Quantitative evaluation of fetal heart rate patterns

Use of automated fetal heart rate analysis to identify risk factors for umbilical cord acidosis at birth

OBJECTIVE

To identify clinical parameters and intrapartum fetal heart rate parameters associated with a risk of umbilical cord acidosis at birth, using an automated analysis method based on empirical mode decomposition.

METHODS

Our single-center study included 381 cases (arterial cord blood pH at birth pHa ≤7.15) and 1860 controls (pHa ≥7.25) extracted from a database comprising 8,383 full datasets for over-18 mothers after vaginal or caesarean non-twin, non-breech deliveries at term (>37 weeks of amenorrhea). The analysis of a 120-min period of the FHR recording (before maternal pushing or the decision to perform a caesarean section during labor) led to the extraction of morphological, frequency-related, and long- and short-term heart rate variability variables. After univariate analyses, sparse partial least square selection and logistic regression were applied.

RESULTS

Several clinical factors were predictive of fetal acidosis in a multivariate analysis: nulliparity (odds ratio (OR) 95% confidence interval (CI)]: 1.769 [1.362-2.300]), a male fetus (1.408 [1.097-1.811]), and the term of the pregnancy (1.333 [1.189-1.497]). The risk of acidosis increased with the time interval between the end of the FHR recording and the delivery (OR [95%CI] for a 1-min increment: 1.022 [1.012-1.031]). The risk factors related to the FHR signal were mainly the difference between the mean baseline and the mean FHR (OR [95%CI]: 1.292 [1.174-1.424]), the baseline range (1.027 [1.014-1.040]), fetal bradycardia (1.038 [1.003-1.075]) and the late deceleration area (1.002 [1.000-1.005]). The area under the curve for the multivariate model was 0.79 [0.76; 0.81].

CONCLUSION

In addition to clinical predictors, the automated FHR analysis highlighted other significant predictors, such as the baseline range, the instability of the FHR signal and the late deceleration area. This study further extends the routine application of automated FHR analysis during labor and, ultimately, contributes to the development of predictive scores for fetal acidosis.

Antepartum fetal heart rate monitoring. A semi-quantitative evaluation of the 'non-stressed' fetal heart rate

1964 recordings (from 246 patients were analysed on the basis of a semi-quantitative evaluation of base-line heart rate variability. All the tracings were obtained in the antepartum period, between the 20th wk of pregnancy and prior to the onset of labour. None of the recordings were made during labour or after induction of uterine activity so that a 'non-stressed' fetal heart rate test was obtained in every case. The percentage of flat tracing in each recording and the presence of sinusoidal patterns were compared with the neonatal outcome. When the tracings included a more than 50% flat pattern, 81.8% of the babies were distressed at birth. When a less than 50% flat recording was present only 11.4% of the neonates were distressed. Sinusoidal patterns were more frequently associated with abnormal tracings including those with more than 50% flat recordings. The presence of sinusoidal features did not significantly influence the neonatal outcome for tracings with a less than 50% anomaly: conversely sinusoidal patterns superimposed on a more than 50% flat tracing were predictive of fetal compromise and neonatal distress in 100% of the cases. These results allow to advocate the use of 'non-stressed' antepartum fetal heart rate monitoring, as a reliable test in the management of high risk pregnancies.

The evaluation and significance of intrapartum FHR-oscillation patterns

The last two hours of 342 intrapartum FHR records of high technical quality were evaluated visually with simple tools (compasses, caliper). All data were entered on punched cards (25 pro fetus) and analyzed using an IBM system 370/135. In 253 fetuses with an actual pH in the umbilical artery (UA) above 7.199 the oscillation frequency (OF) during the last 30 min. preceding delivery amounted to 9.5 +/- 4.5 (4615 minutes without decelerations or accelerations). The 5th percentile, the median and the 95th percentile were 2,9 and 18 per minute respectively. OF refers to the number of visually detectable turning points in baseline FHR. There exists a significant association (2 alpha less than 0.001) between the OF and the oscillation amplitude (OA) per minute: a reduction in OF is followed by a decrease in OA even in vigorous infants. In acidotic fetuses OF is decreased when compared with FHR minutes of equal OA in nonacidotic infants. However this becomes statistically significant only in fetuses with actual pH (UA) below 7.150 i.e. in fetuses with severe acidosis. These findings can explain why the loss of FHR variability as a single symptom may be misleading in individual cases.

A comparison of methods for quantitating fetal heart rate variability

Fetal heart rate (FHR) variability is thought to be an important index of fetal health. In the presence of normal variability, the fetus is vigorous, but lack of beat-to-beat variability may be associated with fetal compromise. A distinction between short-term variability (STV) (beat-to-beat changes between successive beats) and long-term variability (LTV) (rhythmic fluctuations in FHR) has not been made to date. We have utilized computer programs to compare three pairs of mathematical indices and one visual index of FHR variability. Among the three pairs of indices designed for detection of STV and LTV, de Haan's short-term and long-term indices exhibited the least interdependence, and the long-term index was completely insensitive to artifically generated pure STV. Yeh's short-term and long-term indices exhibited substantial positive interdependence. Hon's visual index appears to detect LTV primarily rather than STV. When the effect of progression of labor on FHR variability was examined, no conclusions were possible because of inconsistencies between patients. Ultimately, the clinical value of any one of these indices awaits testing of their ability to define fetal well-being or fetal distress.