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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