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

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

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

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 ↑)

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

o ↓Chest wall compliance -> ↓↓FRC under GA and supine;
↓lung volume -> ↓lung compliance