Ojala et al.1 have shown that in their institution, the practice of ST analysis of the fetal electrocardiography as an adjunct to standard cardiotocography (CTG) over a period of 13 months and 733 cases monitored did not decrease the rate of acidaemia (cord artery pH < 7.10). They also noted an increase in the number of cases with cord artery metabolic acidosis, 1.7% in the CTG + ST arm versus 0.7% among those monitored with conventional CTG. The latter finding is in contrast to previous randomised controlled trials and Cochrane database review and deserves to be clarified further.
Metabolic acidosis is calculated from algorithms using pH and PCO2. Forty years ago, the alignment nomogram based on buffer distribution in the blood was introduced by Siggaard-Andersen.2 These algorithms were found to overestimate the metabolic acidosis component in case of a mixed acidaemia, and the improved acid–base chart was introduced by Siggaard-Andersen in 1971, assessing the distribution of buffers in the whole extracellular fluid.2 Unfortunately, blood gas machines such as the Chiron 348 used in the Finnish study have not adopted the acid–base chart algorithms used in previous STAN work. The implication of this is that metabolic acidosis becomes more frequent as a high PCO2 will cause a high base deficit.3 Thus, any comparison to previous STAN data will have to wait until the data from Oulu have been recalculated using the acid–base chart (Base Deficit in the extra cellular fluid) algorithms.