2017B06 Describe the effects of morbid obesity on the respiratory system.

 

List:

·       Intro

·       Anatomy

·       Lung volumes

·       Mechanics

·       Resistance and compliance

·       Ventilation and perfusion

·       Gas exchange

·       Disease: OSA, OHS

 

Intro: morbid obesity

Definition

·  BMI >35kg/m2

Overall effect

·  ↑Resp demand

·  ↓Resp reserve

Pathophysiology

(1) Mass effect (ME)

o Compression of neck, chest

o Displacement of diaphragm

o Worse if male (↑central and visceral fat)

o Better if female (↑peripheral and subcut fat)

(2) ↑Metabolic rate (↑MR)

o Due to ↑muscle (~20% of extra mass) > ↑fat (~80% of extra mass)

(3) Adipokines from visceral fat (AK)

o Mostly pro-inflammatory: leptin, TNFα, IL6, resistin, angiotensinogen, PAI-1

o Some anti-inflammatory: adiponectin

 

Anatomy:

Upper airway

·  ME -> ↓Radius -> ↑resistance

·  ME -> ↓ROM head and neck

    ->↑risk OSA, OHS, obstruction when sedated

    ->↑risk difficult bag/mask ventilation and intubation

Lower airway

·  ME -> ↓lung volume -> ↓airway radius -> ↑resistance

·  AK -> airway inflammation -> ↑resistance

    ->↑airway pressure

    ->↑risk gas trapping

 

Lung volumes:

Static

·  ME -> ↓chest wall compliance -> ↓TLC, RV, FRC (e.g. ↓25% if erect at BMI 30)

·  Worse if supine, under GA

·  If FRC < closing capacity: small airway closure -> ↑shunt

    -> ± supine hypoxaemia

    -> Rapid desaturation after induction

Dynamic

·  ME -> ↓chest wall compliance -> ↓FVC

·  ME -> ↑resistance -> ↓FEV1

·  ↑MR -> ↑VT 20%

 

Breathing mechanics:

Restriction

·  ME -> ↓chest wall compliance

    ->↑Work of breathing (WOB)

    ->Severely impaired ventilation if Trendelenburg

Obstruction

·  ME -> ↓airway radius

·  AK -> airway inflammation

    ->↑WOB

 

Alveolar time constants: T = R x C

Resistance

·  ↑R due to ME, AK as above

Compliance

·  ↓C: due to ME -> ↓lung volume -> ↓alveolar radius (Tension = pressure x radius / 4)

T

·  ↑Variability

    ->↑slope phase 3 capnogram

    ->↑peak-plateau pressure difference

 

Ventilation and perfusion:

V

·  ↑MR -> ↑RR 40%, ↑TV 25%

·  Accessory muscle use at rest

    ->↑WOB further

    ->↓reserve if unwell e.g. pneumonia, exercise

Q

·  ↑MR -> ↑cardiac output

·  ± Risk OSA/OHS -> chronic ↓PaO2 -> +/- pulmonary hypertension

    ->Risk RV failure peri-op (e.g. spont vent sedation -> ↑PaCO2 -> ↑↑PA pressure)

 

Gas exchange:

↓V/Q matching

·  FRC below closing capacity -> shunt

ABG changes

·  ↓PaO2 (if shunt)

·  ↑PaCO2 (if OHS)

·  ↑HCO3- (can be very high if OHS)

·  pH usually normal if chronic process

 

Disease:

OSA

·  ME + sleep -> airway collapse -> ↓PaO2 -> ↑SNS activity -> arousal -> repeat

    ->↓Sensitivity to ↓PaO2

    ->↑Risk obstruction and apnoea peri-op

    ->↑↑Risk with opioids, benzodiazepines

    ->15% have pulmonary hypertension

OHS

·  ME -> ↑WOB -> ↓VA -> chronic ↑PaCO2

    ->↓Sensitivity to ↑PaCO2

    ->Reliant on hypoxic stimulus (ablated by volatile anaesthetics)

    ->50% have pulmonary hypertension