2020A Question 07

Describe the washout of desflurane from a patient following six hours of general anaesthesia. Draw a graph to illustrate the description.

Examiner Report

The major domains assessed in this question were:

• A description of the shape of the washout curve
• Factors which would affect the time course of washout
• A graph illustrating the description

The most common problem was that the graph did not illustrate the description. It is important to give some idea of the time course of the washout and to indicate in some way that the washout is some form of exponential decay. Marking important points on the graph make it easier to illustrate the description. Other common issues were using the blood gas partition coefficient to explain tissue solubility or to say that low lipid solubility leads to a slow tissue washin but a fast washout.

Model Answer

Structure:

• Summary
• Factors affecting uptake
• Factors affecting removal
• Washout graph
• Decrement graph

Summary

Washout is:

• Multiphasic
  • Distribution: $Excretion \times Distribution_{(V_1 \rightarrow V_{2-5})}$
  • Terminal elimination: ${Excretion \times Metabolism } \over {Distribution_{(V_{2-5} \rightarrow V_1)}}$
• Exponential
  • Rate of drug elimination ∝ partial pressure
• Rapid
  • Low tissue solubility → Minimal uptake → Rapid excretion
  • Almost flat 90% decrement curve hence offset is almost duration-independent
• Variable
  Faster washout if:
  • ↓ Uptake
  • ↑ Rate of removal

Factors Affecting Uptake

Factors Affecting Removal

Washout Graph

Decrement Graph