2018A06 Describe the washout (not offset) of sevoflurane from a patient following
 two hours of general anaesthesia. You may wish to use a graph to illustrate the description.

 

List:

·     Washout modelling

·     Uptake

·     Elimination

 

Emergence modelling:

Description

·      Multiphasic: distribution then terminal elimination

·      Exponential decay: rate of removal ∝ partial pressure

·      Fairly rapid: moderate solubility -> context-sensitive decrement times

·      Pulmonary excretion 95-98% >> hepatic metabolism 2-5%

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 (17 mins)

V2

Moderate

Near complete (7 hours)

V3

High

Minimal (5.4 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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