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In 1958, James et al recognised that umbilical cord blood gas analysis can give an indication of preceding fetal hypoxic stress.1 It has since become widely accepted that umbilical cord blood gas analysis can provide important information about the past, present and possibly the future condition of the infant. Umbilical cord blood gas analysis is now recommended in all high-risk deliveries by both the British and American Colleges of Obstetrics and Gynaecology,2 3 and in some centres it is practised routinely following all deliveries. It is therefore of increasing clinical and medicolegal importance that clinicians caring for newborn infants are familiar with the principles and practice of obtaining and interpreting cord blood gas values, and with the underlying evidence base.
Umbilical cord blood analysis is assumed to give a picture of the acid–base balance of the infant at the moment of birth when the umbilical circulation was arrested by clamping of the cord. However, from this moment onwards the umbilical cord blood, if it remains in continuity with placenta, will demonstrate progressive change in acid–base status due to ongoing placental metabolism and gas exchange. Small changes in umbilical pH occur within 60 s of delivery,4 and over 60 min cord arterial or venous pH can fall by more than 0.2 pH units.5 Similar changes occur in blood sampled from placental surface vessels except that they are larger and less predictable.6 These changes are not observed if the cord is doubly clamped at birth, isolating a segment of cord blood from both the placenta and the environment.4 The pH of the blood then remains relatively constant at room temperature for an hour.5 7–9
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