37. Pregnancy

Question 1

Haematology

9

Answer 1

1
> Plasma volume increases: 
this increases preload and 
VD of polar drugs. 
It increases by 50% by term.

2
> RBC mass increases: 
but plasma increases more than RBC, 
causing
physiological anaemia.

3
> WBC increase: 
to 12 × 109/L by term, 
with a further increase to
30 × 109/L during labour.

4
> Platelets reduce:
due to consumption.

5
> Albumin decreases: 
this increases the 
free active proportion of
plasma-bound drugs.

6
> Plasma oncotic pressure reduces:
increases the risk of oedema.

7
> Plasma cholinesterase reduces:
the effect of suxamethonium is
offset - the increased VD.

8
> Hypercoagulable:
all clotting factors increase

except XI and XIII.

BT, PT and APTT shortened.

High risk of thromboembolic complications.

9
> CRP and ESR increase.

Question 2

Cardiovascular system

Answer 2

1
> Stroke volume and
heart rate increase:

cardiac output increases by
up to 60% by term (8 L /min).

2
> Systemic vascular resistance reduces:
reduction in DBP > SBP leading
to an increased pulse pressure.

This is due to oestrogen and
progesterone.

Maintenance of SVR is
governed by sympathetic drive,
which is diminished by
central neuroaxial blockade.

3
> Left ventricular mass increases: 
ECG shows left axis deviation, 
ST depression and 
even a flat or inverted T-wave. 

Systolic murmur almost universal at term,
but note that a diastolic murmur is not normal.

4
> Vena caval compression: 
reduces venous return and preload,
decreased cardiac output, 
decreased BP and engorged verterbral veins.

5
> Aortic compression:
occurs from 20 weeks gestation.

Increases afterload,
decreases cardiac output
and utero-placental blood flow.
This is decreased by avoiding supine position and using at least 15° lateral tilt.

Question 3

Respiratory system:

Answer 3

1
> Anatomy:

capillary enlargement and
mucosal congestion can lead to
voice changes and
difficulty breathing in some women.

The diaphragm is elevated by 4 cm,

thoracic circumference increases
as the ribs ‘splay’ out
and breathing becomes
largely diaphragmatic by term.

2
> Volumes:

FRC reduces by up to 20%
and closing volume encroaches on FRC,
leading to airway closure and
increasing the risk of hypoxia.

This is made worse when supine,
obese or multiple pregnancy.

VT increases but TLC and VC remains unchanged.

3

> Ventilation:
respiratory rate and minute ventilation increase.

Dead space increases due to bronchodilation.

4
> Mechanics:
chest wall compliance reduces
but lung compliance remains unchanged.

> Oxygen consumption increases:
by up to 60%, increasing the risk of
developing hypoxia
during induction of anaesthesia.

5
> ABG: pH increases (to ∼7.5),
PO2 increases (to ∼14 kPa),
PCO2 reduces due to hyperventilation (to ∼3.5 kPa) and HCO3– reduces (to ∼18 mmol/L).

Question 4

Gastrointestinal system:

Answer 4

> Barrier pressure reduces:
due to increased intragastric pressure.

> Gastric emptying is reduced:
this occurs during labour due to the
effects of pain and opiates.

> Risk of aspiration increases:
this returns to normal levels 48 hours post partum

Question 5

Renal:

Answer 5

> Renal blood flow
and GFR increase:
by up to 50%.

Urea and creatinine levels reduce,

meaning that a ‘normal’ creative level in pregnancy is abnormal.

> Glycosuria and proteinuria: common.

Question 6

Central nervous system:

Answer 6

> Epidural space reduces:
due to the engorged
extradural venous plexus.

CSF volume is also reduced.

Hence reduced volumes of
local anaesthetic agents are
required during central neuroaxial blockade
(reduce dose by one-third).

There is also an increased risk of inadvertent
intravascular catheter placement.

> Anaesthesia:
MAC decreased.
Inhalational induction faster

(as raised MV more significant factor
that raised CO in this case).

Following labour, woman has reduced strength in respiratory muscles for around 4 hours.

Anaesthesia compounds this effect
and reduces ability to cough.