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The fetus depends on the mother for placental exchange of oxygen and carbon dioxide. This in turn relies on adequate maternal blood gas concentrations, uterine blood supply, placental transfer and fetal gas transport. Disruption of any of these can cause fetal hypoxia, which, despite compensatory mechanisms, may lead to acidosis. When severe and acute (lasting hours), but especially if prolonged (days or weeks), hypoxia and therefore acidosis, are associated with significant morbidity and mortality with potential long term sequelae. Whether this damage is primarily due to reduced cell energy availability, as a result of hypoxia, or secondary to cell poisoning, as a result of acidosis, is unclear and indeed acidosis could simply be a marker of the cause and severity of the hypoxia.
Key points
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The causes and consequences ofacute (minutes or hours) andchronic (days or weeks) fetal acidosis are different
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In the past much attention has been paid to acute acidosis during labour, but in previously normal fetuses this israrely associated with subsequent damage
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In contrast, chronic acidosis, which is often not detected antenatally, is associated with a significant increase in neurodevelopmental delay
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The identification of small for gestational age fetuses by ultrasound scans and the use of Doppler waveforms to detect which of these have placental dysfunction mean that these fetuses can be monitored antenatally
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Delivery before hypoxia has produced chronic acidosis, may prevent subsequent damage and good timing of delivery remains the only management option at present.
The very different aetiologies of acute vschronic acidosis and the possible consequences will be reviewed, whether directly caused by the acidosis or indirectly by the hypoxia
What is acidosis?
Acidosis means a high hydrogen ion concentration in the tissues. Acidaemia refers to a high hydrogen ion concentration in the blood and is the most easily measured indication of tissue …