[Measured quantities in perinatal medicine--the pCO(2)]. / Messgrössen in der Perinatalmedizin -- das pCO(2).

ABSTRACT

BACKGROUND: For many years the interest of the obstetrician and neonatologist was focused on the base excess (BE) in umbilical artery (UA) blood. Otherwise, the pCO (2) (mmHg) value which is necessary to compute BE values played only a marginal role in our diagnostic evaluation of the fetus/newborn. This is especially true for pCO (2) values determined in umbilical venous (UV) blood and the feto-matemal interrelations of blood gases and the variables of the acid-base balance (ABB) in the fetus and the mother. In this study we have reevaluated the importance of fetal pCO (2) measurements and analysed the influence of pCO (2) on the actual pH in UA blood, our leading parameter. MATERIAL AND METHODS: The variables of fetal ABB and the actual blood gases measured in UA and UV blood of 7804 neonates, delivered at term without major malformations in vertex presentation by the vaginal route, were used. Several generations of acid-base equipments from RADIO-METER (Copenhagen) were applied (BMS3 - AMS3). In addition arterial acid-base measurements in 101 parturients were performed immediately after delivery for analysis of feto-maternal acid-base dependencies. The software system JMP (SAS, Carolina, USA) and a Think-Pad (IBM) were used for computation. RESULTS: The median pCO (2) in VA blood was 50.5 and in UV blood 36.0 mmHg, respectively. Both distributions are normal. The AV difference amounted to 13.6 mm Hg. The correlation between pCO (2), UA and the actual pH, UA is highly significant (r = -0.720, P << 0.0000). The correlation between pCO (2), UV and the pH, UA is still highly significant (r = -0.560, P<< 0.0000). Analysing only fetuses with an APGAR index of 9 or 10 after 1 min (N = 6304) reveals the important physiological influence of pCO (2), UV on actual pH, UA (r = -0.543, P << 0.0000). Moreover there exists an inverse and also highly significant dependency of pCO (2), UV and pO (2), UV in vigorous infants (P << 0.0000) indicating better fetal oxygenation; this cannot be explained by the weight of the placenta or other feto-placental measures. Taking into account maternal (N = 101) respiration (arterial pH, pCO (2), pO (2)) during the last minutes of delivery leads to convincing evidence (P << 0.000) that pCO (2), UV is determined by maternal pCO (2) and thus by the breathing habits of the future mother (r = 0.514 for pCO (2), UV and arterial pCO (2)). CONCLUSIONS: At the moment of delivery, where we usually determine actual pH and other parameters of the fetal ABB in order to control our obstetrical management, mother and fetus behave functionally as a unit; this fact deeply influences the predictive value of all fetal AB measurements (including BE, UA). This holds true also in fetal pathology. Maternal respiration turns out to be an important co-variable. In legal controversies the feto-maternal associations of blood gases and their implications must be considered carefully.