The clinical significance of base excess (BEB) and base excess in the extracellular fluid compartment (BEEcf) with and without correction to real oxygen saturation of haemoglobin.

BACKGROUND

Besides actual pH, base excess [ctH (+)(B) (mmol/l)] is of major importance since it is meant to reflect lactate acidosis due to foetal hypoxia; In vivo BE (B) is not independent from pCO (2). Independence is achieved by using the extended extracellular fluid (Ecf) for dilution of haemoglobin (cHb (B)) thus reducing cHb (B) to cHb (B)/3 (in the foetus to cHb (B)/4). Correction of ctH (+)(B) from the normally low foetal oxygen saturation by reoxygenation of Hb increases ctH (+)(B), resulting in 4 different variables: ctH (+)(B,act) (=BE (B)), ctH (+)(Ecf,act) (standard BE), ctH (+)(B,ox.) and ctH (+)(Ecf,ox). 3 questions arise: (i) which variable is most appropriate for perinatal acid-base studies? (ii) are there clinical advantages for using BE when compared with actual pH (UA), and (iii) what are the thresholds of the BE parameters?

METHODS

The Apgar 1 min and the WAS score were used thus measuring neonatal vigour and FHR characteristics during the last 30 min of 475 foetuses all delivered by the vaginal route. FHR was evaluated by computation of the WAS index . The WAS index refers to (FHMW1)(OZFW2)(OZA*W3)(-1) where fhm is mean heart frequency (bpm), ozf denotes the number of turning points (N/min) and oza refers to the 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 min of delivery. BE was computed according to the van Slyke/Henderson-Hasselbalch equation using pH and pCO (2) measurements; sO (2) (%) for HbF was determined according to Ruiz et al. .

RESULTS

In vivo foetal ctH (+)(B,act) (UA) is closely correlated with pCO (2). UA: r=-0.288, P<10 (-4), N=475: whereas ctH (+)(Ecf,act) (standard BE) becomes independent from pCO (2): r=-0.0068, P=0.881. In UA blood there is no independence of the 2 blood gases pCO (2) and pO (2): both are inversely correlated: r=-0.291, P<<10 (-4). pO (2) 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 (1 min) is best correlated with pH (UA) (r=0.4078, P<10 (-4)(,) 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 4 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). The threshold of standard BE (ox.)(=ctH (+)(Ecf ox)) in 390 acidotic term infants with still good outcomes is -14.0 mmol/l.

CONCLUSIONS

Actual pH (cH (+)) offers the closest correlation with 2 essential clinical parameters: FHF and Apgar scores; the advantages of ctH (+)(B) and ctH (+)(Ecf), are not self-evident; if determination of the metabolic component becomes necessary standard BE, (ctH (+)(Ecf)) should be used with correction to 100% oxygen saturation (ctH (+)(Ecf,ox.)) of haemoglobin (HbF), because this quantity (after pH (UA)) correlates best with clinical indices. However if the 'correction' is omitted the difference seems clinically irrelevant.