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Hypoxia is now as before a serious danger to the fetus that can occur within a relatively short period of time. Concerning fetal supervision/ particularly during labor/ we must be quite clear about the special conditions that exist here. Direct insight into the blood gas exchange and the acid-base balance of the fetus have up to now only been possible during labor/ that is after the membranes are ruptured. Access exists only to the presenting part/ which in over 90% of the cases is the area of the fetal scalp. Of particular interest is the measurement of the acid-base balance. In the foreground here are fetal blood analyses/ which have been used now for 22 years. During the past few years continuous tissue pH measurement has been tested in an experimental way/ but without success for routine use. Using fetal blood analysis we should be aware that we are examining the situation in the peripheral circulation. The blood composition does not differ too much from arterial blood/ because the arterio-venous difference in the peripheral circulation is estimated as being relatively small on account of the low metabolism in the skin. There is no difference in the composition of the blood between arteries to the brain and those supplying the scalp/ as they both receive blood from the same main branch of the aorta. Of course/ it would be more informative to analyse venous blood from those organs of-special interest/ for instance from the brain. However/ this is scarcely possible in routine clinical work. However/ for diagnostic purposes it is important to know that particularly in subacute and chronic hypoxia the fetus undergoes the so-called O9 conserving adaptation of its circulation ( 9 ) / and as a result of this pathophysiological mechanism the supply to the brain and heart is affected much later. Other organs/ such as the skin, muscles and intestines are subjected to a reduced supply much earlier. The basic concept of the conserving adaptation is the following: faced with a gradual reduction in oxygen supply due to placental insufficiency or to some other subacute or slowly progressive complications/ the fetus reacts by means of a compensatory adaptation of its circulation. The blood flow and hence the oxygen supply to a number of the less important parts of the body such as the extremities/ abdominal viscera and lungs are reduced by vasoconstriction. The oxygen saved in this way now becomes available for use by the two particularly vital organs/ the brain and the heart. These continue to be supplied with the required amount due to the fact that the total O2 consumption of the fetus has been reduced and the O2 content of its circulating arterial blood maintained at normal or nearnormal levels. To a lesser extent this achievement may be helped by an increase in umbilical-placental blood flow? for hypoxemia is known to lead to an increase in fetal systemic blood pressure due to peripheral vasoconstriction on the one hand (2 /3 ) and to dilatation of the umbilical

Patho-physiology, clinical relevance of continuous measurement of pH and/or CO 2 in the fetus