Physiology of Pregnancy

Dr Stephen Hallworth, Consultant Anaesthetist

Royal London Hospital

Physiological Changes of Pregnancy

Airway

Breathing

Circulation

Gastrointestinal

Haematological

Endocrine

Pregnancy

Most physiological adaptations have a purpose

Most adaptations occur in advance of the need for them

Respiratory System- Airway

Capillary engorgement of upper airway

Exacerbated by: URTI

Fluid Overload

PET / Eclampsia

Occasionally severe UAO

Large tongue + breasts

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

2000

4000

Vol[ml]

VitalCapacityNochange

Non-pregnantLate Pregnancy

Insp

ira

rto

ry

Res

erve

Tid

al

Vo

lum

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xpir

ato

ry

Res

erve

Res

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Inspiratorycapacity+ 15-20%

Lung Volumes during Pregnancy

Total lung

capacity

- 5%

+ 45%

- 25%

- 15%

+ 5%

Functional residual capacity- 20%

Respiratory System- BreathingOther Variables

Respiratory rate / / Airways resistance - 50%

Physiological dead space ??

FEV1 and FEV1 / FVC unchanged

Chest wall compliance - 45%

Lung compliance unchanged

Total compliance - 30%

Respiratory System- BreathingMinute 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

Respiratory System- BreathingMinute 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

Oxygen consumption Oxygen consumption + 15 - 20%

40 ml / min increase

Due to BMR

work of breathing

fetus

uterus

placenta

cardiac work

Respiratory System - BreathingOxygen 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

Respiratory System - BreathingCarbon dioxide tensions

Mixed venous PCO2 1 kPa less than non-pregnant level

Respiratory System - BreathingBlood gases (erect)

Trimester

1st 2nd 3rdNon-pregnant

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

Respiratory System - BreathingHaemoglobin dissociation curve

Shifted to the right

P50 increases from 3.6 kPa to 4.0

U

T

E

R

O

P

L

A

C

E

N

T

A

L

C

I

R

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U

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N

Um

bilica

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bili

cal v

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Ute

rine a

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Ute

rine v

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Mother Fetus

Hb02

HHb

HHb

HbO2

SYNCTIOTROPHOBLAST

60

0 M

L/MIN

02

VO2 =5-10 ml/min

DO2 = 40 ml/min

VO2 = 20ml/min

PCO2 7.3

PO2 2.4

SaO2 45%

Ca02 10.0

Hb 17

PCO2 4.2

PO2 13.5

SaO2 98%

Ca02 16

Hb 12

PCO2 6.1

PO2 5.3

SaO2 75%

Ca02 12

PCO2 5.5

PO2 3.9

SaO2 70%

02 content 16.0

C02

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

Cardiovascular SystemHeart 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)

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

Cardiovascular SystemECG

LAD (15%)

Flat Ts / inverted in III

Atrial / ventricular ectopics

Cardiovascular SystemCardiac 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)

Cardiovascular systemRegional blood flow

Uterus 500-600 ml (+ 400%)

Kidney 400-500 ml (+40%)

Skin 500-600 ml (+150%)

Other 300-600 ml

Cardiovascular systemBlood pressure

No change in SBP

20-25% in DBP at 20/40 (normal at term)

Cardiovascular systemTotal peripheral resistance

~ Must

~ 1000 dyne / sec / cm-5 at 20/40 (35% )

~ 1300 dyne / sec / cm-5 towards term (20% )

Cardiovascular systemVenous 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

Cardiovascular systemSupine 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

Cardiovascular systemOedema

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

Haematological SystemPlasma 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

Cardiovascular systemBlood volume - percentage increase

Trimester

T1 T2 T3

Plasma volume +15 +50 +50

RBC volume Falls Normal +30

Total blood volume +10 +30 +40

Cardiovascular systemTypical 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

Significance for the Anaesthetist 3

Hypervolaemia allows for moderate blood

loss at delivery

Avoid aortocaval compression

Total Body Water in Pregnancy

Red Cell Vol1.6 L

Plasma Vol2.6 L

Intracellular Vol 25 LExtracellular Vol 15 L

Blood Vol 4.2 L

Red Cell Vol2 L

Plasma Vol4 L

Intracellular Vol 27 LExtracellular Vol 19 L

Blood Vol 6 L

TBW = 40 L

TBW = 46 L

Increases in Total Body Water

2.5

1.2

0.8 0.80.5

0.3

0

1

2

3

Fetus blood vol Uterus AmnioticFluid

Placenta Breasts

L

6L

Weight gain in pregnancy

3.4

2.7 2.6

1.40.95 0.8 0.65

0.4

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Bre

asts

Wt

(kg

)

Genito-urinary system

~ 50% in RBF

~ 50% in GFR

~ 40% in [creatinine]

Glycosuria (1-10 g/d)

Proteinuria 300 mg/d

UTIs common

Osmoregulation during pregnancy

Plasma osmolality to 280 - 290 mosmol / kg

No decrease in ADH secretion

Decrease in thirst threshold

Fluid ingestion > diuresis

Gastrointestinal systemStomach 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

Gastrointestinal System

1stTrimester

2ndTrimester

3rdTrimester

Labour

BarrierPressure

Decreased Decreased Decreased Decreased

GastricEmptying

No change No change No change Decreased

GastricAcidSecretion

Decreased Decreased No change ?

Dept of Obstetric Anaesthesia / Royal Free Hospital

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 ?

Gastrointestinal systemHeartburn 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

Gastrointestinal systemAcid aspiration prophylaxis

?? Need

Sodium citrate

H-2 antagonist

Metoclopramide

RSI / cricoid pressure

Gastrointestinal systemLiver and bowel

Normal hepatic blood flow

bilirubin / ALT / AST / LDH

gallbladder emptying / gallstones

intestinal motility / constipation

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

Haematological SystemClotting

20% reduction of PT and PTTK

fibrin deposition (esp. uteroplacental circulation)

Fibrinolysis ( FDPs)

Platelets 15%

Haematological SystemWhite cells

PMNs (max at 30/40)

Lymphocyte count normal

cell-mediated immunity

Normal humoral immunity

Conclusion

Pregnancy is associated with multiple

physiological adaptations

Clinical implications for the anaesthetist

Avoid the supine position / think laterally