Fetal Blood Gases

Question 1

Describe the 4 outcomes for substances entering the placenta:

Answer 1

1. Transported intact e.g. alcohol
2. Part consumed e.g. oxygen and glucose
3. Metabolised
4. Not transported

Question 2

What drives the transport of gas across the placenta?

Answer 2

• Gases (oxygen and carbon dioxide) travel down a concentration gradient
• Maternal PaO2 is ~95mmHg and fetal PaO2 is only 25mmHg so there is a clear pressure gradient driving the diffusion of oxygen from the maternal blood into fetal circulation

Question 3

Why is fetal blood oxygen concentration so low when there is a pressure gradient driving the diffusion of oxygen from maternal into fetal circulation?

Answer 3

• The oxygen must diffuse through the placenta which consumes oxygen at a high rate as it is a highly metabolic organ
• There may also be a mismatch of perfusion between maternal and fetal blood (which accounts for 50% of the difference between fetal and maternal PaO2 in sheep)

Question 4

How does the fetus retain reasonable oxygen saturation despite low oxygen concentration?

Answer 4

• As gestation progresses, erythropoiesis begins to take place in the fetal liver
• Within the liver, red blood cells contain fetal haemoglobin which contains 2 alpha and 2 gamma subunits is produced
• The fetal hemoglobin gamma subunits bind a molecule called 2,3-BPG much less efficiently than the beta subunits in adult hemoglobin so the fetal hemoglobin gamma subunits are more avaliable for oxygen binding and therefore have a much higher affinity for oxygen
• This higher affinity for oxygen binding allows the fetal hemoglobin to saturate more highly, even at a lower PaO2 of oxygen in the blood

Question 5

How is fetal oxygen delivery (supply) calculated?

Answer 5

umbilical blood flow x umbilical venous [O2]

Question 6

How is oxygen consumption by the fetus calculated?

Answer 6

umbilical blood flow x (umbilical venous [O2] - umbilical arterial [O2])

Question 7

How is carbon dioxide transported by the placenta?

Answer 7

• The placenta is highly permeable so carbon dioxide is able to diffuse along its pressure gradient from fetal into maternal circulation
• Fetal [CO2] remains higher than maternal [CO2] due to placental CO2 production and inequalities of maternal and fetal placental blood flow

Question 8

How is carbon dioxide buffered in the blood?

Answer 8

• Carbon dioxide in the blood is hydrated by carbonic anhydrase and becomes carbonic acid
• Carbonic acid is buffered in the blood by mechanisms that bind H+ ions to keep fetal blood pH regulated:
  1. Carbonic-acid bicarbonate system (bicarbonate is regulated by fetal kidneys)
  2. Hemoglobin
• The ability for the fetus to regulate this acid base balance is limited compared to an adult, and is only established later in gestation- therefore in the event of asphyxiation a fetus may be unable to buffer the amount of carbonic acid in the blood and easily become acidotic

Question 9

How do nutrients move across the placenta?

Answer 9

• GLUTs transport glucose across the placenta
• FATPs transport fatty acids across the placenta
• Amino acid transporters transport amino acids across the placenta
• Glucose although moved across the placenta by GLUTs requires a concentration gradient to facilitate its movement (so concentration of maternal blood glucose must be higher than fetal blood glucose)
• Additionally the placenta uptakes glucose and helps establish the glucose concentration gradient

Question 10

How is fetal glucose utilisation regulated?

Answer 10

• The fetus is a highly glucose dependent organism
• Placental glucose uptake contributes to fetal hypoglycaemia and helps establish a concentration gradient
• Additionally if the fetus is stressed it will release adrenaline that increases the movement of glucose across the placenta

Question 11

Describe the regulation of lactate in the fetus:

Answer 11

• Lactate is a waste product produced due to anaerobic respiration
• Lactate enters both fetal and maternal circulation in approximately equal amounts
• However if the fetus becomes stressed, this can trigger an increase in anaerobic metabolism and subsequently an increase in lactate levels

Question 12

Explain the terms:

1. Hypoxia:
2. Hypoxemia
3. Asphyxia

Answer 12

1. Hypoxia: low O2 in tissues
2. Hypoxemia: low O2 in blood
3. Asphyxia: low O2 in tissues and blood, high CO2 and decreases pH in blood

Question 13

What physiological measurements indicate asphyxia?

Answer 13

1. Hypercapnia (high CO2)
2. Hypoxemia and hypoxia
3. Metabolic acidosis
4. Respiratory acidosis
5. High lactate
6. High hemoglobin
7. High glucose (acute)
8. Low glucose (chronic)

Question 14

Explain the features of metabolic acidosis:

Answer 14

• Bicarbonate low, pH low

Cause:

• Lactic acidosis secondary to tissue hypoxia
• Inability to excrete/buffer accumulated acids
• Excessive loss of bicarbonate in the urine or gut (common in preterm extremely babies)

Treatments:

• Fluids
• Sodium bicarbonate

Question 15

Explain the features of metabolic alkalosis:

Answer 15

• Bicarbonate high, pH high

Causes:

• Hypocholaremia due to diuretic therapy of upper GI obstruction
• Excess bicarbonate

Treatment:
- Chloride replacement

Question 16

Explain the features of respiratory acidosis:

Answer 16

• pCO2 high, pH low

Causes:

• Inadequate alveolar ventilation
• Depression of breathing centre in brain
• Upper airway obstruction
• Significant ventilation/perfusion imbalance

If Chronic:
- Body will compensate and there will be respiratory acidosis with compensated high bicarbonate

Treatment:
- Respiratory support and increasing oxygenation

Question 17

Explain the features of respiratory alkalosis:

Answer 17

• pCO2 low, ph high

Causes:

• excessive mechanical ventilation
• abnormal control of ventilation e.g. in HIE

Treatment:
- wean mechanical ventilation by reducing tidal volume or respiratory rate