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:

·      Intro

·      Emergence modelling

·      Final end-tidal partial pressure

·      Elimination from V1

 

Emergence modelling:

Determinants of washout time

1. Final partial pressure in effect site prior to washout

2. Rate of elimination from V1

Kinetic model

 

Equilibration time constants:

·   τ= 1/k

·   τ= (V x λ) / Q

·   (V = volume of tissue, λ = tissue: blood partition coefficient, Q = blood flow)

Compartment saturation at two hours

 

 

τ

Capacity

Saturation

V1

5.7 mins

Low

~100%

V2

2.3 hours

Moderate

~50%

V5

1.8 days

High

Minimal

 

 

Washout curve at two hours

Putative effect site

·   Ascending reticular activating system, thalamus frontal cortex

·   Concentration correlates with that in V1

·   May not be included in kinetic model

·   Rate of equilibration between effect site and V1

o Blood flow to brain per unit mass (not % cardiac output to brain)

o 1/(brain-blood partition coefficient)

 

1.Final end-tidal partial pressure: (factors decreasing it)

Infusion

·   ↓Duration

·   ↓Inspired %

Drug

·   ↑Tissue:blood partition coefficients

o ↑Muscle:blood PC (sevo 3.1, des 2.0, iso 2.9)

o ↑Fat:blood PC (sevo 48, de+s 27, iso 45)

·   ↓MAC (iso 1.2%, sevo 2%, des 6.6%)

o Correlates with ↓oil:gas PC (sevo 80, des 29, iso 98)

·   ↑Metabolism

Patient

·   ↑Cardiac output

·   ↑Tissue volumes

 

2.Elimination from V1: (factors accelerating it)

Short case

Offset during distribution phase:

·   ↑Cardiac output

·   ↑Tissue:blood PC

·   ↑Compartment volumes

Long case

Offset during terminal elimination phase: (note bivalent factors!)

·   ↓Cardiac output

·   ↓Tissue:blood PC

·   ↓Compartment volumes

·   ↑Metabolism

·   ↑Excretion:

o ↑ratio VA:FRC (? most important)

o ↑ratio FGF rate : circuit volume

o ↓Inspired %

 

 

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