2019B07 Describe how morbid obesity might affect washout (not time to awakening)
after inhalational anaesthesia.

List:

· Morbid obesity

· Emergence modelling

· Final end-tidal partial pressure

· Elimination from V1

· MAC-awake

Morbid obesity:

Definition	· BMI >35kg/m² body surface area
Changes	· ↑Lean mass (20% of excess) -> ↑BMR, ↑cardiac output, ↑V_A · ↑Fat mass (80% of excess), cephalad diaphragm displacement, ↓FRC · Inflammatory cytokines -> ↓MAC
Overall effects on washout	· Short case: ↑distribution rate -> ↓washout rate but ↓time to emergence · Long case: ↑uptake -> ↓washout rate and ↑time to emergence
Implications	· Prefer desflurane -> ↓accumulation (mainly due to lean mass) · Prefer adjuvants -> ↓MAC -> ↓accumulation

Washout modelling:

Determinants of washout rate

1. Final partial pressure in effect site prior to washout

2. Rate of elimination from V₁

Kinetic model

Equilibration time constants:

· τ= 1/k

· τ= (V x λ) / Q

· (V = volume of tissue, λ = tissue: blood partition coefficient, Q = blood flow)

Significant effects of MO:

· ↑Cardiac output but tissue blood flow per unit mass

· ↑V₂, V₃ -> ↓washout rate after moderate duration case

· ↑↑V₅ -> ↓washout rate after long duration case

Washout curve

1.Final end-tidal partial pressure: (factors decreasing it)

Infusion

· ↓Duration (MO: difficult surgery)

· ↓Inspired % (MO: ↑uptake hence ↑inspired % for given expired %)

Drug

· ↑Tissue:blood partition coefficients (MO ↑ as above)

o ↑Muscle:blood PC (sevo 3.1, des 2.0, iso 2.9)

o ↑Fat:blood PC (sevo 48, des 27, iso 45)

· ↓MAC (MO: cytokines -> ↓)

o Correlates with ↓oil:gas PC (sevo 80, des 29, iso 98)

· ↑Metabolism (MO: +/- liver disease -> ↓)

Patient

· ↑Cardiac output (MO ↑)

· ↑Tissue volumes (MO ↑)

Elimination from V1: (factors accelerating it)

Short case

Offset during distribution phase:

· ↑Cardiac output (MO ↑)

· ↑Tissue:blood PC (MO ↑)

· ↑Compartment volumes (MO ↑)

Long case

Offset during terminal elimination phase:

· ↓Cardiac output (MO ↑)

· ↓Tissue:blood PC (MO ↑

· ↓Compartment volumes (MO ↑)

· ↑Metabolism (MO: +-/ liver disease -> ↓)

· ↑Excretion (MO: complex effect on V_A:FRC ratio)

o ↑metabolic rate -> ↑VCO₂ -> ↑awake ventilation; but

o ↓Chest wall compliance -> ↓ventilation under GA