Fluid and Acid Base Balance Flashcards
How does fluid balance change in pregnancy?
1) The plasma volume typically increases by 1.5L from 3 to 4.5L.
2) Even interstitial volume increases typically from 11 to 15L.
Why do we get more oedema in pregnancy?
There is a rise in H2O in the plasma, causing a fall in conc of albumin. Meaning imabalance of starling forces, so more oedema.
Esp so in the third trimester when arterial pressure rieses.
What happens to plasma volume in pre-eclampsia
It falls.
How does Acid Base change in pregnancy?
There is a slight resp alkalosis = This is because a low CO2 is needed to maintain a diffusion gradient for CO2 across the placenta.
This causes a compensatory drop in the HCO3, which can put the pH back into a normal range.
What happens to Acid base balance in labour?
The normal resp alkalosis of pregnancy, gets worse during labour because of maternal hyperventilation. Renal compensation cannot occur so resp alkalosis looks worse with higher pH
How can maternal resp alkalosis affect the pH of the foetus
Remember mothers tend to have resp alkalosis due to maternal hyperventilation.
1) Alkalosis shifts dissociation curve to the left. Meaning that O2 has a higher affinity for maternal Hb so it binds easier. This causes a drop of PaO2 in the umbilical vein (placenta to foetus), which can cause more anaerobic resp in foetus and can a foetal metabolic acidosis
2) THe increased proportion of deoxygenation Hb in the foetus also means more CO2 can be carried in solution. And this contirbutes to a resp acidosis int he foetus as well.
Therefore maternal hypervetilation can contribute to both a metabolic acidosis and resp acidosis.
What is the acid base balance like in a foetus
Usually an acidaemia that is mixed resp and metabolic. With a low pH, raised CO2, low HCO3, and negative BE. Need to treat underlying cause or the aggravating factor, which could be maternal shock
