2016B13 Describe the effect of pregnancy, at term, on wash-in (not rate of onset)
of volatile anaesthetic agents.

 

List:

·         Intro

·         Respiratory

·         Cardiovascular

·         Haematological

·         Nervous

 

Intro:

Wash-in

·  Correlates with equilibration of inspired and alveolar (and effect site) partial pressures

·  i.e. rate at which FA/FI approaches 100%

·  delivery rate / uptake rate

Summary of effects

·  Resp changes -> ↑↑rate (most important; ratio VA:FRC 6:1 cf. 2:1)

·  CVS changes -> ↓rate (less important)

·  Haem changes -> ↔rate (less important)

·  CNS changes -> ↑rate of onset but no effect on wash-in

Main causes

·  ↑Metabolic rate (MR): due to foetus, placenta, cardiac and resp work

·  Cephalad displacement of diaphragm by gravid uterus

·  Hormones: progesterone, relaxin

Considerations

·  Almost always IV induction, paralysis, mechanical ventilation

o High risk aspiration

o Aim to minimize effect of drugs on offspring at GA LUSCS

·  Hence factors described may not be relevant

 

Respiratory:

↑Ratio VA:FRC

·  Changes and causes:

o ↑RR 10% and ↑VT 40% (progesterone sensitises central chemoreceptors; also ↑BMR)

o ↑VD (bronchodilation) but ↔VD/VT

o ↓FRC -20% erect, -30% supine, further under GA (mass effect)

·  Implication:

o VA:FRC 6.1 cf. 2:1

o ↑Rate of rise FA/FI

Improved V/Q matching

·  Changes and cause:

o ↑Pulmonary blood flow -> recruitment and distension (↑cardiac output)

·  Implication:

o ↑Rate of drug uptake

o ↑Rate of rise FA/FI (especially insoluble agents)

 

Cardiovascular changes:

↑Cardiac output

·  Changes:

o ↑HR 25% (↑15% by end T1)

o ↑SV 40% (↑25% by end T1)

o ↑CO 45% (↑40% by end T1)

·  Causes:

o ↑Metabolic rate -> ↑preload

o Oestrogen -> ↑RAAS -> ↑plasma volume 40% -> ↑preload

o Oestrogen -> ↑Epo -> ↑RBC volume 20% -> ↑preload

o Progesterone -> ↓SVR, ↓afterload

·  Implication:

o ↑Rate of uptake

o ↑Rate of distribution

o ↓Rate of rise of Pv(Gas)

o ↓Rate of rise FA/FI

↓%Cardiac output to brain

·  Changes and causes:

o Absolute CBF unchanged (autoregulation)

o Relative CBF decreased 15% -> 10% (↑cardiac output)

·  Implication: ↔ rate of rise FA/FI


Addit:

·  I would argue that

o For (continuous) volatile onset, absolute brain blood flow matters more

o For (bolus) IV anaesthetic onset, relative brain blood flow matters more

 

Haematological:

Haemodilution

·  Cause: oestrogen as above

·  Implication: ↓blood:gas partition coefficient -> ↑rate of rise FA/FI

Hyperlipidaemia

·  Cause: human placental lactogen -> insulin resistance

·  Implication: ↑blood:gas partition coefficient -> ↓rate of rise FA/FI

 

Central nervous system:

↓MAC

·  ↑Sensitivity to hypnotics -> ↓MAC ~25%

·  Cause: progesterone

·  Implication: ↑rate of onset but rate of rise FA/FI

 

 

 

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