Continuous Measurement of Scalp Tissue Oxygen Tension and Carotid Arterial Oxygen Tension in the Fetal Lamb

Abstract

Scalp tissue P_O2 carotid arterial P_O2 and fetal heart rate were continuously measured in the anaesthetized fetal lamb in utero while variations in oxygen supply were brought about. In some experiments the transcutaneously measured fetal scalp P_O2 was recorded in addition. Scalp tissue P_O2 was measured using a specially designed miniature needle-type oxygen electrode, incorporated in an easily applicable spiral scalp electrode as commonly used for fetal heart rate monitoring. The measurements showed that fetal carotid arterial hypoxaemia is always nearly immediately followed by fetal scalp tissue hypoxia, and that the recovery of scalp tissue P_O2 after a hypoxic period has a remarkably varying time course. Fetal heart rate usually decreased during hypoxia, but in some instances it did not change or even increased, demonstrating that heart rate is not always a reliable indicator of fetal hypoxia. P_O2 values obtained with the transcutaneous method were higher than those with the needle electrode, because of the effect of the heating system of the transcutaneous electrode on tissue blood flow and haemoglobin oxygen affinity. It would seem that during hypoxaemia the decrease in scalp tissue P_O2 is possibly the combined result of the fall in arterial P_O2 and a concomitant decrease in blood flow through the skin.

Influence of Variations in pH and P_CO₂ on Scalp Tissue Oxygen Tension and Carotid Arterial Oxygen Tension in the Fetal Lamb

“…As compared with earlier experiments [8], Po,sc levels were found to be considerably higher in the present study. This can be ascribed to a better fixation of the needle tip in the deeper layers of the subcutaneous tis sue [9].…”

“…The results of an initial study on the human fetus during labour seemed promising indeed [3]. In recent arti cles, however, the reliability of this technique when used during labour has been ques tioned, because strong evidence was found for reduced capillary blood flow under the electrode due to mechanical factors [4][5][6], Continuous measurement of fetal subcuta neous Po, (P0,sc) with a small needle elec trode in the scalp or the breech is another recent technique for monitoring the fetus during labour [7,8], The small needle-elec trode can easily be applied during early la bour, when local capillary blood flow is not yet affected by mechanical factors. The re sults obtained in a first clinical study with this needle-electrode were encouraging [9], Oxygen delivery to tissue does not only depend on capillary blood flow and blood oxygen content, but is also determined by oxygen unloading factors, one of which is the Bohr effect.…”

THE RELATIONSHIP BETWEEN ARTERIAL AND TISSUE PO2 IN THE FETAL LAMB

“…The major difference which emerges between mother and fetus is the low PO 2 of fetal blood compared with maternal blood. This is likely to impose an even lower tissue PO 2 on the fetus than that found in the adult and values reported for fetal tissues have ranged from 1 to 20 mmHg (1)(2)(3)(4)(5)(6).…”

Contribution of the BOHR effect to the fall in fetal Po2caused by maternal alkalosis

“…Recent experimental findings suggest that this may be part of the normal physiological response in moderate to heavy exercise [6,17], and it is therefore of interest in the general clinical context of pregnancy. The fall in fetal Ρθ2 during respiratory alkalosis has been demonstrated experimentally in the guinea pig [22] and sheep [1,15,24] and three mechanisms have been proposed to explain it. Firstly, a rise in pH will increase the oxygen affinity of maternal blood (the BOHR effect), and the closer approximation of the maternal and fetal oxygen dissociation curves may entail that equilibrium between the blood of the mother and the fetus is reached at a lower Ρθ2.…”