Fetal ECG waveform analysis for intrapartum monitoring
Abstract: Waveform analysis of the fetal electrocardiogram has been studied since the early seventies from both physiological, signal processing and clinical aspects. It has been found to contain more useful information than the mere detection of RR-intervals (fetal heart rate), e.g., the ST-segment giving evidence of the ability of fetal myocardium to respond to the stress of labor. In the Swedish RCT, STAN S21® was used to monitor 4966 labors either with cardiotocography (CTG) only or with CTG in combination with the ST waveform analysis of the fetal electrocardiogram. A computerized algorithm, the ST Log, was used to identify the qualitative changes in the ST waveform automatically. Metabolic acidosis in cord artery blood defined as pH < 7.05 and BDecf > 12 mmol/L, decreased with 54% in the CTG+ST group according to intention to treat analysis. The number of operative deliveries for fetal distress decreased by 19%. A blinded assessment of neonatal outcome revealed an improvement of Apgar score 1 and 5 min, need for intensive care and neonatal encephalopathy in the group monitored with CTG in combination with ST waveform analysis. Detailed evaluation of the CTG- and ST-traces of the fetuses with acidemia at birth and three controls for each from the randomized controlled trial gave a higher inter-observer agreement for ST analysis than for CTG alone. ST waveform analysis in combination with CTG increased the specificity in identifying fetal hypoxia without losing sensitivity and gave a more conform indication to intervene. Interim analysis during the trial revealed several protocol violations and therefore additional educational efforts were made. The experimental study on fetal lamb using microdialysis technique to monitor cerebral energy metabolites simultaneously with ECG recording confirmed earlier studies on the increase in T/QRS ratio with asphyxia. The increase preceded the liberation of excitotoxic transmitters and signs of cell membrane degradation. Thus, during clinical conditions, STAN monitoring should indicate fetal hypoxia before irreversible brain damage has occurred. ST analysis of the fetal ECG in combination with CTG, reduced the incidence of newborns with adverse outcome and concomitantly reduced operative interventions for fetal distress — it is ready for introduction into standard clinical practice
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