2020A07 Describe the washout of desflurane from a patient following six hours of general anaesthesia.
Draw a graph to illustrate the description.

· Summary

· Determinants

· Kinetic model

· Washout graph

· Decrement graph

Multiphasic

· Distribution: (excretion) x (distribution V1 -> V2-5)

· Terminal elimination: (excretion) x (metabolism) / (redistribution V2-5 -> V1)

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

Tissue -> blood	↑Tissue blood flow (i.e. cardiac output) ↓Compartment volume (i.e. not obese) ↓Duration of anaesthesia
Blood -> alveolus	↑Cardiac output
Alveolus -> circuit	↑Alveolar ventilation : FRC (e.g. child or pregnant)
Circuit -> atmosphere	↑Fresh gas flow rate : circuit volume

N.B. high cardiac output mainly slows washout due to greater drug uptake

Muscle:blood partition coefficient 2.0 (cf. sevo 3.1)
Fat:blood partition coefficient 29 (cf. sevo 48)