2020B15 Discuss factors that determine the time to awakening after cessation of a propofol target infusion (TCI).

 

List:

·        Emergence modelling

·        Uptake

·        Elimination

·        Threshold

 

Emergence modelling:

Determinants

1.        Uptake (into all compartments)

2.        Elimination (from effect site via central compartment)

3.        Threshold (median effect site concentration for emergence – i.e. Ce50)

Kinetic model

 

 

Perfusion

Capacity

V1

+++

+ (~4L)

V2

++

++ (~30L)

V3

+

+++ (~200L)

Washout curve

 

Half lives:

·     t1/2α fast: 2 mins

·     t1/2α slow: 20-50 mins

·     t1/2β: 5-12 hours

 

Decrement times

 

·     Fast offset after short infusion

·     Slow offset after long infusion

 

 

1. Uptake (factors increasing it)

Infusion

·     ↑Duration

·     ↑Infusion rate / target concentration

Patient

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

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

Algorithm

·     Marsh > Schnider

·     Male > female

·     weight, height, youth

Logistics

·     Factors impeding titration:

o  Neuromuscular blockade

o  Absence of processed EEG

 

2. Elimination (factors slowing it)

Short case

Offset during distribution phase:

·     ↓Distribution to V2, V3

o   ↓Cardiac output (old age, heart failure, hypovolaemia)

o   ↓Compartment volumes (cachexia)

Long case

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

·     ↑Redistribution from V2, V3

o   ↑Cardiac output

o   ↑Compartment volumes

·     ↓Metabolism (↓mAP -> ↓hepatic blood flow; liver disease if severe)

·     ↓Excretion (<1%)

 

3. Threshold (factors lowering it)

Pharmacology

·     Synergistic: opioids, benzo

·     Additive: sevoflurane, N2O

Physiology

·     Old age

·     Pregnancy (↓30%)

·     Night time (↓10%)

Pathology

·     ↓↓PaO2

·     ↑↑PaCO2

·     ↓↓mAP

·     ↓↓pH

·     ↓↓Temp

 

 

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