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

 

List:

·        Washout modelling

·        Uptake

·        Elimination

 

Washout modelling:

Description

·          Multiphasic: distribution then terminal elimination

·          Exponential decay: rate of removal ∝ partial pressure

·          Rapid: highly insoluble -> (near) context-insensitive decrement times

·          Pulmonary excretion 99.8% >> hepatic metabolism 0.2%

Determinants

1.        Uptake (into all compartments)

2.        Elimination (via central compartment)

Kinetic model

 

Drug solubility:

 

Desflurane

Sevoflurane

Muscle:blood

2.0

3.1

Fat:blood

29

48

 

Compartment saturation:

 

Capacity

Equilibration time (3τ)

V1

Low

Complete (13 mins)

V2

Moderate

Complete (4.6 hours)

V3

High

Minimal (3 days)

Washout curve

Decrement time

 

1.Uptake (factors increasing it)

Administration

·     ↑Duration

·     ↑Inspired concentration (↑MAC in children)

Patient

·     ↑Cardiac output -> ↑tissue blood flow (child, pregnant, obese, septic)

·     ↑Compartment volumes -> ↑capacity (lean tissue, fat)

 

2.Elimination (factors slowing it)

Tissue -> blood

·     ↑Redistribution rate

o   ↑Tissue blood flow = ↑cardiac output (major)

o   ↑Compartment volumes

Blood -> alveolus

·     ↓Cardiac output (minor)

Alveolus -> circuit

·     ↓Ratio VA:FRC

Circuit -> exhaust

·     ↓Ratio FGF rate : circuit volume

·     ↓Inspired concentration

 

 

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