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:

o   (Excretion) x (distribution V1 -> V2-5)

·      Terminal elimination:

o   (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

·      i.e. offset is almost duration-independent

Variability

·      Washout faster if

o   ↓Extent of uptake

o   ↑Rate of removal

 

↓Uptake if:

Administration

·      ↓Inspired concentration (cf. ↑MAC in paediatrics)

·      (↓Duration)

Circulation

·      ↓Cardiac output -> ↓tissue blood flow (cf. ↑CO in paediatrics)

*Most important effect of cardiac output*

Tissues

·      ↓Muscle volume – more important (cf. ↑muscle in the obese)

·      ↓Fat volume – less important unless very long duration

 

↑Removal if:

Tissue -> blood

·      ↑Tissue blood flow  = ↑cardiac output

·      ↓Muscle and fat volume

Blood -> alveolus

·      ↑Cardiac output

Alveolus -> circuit

·      ↑Alveolar ventilation : FRC (e.g. child or pregnant)

Circuit -> exhaust

·      ↑Fresh gas flow rate : circuit volume

 

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:

 

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