Physiological changes during pregnancy

Question 1

How much weight should women gain in pregnancy?

Answer 1

10-20 kg

Due to enlargement of:

• fetus
• placenta
• amniotic fluid
• enlargement of maternal organs and body compartments (uterus, breasts, fat, blood, extracellular fluid)

Question 2

What happens if pregnant women lie supine?

Answer 2

Aortic and IVC compression

• maternal BP is normal or raised above the level of compression but dramatically reduced below that level
• blood supply to the uterus is below the level of aortic compression
• fetal compromised can occur without maternal hypotension
• the azygos venous sstem is dilated when the IVC is compressed - leads to engorgement of veins in spinal canal

Question 3

What is uterine blood flow at term?

Answer 3

700 ml/min

Question 4

Why is there a rise in cardiac output following felivery?

Answer 4

Due to autotransfusion of blood from the uterus

Question 5

How does the spirometry trace differ in pregnant women compared to non-pregnant?

Answer 5

REDUCED

• FRC
• expiratory reserve volume
• residual volume
• total lung capacity

INCREASED

• tidal volume
• respiratory rate
• minute ventilation

Question 6

Why is there a rapid decline in maternal PaO2 during apnoea?

Answer 6

• FRC is reduced
• O2 demand is increased
• therefore rapid fall in PaO2

Question 7

What is the recommended pre-O2 strategy in pregnancy?

Answer 7

3 mins tidal breathing.

There is a faster rise in maternal end-tidal O2 concentration with pre-oxygenation due to the rise in minute ventilation and fall in FRC.

Question 8

How does the maternal blood gas change in pregnancy?

Answer 8

PaCO2 falls early in pregnancy.

PaO2 is slightly raised.

pH becomes slightly more alkaline.

Question 9

What ventilator settings should you use in a pregnant woman weighing 55kg?

Answer 9

RR - 15

Tidal volume - 480-460 mls

PaCO2 - 4.1 kPa

PaO2 - 14 kPa

Question 10

What is the alveolar gas equation?

Answer 10

P_AO₂ = P_IO₂ - (PaCO₂/R)

P_AO_{2 -} alveolar partial pressure of O2

PIO2 - inspired partial pressure of O2

PaCO2 - arterial partial pressure of CO2

R - respiratory quotient

Question 11

Why is endobronchial intubation more likely in pregnant women?

Answer 11

The distance from teeth to carina is shortened because the diaphragm and chest contents are displaced cephalad as pregnancy advances.

Question 12

How does the gastric pH and volume alter during pregnancy?

Answer 12

pH 2.4 (pregnant)

pH 3 (non-pregnant)

gastric volume 0.49 ml/kg (pregnant)

0.24 ml/kg (non-pregnant)

Question 13

Is there any point in pregnancy that pregnant women don’t need an RSI?

Answer 13

Prior to 16-18 weeks gestation (if they have no other risk factors)

Question 14

How long after delivery can you induce GA without an RSI in pregnancy?

Answer 14

48hrs after delivery

Question 15

What are the blood changes in pregnancy?

Answer 15

40-50% increase in plasma volume

20-30% increase in red cell volume

Fall in Hb concentration

During labour WBC count increases

Question 16

Why are pregnant women hypercoagulable?

Answer 16

Raised plasma levels of

• fibrinogen
• factors VII, X, XII
• decreased fibrinolysis

Question 17

How does pregnancy affect the dose required of local anaesthetic?

Answer 17

Less local anaesthetic is needed for regional anaesthesia.

From late 1st trimester a dose reduction of 30% is required.

Question 18

What are the renal changes in pregnancy?

Answer 18

• GFR increases 50-70%
• kidneys increase in size
• urinary collecting system dilates - leads to stasis of urine/acute pyelonephritis
• glycosuria is common (tubular reabsorption of glucose is overwhelmed)
• proteinuria is common
• plasma albumin falls

Question 19

What are the normal values for urea and creatinine in pregnancy?

Answer 19

Urea 3.3 mmol/l

Creatinine 53 µmol/l

Question 20

When does HCG peak during pregnancy?

Answer 20

At around 12 weeks (50 IU/ml)

Question 21

When does progesterone peak in pregnancy?

Answer 21

Right before labour

Question 22

What is hPL?

Answer 22

Human placental lagtogen (chorionic somatomammotrophin).

Peptide hormone with a structure and function similar to growth hormone. It causes impaired glucose tolerance due to insulin resistance.

Question 23

What is hCG?

Answer 23

Peptide hormone produced by the embryo and later by the placenta.

It maintains the corpus luteum and hence maintains progesterone production.

Question 24

When do heart rate and stroke volume peak in pregnancy?

Answer 24

HR peaks at 16 weeks

SV at 32 weeks

Question 25

How much does cardiac output increase by in pregnancy?

Answer 25

40-50%

Question 26

When is blood pressure the lowest in pregnancy?

Answer 26

20 weeks