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Fetal acid-base physiology

Fetal acid-base physiology

Authors

Edward R Yeomans, MDSusan M Ramin, MD

Section Editor

Charles !oc"wood, MDDeputy Editor

#anessa A $arss, MD

DisclosuresAll topics are updated as new e%idence becomes a%ailable and ourpeer re%iew processis

complete&

!iterature re%iew current through' (ct )*+)& his topic last updated' ul )*, )*+)&

./R(D0C.(/ 1 /ormal 2etal metabolism results in the production o2 acids which arebu22ered to maintain e3tracellular p4 within a critical range& #ery small changes in p4 may

signi2icantly a22ect 2unction o2 %arious 2etal organ systems, such as the central ner%ous system

and the cardio%ascular system 5+6& he ma7or bu22ers utili8ed by the 2etus 2or neutrali8inghydrogen ion production are plasma bicarbonate and hemoglobin 5)6& .norganic phosphates and

erythrocyte bicarbonate are also potential bu22ers, although they play a lesser role in 2etal acid-

base homeostasis 5)6&

Fetal hypo3ia can occur when maternal o3ygenation is compromised, maternal per2usion o2 theplacenta is reduced, or deli%ery o2 o3ygenated blood 2rom the placenta to the 2etus is impeded&

9hen ade:uate 2etal o3ygenation does not occur, complete o3idati%e metabolism o2

carbohydrates to carbon dio3ide ;C()< and water is impaired and metabolism proceeds along ananaerobic pathway with production o2 organic acids, such as lactic acid, which are not readilye3creted or metaboli8ed& Accumulation o2 lactic acid can deplete the bu22er system and result in

metabolic acidosis&

Fetal acid-base physiology will be re%iewed here& .nterpretation o2 umbilical cord p4 and bloodgas %alues assessed immediately a2ter deli%ery is discussed elsewhere& ;See =0mbilical cord

blood acid-base analysis=&4YS.(!(?Y 1 he 2etus produces both %olatile ;carbonic acid< and

non%olatile acids ;noncarbonic or organic acids

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For the most part, the 2etus can handle the amount o2 carbonic acid produced daily 2rom aerobic

metabolism since carbonic acid dissociates to water and C(), which readily di22uses across the

placenta& Di22usion o2 C() across the placenta is 2acilitated by a lower >C() in the motherduring pregnancy, secondary to hyper%entilation 5+6&

(rganic acids 1 /oncarbonic or organic acids result 2rom 2etal anaerobic metabolism, whichoccurs when placental trans2er o2 o3ygen is restricted& 0nli"e carbonic acid, the organic acids are

cleared %ery slowly across the placenta and there2ore accumulate in the 2etus& Metabolicacidemia de%elops when the primary bu22er, bicarbonate ;4C(@