BACKGROUND: Besides actual pH, base excess (ctH⁺_B (mmol/L)) is of major importance since it is meant to reflect lactate acidosis due to fetal hypoxia; in vivo BE_B is not independent from pCO₂. Independence is achieved by using the extended extracellular fluid (Ecf) for dilution of hemoglobin (cHb_B), reducing cHb_B to cHb_B / 3 (in the fetus to 1/4). Correction of ctH⁺_B from the normally low fetal oxygen saturation by reoxygenation of Hb increases ctH⁺_B resulting in four different variables: ctH⁺_B,act (= BE_B), ctH⁺_Ecf,act (Standard BE), ctH⁺_B,ox. and ctH⁺_Ecf,ox. The question is which variable is most appropriate for perinatal acid-base studies?

METHODS: The APGAR 1 min. and the WAS score were used, thus measuring neonatal vigor and FHR (Fetal Heart Rate) characteristics during the last 30 minutes of 475 fetuses all delivered vaginally.
FHR was evaluated by computation of the WAS index. 
The WAS index refers to: (fhm ×  w1) × (ozf × w2) × (oza × w3)^–1 where fhm is mean heart frequency (bpm), ozf is the number of turning points (N/min.) and oza denotes oscillation amplitude/min. (bpm). The weighting functions w1, w2 and w3 were computed using optimizing software. The WAS score denotes the mean of the WAS indices of the last 30 minutes of delivery.

RESULTS: In vivo fetal ctH⁺_B,act (UA) is closely correlated with pCO₂, UA: r = –0.288, P < 10^–4, N = 475, whereas ctH⁺_Ecf,act (Standard BE) becomes definitely independent from pCO₂: r = –0.0068, P = 0.881. In UA blood there is no independence of the two blood gases pCO₂ and pO₂: both are inversely correlated: r = –0.291, P << 10^–4. pO₂ shows no correlation with ctH⁺_B,act (r = –0.074,P = 0.105) but correlates well with ctH⁺_Ecf,act: r = –0.1722, P = 0.0002.

The APGAR score is best correlated with pH_UA (r = 0.4078, P < 10^–4, N= 475, Spearman's Rho = 0.307, P < 10^–4). Correction of ctH⁺_B,act or ctH⁺_Ecf,act to 100 % oxygen saturation always leads to higher coefficients. Using ctH⁺_B,ox., ctH⁺_B,act, ctH⁺_Ecf,ox. and ctH⁺_Ecf,act: Rho = 0.2597, 0.2394, 0.1838 and 0.1763, respectively; P all < 10^–4. The same holds true for APGAR 5 min. Rho = 0.2307, 0.2168, 0.1811 and 0.1771, respectively (P < 10^–4 for all).

The WAS score is closely correlated with pH_UA: r = 0.656, P << 10^–4, N = 475. The correlation with the four variables under investigation: ctH⁺_B,ox., ctH⁺_B,act, ctH⁺_Ecf,ox. and ctH⁺_Ecf,act leads to r = –0.587, r = –0.565, r = –0.437 and r = –0.427, respectively (P < 10^–4 for all).

CONCLUSIONS: Actual pH (cH⁺) offers the closest correlation with two essential clinical parameters: FHF and APGAR scores; the advantages of ctH⁺_B and ctH⁺_Ecf are not evident; if determination of a metabolic component becomes necessary, Standard BE, (ctH⁺_Ecf) should be used with correction to 100 % oxygen saturation (ctH⁺_Ecf,ox.) of hemoglobin (HbF), because this quantity correlates best with clinical indices.