Obesity & AnaesthesiaDr Ikhwan Wan Mohd Rubi MD (UKM)MO Anaesthesiology, HSNZ

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1/6 Malaysians are either overweight/obese- Malaysia Ministry of Health

• “Malaysia is leading in the prevalence of obesity among Southeast Asian countries. Almost one in two Malaysians are either overweight or obese, placing them at a high risk for diabetes,”says Datin Paduka Santha Kumari, chairman of the Selangor branch of the Malaysian Diabetes Association.

Classical description of obesity

Intraperiotenal fat

(liver,omentum)

Peripehral fat(arms,legs,buttoc

k)

Waist hip ratio: >0.94 in men >0.8 in women

Health risk associated with obesity (Evidence Level B)

Comorbid• BMI alone is a poor predictor of comorbidity, surgical and

anesthetic difficulty• Fat distribution (waist/collar circumference) > predictive of

CVS/Respiratory comorbid• Android fat distribution

• Makes intra-abdominal surgery > difficult• Greater difficulty in airway management/ventilation• Greater risk of metabolic and CVS complication

• Risk of comorbid increases the duration of obesity (fat years)• Presence and severity of comorbid may be masked by

sedentary lifestyle• TRUE significance obesity related illness may only emerge

during perioperative phase.

RESPIRATORY SYSTEM• OSA• Obesity Hypoventilation Syndrome• Airway Assessment• Obesity and gas exchange• Lung volume• O2 consumption and CO2 production• Gas exchange• Lung Compliance and resistance• Respiratory efficiency and work of breathing• Implication for anesthesia

CVS SYSTEM• Cardiovascular derangemnt• HPT• IHD• Blood Volume• Cardiac arrhythmia• Cardiac function

Obstructive Sleep Apnea• Up to 5% of obese

patients have clinically significant obstructive sleep apnea

• Apnea is defined as 10 seconds or more of total cessation of airflow despite continuous respiratory effort against a closed glottis

ObesityEffects on Blood Volume

• Total blood volume is increased in the obese, but on a volume-to-weight basis, it is less than in nonobese individuals(50ml/kg compared to 70ml/kg)

• Most of this extra blood volume is distributed to the fat organ

ObesityCardiovascular Effects

• Cardiac output increases as much as 20 – 30 ml/kg of excess body fat secondary to ventricular dilatation and increasing stroke volume

• The increased left ventricular wall stress leads to:

• Hypertrophy• Reduced compliance• Impaired left ventricular filling • Obesity cardiomyopathy

ObesityEffects on Gastrointestinal System

• Gastric volume and acidity are increased• Most fasted morbidly obese patients presenting for

elective surgery have gastric volumes in excess of 25 ml and gastric fluid pH less than 2.5 ( the generally accepted volume and Ph indicative of high risk for pneumonitis should regurgitation and aspiration occur).

• Gastric emptying may actually be faster in the obese, but because of their larger gastric volume (up to 75% larger), the residual volume is larger.

ObesityObesity and Diabetes

• Impaired glucose tolerance in the morbidly obese is reflected by a high prevalence of type II diabetes mellitus as a result of resistance of peripheral fatty tissues to insulin

• Greater than 10% of obese patients have an abnormal glucose tolerance test, which predisposes them to wound infection and an increased risk of myocardial infarction during periods of myocardial ischemia

ObesityEffects on the Airway• Anatomic changes that contribute to potential for

difficult airway management• Limitation of movement of the atlantoaxial joint and cervical

spine by upper thoracic and low cervical fat pads• Excessive tissue folds in the mouth and pharynx• Short thick neck• Suprasternal, presternal and posterior cervical fat• Very thick submental fat pad

• Obstructive sleep apnea• Predisposes to airway difficulties during anesthesia• OSA patients have excess tissue deposited in their lateral

pharyngeal walls which may not be recognized during routine airway examination

Preoperative Assessment(AAGBI/SOBA)• Obese patient may present for

ELECTIVE/EMERGENCY surgery/ Obstetric Analgesic/Anesthesia

• Similar between ELECTIVE/EMERGENCY• Multidiscipline- where deem necessary• Respiratory physician, Cardiologist, Endocrinologist,

Dietitian• Specific attention to comorbid• Cardiovascular/Respiratory/Metabolic• Obese patient may have limited mobility, may appear

relatively asymptomatic despite significant cardiorespiratory dysfunction

Preop. Assessment (Respiratory)

• Previous anesthetic experiences• Attention should focus on the cardiorespiratory system

and airway• Assess for Obstructive Sleep Apnea and Obesity

Hypoventilation Syndrome• STOP – BANG (5 or more)• Snoring (loudly)• Tired (often tired/sleepy at day time)• Observed (has anyone observed you stop breathing during sleep)• Pressure (has you been treated for/ has high blood pressure)• BMI>35kg/m2• Age>50 years old• Neck Circumference >40cm• Gender-Male

Preoperative Assessment (Respiratory)

Preoperative Assessment (Cardiovascular)• Check for HPT• Assess for IHD (angina/Exertional dyspnea)• Assess symptoms and sign of cardiac failure• Effort tolerance (walk to the length of the ward)• Ability to lie flat/supine• Position of sleeping• Orthopnea/ Paroxysmal Nocturnal Dyspnea

Airway Assessment• Head and Neck flexion/Extension/lateral rotation (cervical limit)• Jaw mobility/Mouth opening• Oropharynx (excessive palatal and pharnygeal soft tissue) and

dentition • Patency of nostril• Previous anaesthetic experiences• Mallampati score• Neck Circumference (>17.5in/40cm)

• The single biggest predictor of problematic intubation in morbidly obese patients

• 40 cm neck circumference = 5% probability of a problematic intubation• 60 cm neck circumference = 35% probability of a problematic intubation

• Fat face & cheeks, large breast, short neck, large tongue, high anterior larynx

Airway assessment• Role of imaging if time permit - soft tissue xray/CT

scans with consultation with Otolaryngologist for direct/indirect laryngoscopy

• Consider and discuss re: Awake fibre optic intubation with patient

Anticipate difficult ventilation/intubation

Investigation• Tailored to individual (comorbid/type/urgency of

surgery)• FBC, Electrolytes, Renal, Liver function, Blood

Glucose (Basic)• Arterial Blood Gas (maybe useful) in suspected

respiratory comorbid (OSA, OHS, pulmonary disease)-provide guide to weaning and expecting postoperative respiratory support

• Preoperative ECG (to exclude significant rhythm disturbances, cor pulmonale, guide for further extensive study

• Echocardiography• Transthoracic may be difficult (poor window)• May estimate systolic and diastolic function, chamber

dimension• CXR - assess CTR, evidence of cardiac failure• Pulmonary function test – may reveal restrictive

pattern but not done on all patients• Exercise ECG testing (stress test)- impracticle

Pharmacokinetics of anaesthetic agenst• Calculation of appropriate dose may be difficult• Should based Actual Body Weight or Ideal Body

Weight?• Most PF of anaesthetic agents influenced by mass

of adipose tissue, producing prolonged and less predictable effect

• Volume of central compartment is largely unchanged

• BUT dosages of lipophilic drugs need to be adjusted due to changes in Vd

• Less fat soluble drugs show little or no change in Vd• lean body mass/ IBW + 20%

• Exception: Scolene based on ABW, sames as

ObesityEffects on Drug Distribution

• Volume of Distribution in Obese patients is affected by:

• Reduced total body water• Increased total body fat• Increased lean body mass• Altered protein binding• Increased blood volume• Increased cardiac output

ObesityEffects on Drug Elimination

• Hepatic clearance is not usually effected• Renal clearance of drugs is increased in obesity

because of increased renal blood flow and glomerular filtration rate

ObesityHow does it effect drug dosing?• Highly Lipophilic

• Barbiturates and benzodiazepines have an increased volume of distribution

• Less Lipophilic• Little or no change in volume of distribution with obesity

• Increased blood volume in the obese patient decreases the plasma concentrations of rapidly injected intravenous drugs.

• Fat has poor blood flow and doses calculated on actual body weight could lead to excessive plasma concentrations.

* Review Barash et al table 47-5*

ObesityInduction of General Anesthesia• Adequate preoxygenation

• Rapid desaturation because of increased oxygen consumption and decreased FRC

• Positive pressure ventilation during preoxygenation decreases atelectasis formation and improves oxygenation

• Patient position• The head-up (reverse tredelenburg) position provides

the longest safe apnea period during induction of anesthesia

ObesityPatient positioning• Supine

• Causes ventilatory impairment and inferior vena cava and aortic compression

• Trendelenburg• Further worsens FRC and should be avoided

• Reverse tredelenburg• Increased compliance results in lower airway pressures

• Prone • Detrimental effects on lung compliance, ventilation and

arterial oxygenation• Increased intra-abdominal pressure worsens IVC and

aortic compression and further decreases FRC

Obesity Ventilating the obese patient

• Tidal volumes greater than 13 ml/kg offer no added advantage

• Increasing tidal volume beyond 13 ml/kg increases PIP without improving arterial oxygen tension

• Positive end-expiratory pressure (PEEP) is the only ventilatory parameter that has consistently been shown to improve respiratory function in obese patients

• PEEP may reduce venous return and cardiac output

The Future is BIG!