2019B Question 07

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

Examiner Report

63.8% of candidates achieved a pass in this question.

The main domains assessed in this question were:

  • Definition of morbid obesity
  • Pharmacokinetics associated with morbid obesity
  • Effect of specific inhalational agent solubility on washout
  • Inhalational washout curve
  • Effect of anaesthetic duration
  • Respiratory problems of morbid obesity during recovery affecting washout

Credit was given for other relevant correct material such as context sensitive recovery from inhalational anaesthesia, tissue equilibration times, decrement times of inhalational agents and the effect of controlled ventilation by the anaesthetist during emergence.

Common problems were:

  • Poor knowledge of respiratory changes of morbid obesity in the awake state and upon emergence of anaesthesia
  • Misconception about the effect of cardiac output on inhalational washout
  • Washout curves with wrongly labelled y-axis

Better answers incorporated the effects of morbid obesity and specific inhalational agents into the washout curve.

Model Answer

Structure:

  • Morbid obesity
  • Emergence modeling
  • Final end-tidal partial pressure
  • Elimination from V1
  • MAC-awake

Morbid Obesity

Factor Details
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 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 Modeling

Factor Details
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

Final End-tidal Partial Pressure

Factor Decreased partial pressure with:
Infusion

- ↓ Duration (MO: Difficult surgery)

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

Drug

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

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

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

- ↓ MAC (MO: Cytokines → ↓ )

 - 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

Factor Increased elimination with:
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)

 - ↑ Metabolic rate → ↑ VCO2 → ↑ Awake ventilation; but

 - ↓ Chest wall compliance → ↓ Ventilation under GA

 - ↓ Chest wall compliance → ↓↓ FRC under GA and supine;
  ↓ lung volume → ↓ Lung compliance


Last updated 2021-08-23

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