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

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

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

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 -> ↑V_T 20%

Restriction

·   ME -> ↓chest wall compliance

    ->↑Work of breathing (WOB)

    ->Severely impaired ventilation if Trendelenburg

Obstruction

·   ME -> ↓airway radius

·   AK -> airway inflammation

    ->↑WOB

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

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)

↓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

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