How to Define a Non-Reassuring FHR Tracing Online

Background: Sub partu foetal heart-rate (FHR) patterns are difficult to evaluate. Until now we have no numerical criteria to reliably define what is a non-reassuring or even a 'pathological ' FHR tracing. Computer-aided FHR analysis using the new WAS score offers the unique opportunity to assess numerical boundaries for these definitions. Methods: Direct FHR tracings of 475 foetuses, all delivered by the vaginal route, were recorded electronically and the last 30 min of each tracing were used for computation of the WAS score. The WAS score refers to FHR frequency, microfluctuation, and oscillation amplitude per minute. Acid-base and blood-gas analyses were performed in blood of the umbilical artery (UA) and vein (UV) immediately post partum. pH(UA) and the WAS score were correlated (r) and the 4 variables sensitivity, specificity, the false positive rate (FPR) and the false negative rate (FNR) were determined. ROC plots for different threshold pH(UA) values were performed. Results: pH(UA) and the WAS score are normally distributed: mean pH(UA) = 7.263 +/- 0.064 and mean WAS score 2.78 +/- 0.81, respectively. The correlation coefficient, r amounts to 0.657, P < < 10(-4). Using the mean pH(UA) and the mean WAS score leads to a sensitivity and specifi city both of 72 % with an FPR and FNR both of 28 %. Using a WAS score of 1.816 and the threshold pH(UA) of 7.122 sensitivity becomes the first time 100 % (FNR = 0 %) and FPR decreases to 10.7 %. These 2 values are chosen to separate a normal (score > 1.816) from an abnormal (non-reassuring) CTG (score = 1.816). According to this definition newborns with a reassuring (normal) FHR tracing (N = 417) have a pH(UA) = 7.275 +/- 0.055, pCO(2) = 52.3 +/- 7.9, BEEcf,oxy. = -3.0 +/- 2.3, sO(2) = 24.9 +/- 12.3 with an acidotic risk (pH(UA) < 7.100) of 0 % and APGAR 1 min < 7 = 0.96 %, respectively. Neonates with an abnormal (non-reassuring) FHR tracing (N = 58) present a mean pH(UA) = 7.178 +/- 0.066, pCO(2) = 62.6 +/- 9.6, BEEcf,oxy. = -5.4 +/- 2.6, sO(2) = 18.3 +/- 12.0 and an acidotic risk of 6.9 %. No neonate in this group was severely depressed (APGAR 1 min <= 3). Separation of the true positive (pH(UA) < 7.122) from the false positive (pH(UA) >= 7.122) cases yields 10 babies with a mean pH(UA) = 7.084 +/- 0.053, pCO(2) = 71.8 +/- 10.7, BEEcf,oxy. = -8.4 +/- 2.0, sO(2) = 20.2 +/- 14.5 and an acidotic risk of 44.4 %; in this group also not one baby was severely depressed (APGAR 1 min <= 3) but 3 were already moderately affected (< 7 after 1 min). No infant scored < 7 after 5 min. There must be a lower limit for the definition of a true 'pathological' FHR tracing; this boundary should not be identical with the threshold pH value where hypoxic injuries in term infants usually commence. For FHF this is a WAS score of 0.630 and a threshold pH(UA) of 7.075. Conclusion: Using sensitivity, specificity, FPR and FNR of the FHR sub partu, it is possible to discriminate online a normal (ressuring) from an abnormal (non-reassuring) FHR tracing by a simple computer-aided procedure which we call WAS scoring.