3. Old stuff
          3.2. Old physio stuff (around 2005)
              3.2.3. Physiology
                  3.2.3.1. Acid and base
                      3.2.3.1.4. From Kerry's book
 3.2.3.1.4.10. Pregnancy 

 

 

Hyperventilation

  • Probably due in part to progesterone stimulating the respiratory center
  • Lung volume changes and altered compliance may also contribute

 

Thus,

  • Chronic respiratory alkalosis which is compensated by renal excretion of bicarbonate

 

Typical blood gases results in the third trimester are:

  • pH 7.43
  • pCO2 33mmHg
  • [HCO3] 21mmHg
  • pO2 104 mmHg

 

Reduction in bicarbonate
--> Slightly reduced ability to buffer a metabolic acid load

Changes in ODC

Lower pCO2 would shift the oxygen dissociation curve to the left

But minimal change in pH

Increased 2,3 DPG levels during pregnancy

Overall,

  • ODC is little altered in position

Hyperemesis

  • Nausea and vomiting occur commonly in the first trimester
  • May be severe (hyperemesis gravidarum) and intractable vomiting
    --> Fluid loss and electrolyte disturbances

 

Typically causing metabolic alkalosis

NB:

  • The actual acid-base effect of vomiting depends on the actual mix of acidic gastric fluid and alkaline intestinal secretions in the vomitus.
  • Alkalosis does not always occur with prolonged vomiting.

Maternal Ketosis

The pregnant woman is prone to develop elevated ketone levels because:

  • Fasting during pregnancy more rapidly results in hypoglycaemia and low insulin levels
  • Insulin resistance develops as pregnancy progresses (probably due to placental hormones)
  • Fasting ketosis develops in less than 16 hours in late pregnancy
    * Compared to usually > 24 hours in the non-pregnant female

 

Ketones

  • Ketones can cross the placenta and the foetus can adapt to use them as an energy source
  • Ketones may be important in myelination in the developing central nervous system
  • This mild ketosis that occurs with fasting does not seem to have any adverse effect on the mother
  • Effect of ketosis on foetus is uncertain

However,

  • Ketoacidosis due to maternal DM is more serious and has very serious adverse effect on the foetus

 

Others

Diuretic use may cause a metabolic alkalosis
--> a mixed alkalosis because the hyperventilation has already reduced the pCO2