Changes in Obstetric Physiology

Question 1

Plasma Volume

Answer 1

Increases by 45% due to progesterone/oestrogen causing renin release and therefore activation of renin-angiotensin axis causing sodium retention and increase in total body water

Question 2

Red cell mass

Answer 2

Increases by 20% due to increase in renal erythropoietin

Question 3

Haemoglobin concentration

Answer 3

Falls to around 120 g/L - physiological anaemia of pregnancy (the increase in plasma volume is more than the increase in red cell mass)

Question 4

White cell count

Answer 4

Increases throughout pregnancy

Question 5

Systemic vascular resistance

Answer 5

Reduces by 20% due to increased levels of oestrogen and progesterone

Question 6

Systolic and diastolic blood pressure

Answer 6

Reduced due to lower SVR

Question 7

Heart rate

Answer 7

Increases by 25% as a response to reduced SVR (by 20%)

Question 8

Stroke volume

Answer 8

Increases by 25%

Question 9

Cardiac output

Answer 9

Increases by 50% by third trimester

Question 10

Aortocaval compression: compensation

Answer 10

Occurs typically after 20 weeks, may cause maternal hypotension - compensation is an increase in sympathetic tone causing vasoconstriction and tachycardia and utilisation of azygos veins and vertebral plexus.

Question 11

Aortocaval compression: complications

Answer 11

In 10% of mothers, causes inadequate blood pressure in the supine position (and may cause mother to lose consciousness)
However, may cause foetal hypoxia in the asymptomatic mother

Question 12

Coagulation: Factors and Fibrinogen

Answer 12

Gradually increase therefore pregnancy is a hypercoagulable state

Question 13

Platelets

Answer 13

Function remains normal. Increased turnover.

Question 14

Anatomical changes of the respiratory system

Answer 14

Capillary engorgement and increased oedema of the upper airway (therefore bleeding is more likely and intubation may be more difficult)

Question 15

Minute ventilation

Answer 15

Increased

Question 16

Functional Residual Capacity

Answer 16

Reduced by 20%

Question 17

Residual volume and expiratory reserve volume

Answer 17

Decreased

Question 18

Inspiratory reserve volume

Answer 18

Increased

Question 19

FEV1, Vital Capacity, Total Lung Capacity

Answer 19

Unchanged

Question 20

Pre-oxygenation

Answer 20

Less effective due to reduced FRC

Question 21

Respiratory Rate

Answer 21

Increased by 10% (progesterone mediated hypersensitivity to CO2)

Question 22

Renal function

Answer 22

Increased (therefore reduced serum urea, creatinine and bicarbonate)

Question 23

Propensity to UTI

Answer 23

Increased due to progesterone mediated uterine smooth muscle relaxation causing urinary stasis

Question 24

Acid-base regulation

Answer 24

Increased RR causes reduced PCO2 causing leftward shift in oxyhaemoglobin dissociation curve

Question 25

2,3-DPG concentration

Answer 25

Increased by 30%

Question 26

Lower oesophageal sphincter tone

Answer 26

Reduced, increasing propensity to heartburn (worsened by displacement by gravid uterus) - progesterone induced smooth muscle relaxation

Question 27

Upper oesophageal sphincter tone

Answer 27

Not affected

Question 28

Gastric emptying

Answer 28

Pregnancy does not affect gastric emptying

However, labour increases gastric volume and decreases gastric emptying

Question 29

Insulin production

Answer 29

Rises

Question 30

Insulin resistance

Answer 30

Rises

Question 31

Does insulin cross the placenta?

Answer 31

No

Question 32

Superior boundary of the epidural space

Answer 32

Foramen magnum

Question 33

Inferior boundary of the epidural space

Answer 33

Sacrococcygeal membrane

Question 34

Anterior boundary of the epidural space

Answer 34

Posterior longitudinal ligaments, vertebral bodies

Question 35

Posterior boundaries of the vertebral space

Answer 35

Vertebral laminae, ligamenta flava

Question 36

Lateral boundaries of the vertebral space

Answer 36

Open, pedicles and intervertebral foraminae

Question 37

Layers of ligament to the epidural space

Answer 37

Supraspinous ligament (crunchy, approx 2cm)
Interspinous ligament (spongy, up to 6cm)
Ligamentum flavum (variable thickness, tough, loss of resistance technique here)

Question 38

Epidural test dose

Answer 38

Small enough that it will do no harm if in the wrong place e.g. subarachnoid