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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