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

 

List:

·         Summary

·         Determinants

·         Kinetic model

·         Washout graph

·         Decrement graph

 

Summary:

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

 

Determinants: washout faster if

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

 

Volatile anaesthetic kinetic model:

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

 

Washout graph:

 

Decrement graph:

 

Feedback welcome at ketaminenightmares@gmail.com