2015B05 Describe how morbid obesity affects time to awakening (not wash-out)
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, ↑VA

·   ↑Fat mass (80% of excess), cephalad diaphragm displacement, ↓FRC

·   Inflammatory cytokines -> ↓MAC

Overall effects on time to emergence (TTE)

·   Short case: ↑distribution rate -> ↓time to emergence (TTE)

·   Long case: ↑uptake -> ↑TTE

·   Washout of drug is invariably slower

Implications

·   Prefer desflurane -> ↓accumulation (mainly due to lean mass)

·   Prefer adjuvants -> ↓MAC -> ↓accumulation

 

Emergence modelling:

Determinants of TTE

1.   Final partial pressure in effect site prior to washout

2.   Rate of elimination from V1

3.   Partial pressure in V1 associated with emergence (MAC-awake)

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 -> ↑TTE after moderate duration case

·   ↑↑V5 -> ↑TTE after long duration case

Washout curve

Putative effect site

·   Ascending reticular activating system, thalamus frontal cortex

·   Concentration correlates with that in V1

·   May not be included in kinetic model

·   Rate of equilibration between effect site and VRG ∝

o Blood flow to brain per unit mass (not % cardiac output to brain)

o 1/(brain-blood partition coefficient)

 

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 ↑)

 

2.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 VA: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

 

3.MAC awake: (normal ~0.33)

Patient factors

·   ? Adipokines -> ↓MAC-awake

·   ? Sleep-disordered breathing -> ↑narcotic sensitivity -> ↓MAC-awake

 

 

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