biggy
Other Techniques for Intubating A Biggy
– Intubating stylets – Eschmann stylet, Cook Airway Exchange catheter, bougie
– Intubating LMAs
– Video laryngoscopes – Glide Scope, C Mac
– Repeated laryngoscopy and attempts at
intubation increase airway and hemodynamic complications
– Limit conventional intubation attempts to three

Aspiration Risk in Obese Patients
– Major concerns: hypoxemia, regurgitation,
– Increased gastric volume & acidity and delayed
gastric emptying
– Increased abdominal mass causes antral
distension, gastrin release, and decreased pH
– Increased intragastric pressure, increases
frequency of transient lower esophageal
sphincter relaxation, and/or hiatal hernia
– At risk for severe pneumonitis with aspiration

T/F – You should RSI every obese patient
FALSE – it should be individualized

Induction of Obese Patients
– Induction agents: larger doses d/t increased blood volume, muscle mass, and cardiac output – Propofol & thiopental: based on LBW
– Muscle relaxants: Succinylcholine is the MR of choice due to rapid onset & limited duration of action – increased dose on TBQ due to increased psudocholinesterase activity
– NDMRs Rocuronium & vecuronium: based on LBW, Atracurium and cisatracurium: based on TBW (ROC/VEC LBW, AT/CIS = TBW)

Maintenence of Obese Patients
– Inhalation agents: desflurane, sevoflurane, & isoflurane are minimally metabolized
– Desflurane: faster washout, possibly better
hemodynamic stability
– Use of N2O limited by high O2 demand
– Intravenous agents: short-acting opioids are preferred (remifentanil and fentanyl)
– Remi/Fent are based on LBW and are titrated to clinical effect; most common choices
– Dexmedetomidine: based on TBW

Which meds are based on TBW?
– Precedex, propofol (maintenence), sux, reversal

What meds are based on LBW
– Remi, sufentanil, fentanyl, propofol (induction), NDMRs, LOCAL ANESTHETICS

Use of OGTs and Temp Probes
– Anesthesia will place various devices to help with surgery (intragastric bougie or baloon, OGTs, etc)
– When placing ensure adequane MR, ensure a tight seal on cuff, and ensure all devices and tubes are COMPLETELY removed before they staple. Do not just pull them back (this is how a OGT gets stuck in someones belly and they have to open again)

Supine Positioning in Obesity
– Ventilatory impairment – decreased FRC and oxygenation
– PEEP & head-up position decrease alveolar-
arterial oxygen tension difference
– PEEP & head-up position decrease CO – counteracting the benefits
– Compression of IVC & arota

Prone Positioning in Obesity
– Ensure freedom of abdomin to prevent impaired
ventilation, & oxygenation

Lateral Positioning in Obesity
– Allows for better diaphragmatic excursion
– Preferred over prone, if surgical procedure permits

Padding the obese pt
– Susceptible to pressure sores, neural injuries and

Nerve injuries in the obese
– Brachial plexus & LE nerve injuries occur frequently
– Carpal tunnel syndrome is the most common
mononeuropathy after bariatric surgery
– Other: encephalopathy (Wernicke) & optic neuropathy

Wernicke encephalopathy (WE)
– A neurological disorder induced by thiamine, vitamin B1, deficiency
– WE is the most important encephalopathy due to a single vitamin deficiency.

Fluid Management in the Obese
– Difficult to assess
– Blood loss usually greater d/t technical difficulties
– Normovolemia should be the goal
– Avoid hypervolemia-associated complications –> congestive heart failure, peripheral edema, &
pulmonary complications.
– Rapid infusion of intravenous fluids should be
avoided because pre-existing congestive cardiac
failure (they are fat, dont just expect their hearts to work normal)

Mechanical Ventilation on Obese Pts
– Exposed to higher TVs d/t miscalculation of body
– Experience higher airway pressures d/t decreased compliance
– Greater inflation pressures may be tolerated as extra adipose tissue partially attenuates lung over-distension
– TVs should be maintained at less than 13 mL/kg
– No specific ventilatory mode (e.g., volume vs. pressure control ventilation [PCV]) has been found significantly better
– Like any person, you have to see how they handle different modes

PEEP in Obese Pts
– PEEP has been shown to consistently improve
respiratory function
– Moderate PEEP (10 cm H2O) w/ recruitment
– Recruitment maneuvers = intermittent large TV breaths during the case that open and maintain patency of small airways
– They improve V/Q & oxygenation
– Inspired O2 should be titrated to minimum levels

Emergence on Obese Pts
– Prompt extubation reduces likelihood of post-op
mechanical post op ventilation
– Extubated in a sitting position
– Consider extubating over exchange catheter for
difficult intubations

Regional Anesthesia for Obese Pts
– Can use any technique alone or in combo with GA
– Usual adavantages of RA
– Disadvantages – increased block failure, exaggerated pulm and CV compromise, catheters may become misplaced with mocement
– Decrease dose of LA for SAB
– LA dose is based on IBW for PNBs

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