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; |