The effect of sampling technique on measurement of fetal blood pH and gases--an in vitro system.

OBJECTIVE

Fetal scalp blood for intrapartum pH monitoring is usually collected aerobically in small volumes and may therefore be affected by the mode of sampling. The effect of specimen collection and handling on measured index values of fetal blood acid-base balance was evaluated in an in vitro simulated system.

STUDY DESIGN

The system consisted of an air-tight syringe filled with either arterial or venous fresh cord blood and an attached needle with a blunted tip. The rate of blood drop formation at the tip was determined by a variable-speed infusion pump. Two techniques, simulating actual scalp blood sampling, were evaluated: (1) commonly used technique--aspiration of individual drops into a glass tube, transfer into a syringe, and measurement and (2) direct technique--collection as a continuous column of blood into a capillary and subsequent measurement.

RESULTS

Control acid-base values obtained before and immediately after completion of each test were not significantly different. Significant changes from control values were noted when sampling by the commonly used technique: mean arterial pH (7.21 to 7.26 and 7.24) and PO2 (19 to 25 and 23 mm Hg) increased, whereas PCO2 decreased (56 to 44 and 47 mm Hg) at slow and fast drop rates, respectively. Changes in venous blood samples were of similar direction and magnitude. Absolute changes from control during sampling by the commonly used technique averaged 21% +/- 2% and 15% +/- 1% (+/- SEM) for slow and fast drop rates, respectively (p < 0.01). On the other hand, sampling by the direct technique did not have a significant effect on measured acid-base values at slow and fast drop rates: absolute changes from control averaged 4.5% +/- 0.4% and 4.7% +/- 1.3% (+/- SEM), respectively.

CONCLUSIONS

The mode of fetal scalp blood sampling and handling affects measured acid-base index values. Such changes reflect aeration of small blood volumes and may result in falsely elevated capillary scalp pH. This effect may be prominent when blood drops are slow forming or difficult to obtain.