By Dr Steve Hallworth

1. Physiology of Pregnancy
Dr Stephen Hallworth, Consultant Anaesthetist
Royal London Hospital

2. Physiological Changes of Pregnancy
Airway
Breathing
Circulation
Gastrointestinal
Haematological
Endocrine

3. Pregnancy
Most physiological adaptations have a purpose
Most adaptations occur in advance of the need
for them

4. Respiratory System- Airway
Capillary engorgement of upper airway
Exacerbated by: URTI
Fluid Overload
PET / Eclampsia
Occasionally severe UAO
Large tongue + breasts

5. Significance for the Anaesthetist 1
Extreme care with : manipulation of airway
suctioning
use of airways
laryngoscopy
Anticipate difficult intubation
Use smaller COTT due to glottic oedema
UAO may occur early after induction

6. Lung Volumes during Pregnancy
4000
Inspirartory
Reserve
Total lung Vital Inspiratory
Capacity capacity
+ 5%
capacity No + 15-20%
- 5% change
Volume
+ 45%
Tidal
2000
Expiratory
Reserve
Functional - 25%
residual
Vol capacity
- 20%
Residual
[ml]
Volume
- 15%
Late Pregnancy
Non-pregnant

7. Respiratory System- Breathing
Other Variables
Respiratory rate ↑/↓/↔
Airways resistance - 50%
Physiological dead space ??
FEV1 and FEV1 / FVC unchanged
Chest wall compliance - 45%
Lung compliance unchanged
Total compliance - 30%

8. Respiratory System- Breathing
Minute Volume
45% increase in MV
Progesterone ± oestrogen effect
Direct effect on respiratory centre
Increased sensitivity to respiratory centre to CO2
Increased level of carbonic anhydrase B in RBCs
Also due to increased CO2 production

9. Respiratory System- Breathing
Minute Volume
Non-pregnant state: Increased ventilation
by 1.5 L/min for each
1mmHg
PaCO2 rise
Pregnant state: Increased ventilation
by 6 L/min for each
1mmHg PaCO2 rise

10. Oxygen consumption
Oxygen consumption + 15 - 20%
≈ 40 ml / min increase
Due to ↑ BMR
↑ work of breathing
fetus
uterus
placenta
↑ cardiac work

11. Respiratory System - Breathing
Oxygen tensions
In 1/3 to 1/2 of women at term airway closure
occurs during normal tidal breathing when supine
PAO2 : PaO2 gradient ∼ 2 kPa sitting
∼ 3 kPa supine
↑ PaO2 due to ↓ PaCO2 + ↓AVO2 difference

12. Respiratory System - Breathing
Carbon dioxide tensions
Mixed venous PCO2 1 kPa less than non-
pregnant level

13. Respiratory System - Breathing
Blood gases (erect)
Trimester
Non-pregnant 1st 2nd 3rd
PaCO2 (kPa) 5.3 4.0 4.0 4.0
PaO2 (kPa) 13.3 14.3 14.0 13.7
pH 7.40 7.44 7.44 7.44
[HCO3] (mEq/L) 24 21 20 20
SBE (mEq/L) 0 -2 -3 -3

14. Respiratory System - Breathing
Haemoglobin dissociation curve
Shifted to the right
P50 increases from 3.6 kPa to 4.0

15. Mother Fetus
U
T C
Uterine artery
Umbilical artery
Umbilical vein
Uterine vein
PCO2 4.2 PCO2 7.3
E PO2
SaO2
13.5
98%
PO2
SaO2
2.4
45% I
Ca02 16
R
SYNCTIOTROPHOBLAST Ca02 10.0
Hb 12 Hb 17
R
VO2 =5-10 ml/min
O
600 ML/MIN
C
P U
DO2 = 40
L ml/min
L
A 02 A
Hb02 HHb
C T
E HHb HbO2 I
C02
N O
T VO2 = 20
ml/min N
A
PCO2 6.1 PCO2 5.5
PO2 5.3 PO2 3.9
SaO2 75% SaO2 70%
L
Ca02 12 02 content 16.0

16. Significance for the Anaesthetist 2
Rapid maternal desaturation following
induction for GA (10 kPa / min faster than
non-pregnant)
Pre- O2 for 5 mins recommended
Avoid aortocaval compression at all times
Epidurals may prevent fetal hypoxaemia
during labour

17. Cardiovascular System
Heart position
Pushed upwards and forwards
AB ⇒ 4th intercostal space
Gives impression of cardiac enlargement on
CXR
But - it is enlarged by ~ 12% (70 - 80 ml)

18. Cardiovascular System / Heart sounds
1st: Louder & exaggerated splitting
2nd: Not affected
3rd: Heard loudly in majority
4th: Detected by phonocardiography in ~16%
Early- to mid-systolic ejection in most at LS
Diastolic murmurs also fairly common due to
tricuspid flow murmur

19. Cardiovascular System
ECG
LAD (15%)
Flat Ts / inverted in III
Atrial / ventricular ectopics

20. Cardiovascular System
Cardiac output
25% ↑ by 13/40
50% ↑ by 20/40 to term ~ 2 L / min (e.g 4.5 - 6.5)
20% ↑ in heart rate ~ 15 bpm (e.g 70 - 85)
13/40 until term
20% ↑ in stroke volume ~ 12 ml (e.g 64 - 76)

21. Cardiovascular system
Regional blood flow
Uterus ↑ 500-600 ml (+ 400%)
Kidney ↑ 400-500 ml (+40%)
Skin ↑ 500-600 ml (+150%)
Other ↑ 300-600 ml

22. Cardiovascular system
Blood pressure
No change in SBP
20-25% ↓ in DBP at 20/40 (normal at term)

23. Cardiovascular system
Total peripheral resistance
~ Must ↓
~ 1000 dyne / sec / cm-5 at 20/40 (35% ↓)
~ 1300 dyne / sec / cm-5 towards term (20% ↓)

24. Cardiovascular system
Venous pressures
No change in CVP / RA / arm veins
2.5 ↑ in femoral / IVC / leg veins at term
Causes: weight of uterus on iliacs / IVC
pressure of fetal head on iliacs
hydrodynamic obstruction

25. Cardiovascular system
Supine hypotensive syndrome
From 20/40
Majority of women placed in supine position
at term get a 30-50% ↓ in CO but don’t
become hypotensive due to ↑ TPR
10% get a 30% ↓ in SBP
A / w ↓ RAP / ↓ CO / ↓ MAP

26. Cardiovascular system
Oedema
Pedal oedema in 40% of normotensives
Colloid osmotic pressure 22 mmHg at onset
of labour and 16 mmHg 6 hr post delivery
Non-cardiogenic pulmonary oedema can
occur at 13-16 mmHg

27. Haematological System
Plasma proteins
Trimester
Pre-pregnancy T1 T2 T3
Total protein (g/L) 78 69 69 70
Albumin (g/L) 45 39 36 33
Globulin (g/L) 33 30 33 37
Albumin/globulin ratio 1.4 1.3 1.1 0.9
Oncotic pressure 27 25 23 22

28. Cardiovascular system
Blood volume - percentage increase
Trimester
T1 T2 T3
Plasma volume +15 +50 +50
RBC volume Falls Normal +30
Total blood volume +10 +30 +40

29. Cardiovascular system
Typical values for a 65kg woman
Pre-pregnancy Term
Total blood volume (L) 4.2 6.0
Plasma volume (L) 2.6 4.0
RBC volume (L) 1.6 2.0
[Hb] (g /dl) 12.5 11.0
Hct (%) .38 .34
Total oxygen carrying capacity 10.5 13.2

30. Significance for the Anaesthetist 3
Hypervolaemia allows for moderate blood
loss at delivery
Avoid aortocaval compression

31. Total Body Water in Pregnancy
Plasma Vol Red Cell Vol
2.6 L Blood Vol 1.6 L
4.2 L TBW = 40 L
Extracellular Vol 15 L Intracellular Vol 25 L
Plasma Vol Red Cell Vol
4L 2 L
Blood Vol
6L TBW = 46 L
Extracellular Vol 19 L Intracellular Vol 27 L

32. Increases in Total Body Water
3
2.5
2
L 1.2
1 0.8 0.8
0.5
0.3
0
Fetus blood vol Uterus Amniotic P lacenta B reas ts
Fluid
6L

33. Wt (kg)
4
0
2
3
1
Fetus
3.4
maternal
s tores
2.7
E CF
2.6
P las ma
volume
1.4
Uterus
0.95
Amniotic
fluid
0.8
Weight gain in pregnancy
P lacenta
0.65
B reas ts
0.4

34. Genito-urinary system
~ 50% ↑ in RBF
~ 50% ↑ in GFR
~ 40% ↓ in [creatinine]
Glycosuria (1-10 g/d)
Proteinuria 300 mg/d
UTIs common

35. Osmoregulation during pregnancy
Plasma osmolality ↓ to 280 - 290 mosmol / kg
No decrease in ADH secretion
Decrease in thirst threshold
Fluid ingestion > diuresis

36. Gastrointestinal system
Stomach
Stomach displaced upwards
⇒ changes angle of GO junction
⇒ reflux (in 50 - 80%)
↑ progesterone
↓ gastrin and pepsin
No difference in gastric volumes > 25ml *
No difference in gastric pH< 2.5*
* relative to non-pregnant women

37. Gastrointestinal System
1st 2nd 3rd Labour
Trimester Trimester Trimester
Barrier Decreased Decreased Decreased Decreased
Pressure
Gastric No change No change No change Decreased
Emptying
Gastric Decreased Decreased No change ?
Acid
Secretion

38. Gastric Emptying during Labour
Labour minimum delay
Labour + IM opioids marked delay
Labour + epid opioids [bolus] marked delay
Labour + epid opioids [infusion] minimum delay
Postpartum ?
Dept of Obstetric Anaesthesia / Royal Free Hospital

39. Gastrointestinal system
Heartburn
All have raised intragastric pressure
↑ GO junction pressures in 20-50%
⇒ barrier pressure ≥ normal
⇒ no reflux
↓ GO junction pressures in 50-80-%
⇒ barrier pressure is < normal
⇒ reflux

40. Gastrointestinal system
Acid aspiration prophylaxis
?? Need
Sodium citrate
H-2 antagonist
Metoclopramide
RSI / cricoid pressure

41. Gastrointestinal system
Liver and bowel
Normal hepatic blood flow
↑ bilirubin / ALT / AST / LDH
↓ gallbladder emptying / gallstones
↓ intestinal motility / constipation

43. Nonplacental endocrinology
Thyroid ↑ total T3 and T4
Normal free T3 and T4
Adrenal cortex 200% ↑ in free / total cortisol
Pancreas ↓ tissue sensitivity to insulin
↓ GTT
↓ fasting [glucose]
↑ ketosis

44. Haematological System
Clotting
⇒ 20% reduction of PT and PTTK
↑ fibrin deposition (esp. uteroplacental
circulation)
↑ Fibrinolysis (↑ FDPs)
Platelets ↓ 15%

45. Haematological System
White cells
↑ PMNs (max at 30/40)
Lymphocyte count normal
↓ cell-mediated immunity
Normal humoral immunity

47. Conclusion
Pregnancy is associated with multiple
physiological adaptations
Clinical implications for the anaesthetist
Avoid the supine position / think laterally