2018A Question 06

Describe the washout (not offset) of sevoflurane from a patient following two hours of general anaesthesia. You may wish to use a graph to illustrate the description.

Examiner Report

54% of candidates achieved a pass in this question.

To achieve a pass in this question the following were required :

  • A clear explanation of the elimination kinetics of Sevoflurane with due consideration to a time course appropriate to the termination of anaesthesia after two hours
  • A washout graph including appropriate axis labelling, a multi exponential decay curve, time course and half times consistent with volatile washout
    It was very difficult to pass the question without a washout graph. Wash-in graphs did not score points.
  • A description of the movement of Sevoflurane out of the alveolus, from plasma to alveolus, from the various compartments to plasma and their respective time courses and a description of factors which underpin those kinetics; including depth and duration of anaesthesia, alveolar ventilation and fresh gas flows, compartmental blood flows and solubilities

Common errors included:

  • Confused graphs with no or inappropriate time courses indicated, nonsensical axis labelling or graphing of wash-in and wash-out on the same graph
  • Detailed descriptions of Fick’s Law which wasted time and did not score points
  • An overemphasis on the role of cardiac output in washout and incorrect assertions that increased cardiac output speeds washout
  • Conflation of washout and awakening from anaesthetic as the same thing
  • The incorrect assertion that following two hours the fat is completely saturated with Sevoflurane
  • The confusion of partial pressures, concentrations and amounts

Model Answer


  • Emergence modeling
  • Final end-tidal partial pressure
  • Elimination from V1

Emergence Modeling

Washout time is a function of:

  • Final partial pressure in effect site prior to washout
  • Rate of elimination from V1

Compartment saturation at two hours:

Compartment τ Capacity Saturation
V1 5.7 mins Low ~100%
V2 2.3 hours Moderate ~50%
V5 1.8 days High Minimal
  • Equilibration time constants:
    • τ= 1/k
    • τ= (V x λ) / Q
    • Where: (V = volume of tissue, λ = tissue: Blood partition coefficient, Q = blood flow)

Final Effect-Site Partial Pressure

Effect site partial pressure:

  • Correlates with that in V1
  • May not be included in all kinetic models
  • Rate of equilibration between effect site and V1 is:
    • CBF (per unit mass, not percentage CO to brain)
Factor Partial pressure decreased with:

- ↓ Duration

- ↓ Inspired %


- ↑ Tissue:blood partition coefficients

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

 - ↑ Fat:blood PC (sevo 48, de+s 27, iso 45)

- ↓ MAC (iso 1.2%, sevo 2%, des 6.6%)

 - Correlates with ↓ oil:gas PC (sevo 80, des 29, iso 98)

- ↑ Metabolism


- ↑ Cardiac output

- ↑ Tissue volumes

Elimination from V1

Factor Elimination accelerated with:
Short case

Offset during distribution phase:

- ↑ Cardiac output

- ↑ Tissue:blood PC

- ↑ Compartment volumes

Long case

Offset during terminal elimination phase: (note bivalent factors!)

- ↓ Cardiac output

- ↓ Tissue:blood PC

- ↓ Compartment volumes

- ↑ Metabolism

- ↑ Excretion:

 - ↑ Ratio VA:FRC (?Most important)

 - ↑ Ratio FGF rate : Circuit volume

 - ↓ Inspired %

Last updated 2021-08-23

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