Quantification of ST waveform charges of the fetal electrocardiogram and their relationship to asphyxia

Greene, Keith Richard (1983) Quantification of ST waveform charges of the fetal electrocardiogram and their relationship to asphyxia. University of Southampton, Doctoral Thesis.

Abstract

In clinical obstetrics, there remains a need for a reliable and more precise system of detecting fetal asphyxia. Recent studies in the acutely exteriorised and anaesthetised fetal lamb have shown progressive changes in the ST waveform of the electrocardiogram with hypoxia. These changes were related to myocardial glycogenolysis. The objectives of this Thesis were: i) to establish that such changes occurred in the unanaesthetised fetal lamb in utero and determine what other factors might affect the ST waveform; ii) to ascertain if changes could be observed on the signal from fetal scalp electrodes in human labour and that they could provide additionally useful clinical information; iii) to examine the possibility of automatic quantification of ST waveform change in the fetal lamb and human fetus. Chronically instrumented fetal lambs were studied for 3 to 26 days, whilst undisturbed, and during hypoxia induced by maternal hypoxaemia. After signal averaging, changes in the ST waveform of the fetal praecordial ECG were quantified by the T/QRS ratio and related to changes in heart rate, blood pressure, blood gases, acid-base, blood glucose and lactate. Minor changes of the ST waveform were observed with fetal breathing and fetal movement, but there were no regular periodic changes. The normal T/QRS ratio was established as 0.25 or less, but some fetuses showed persistently elevated ratios (range 0.32 to 0.80) in association with hypotension and anaemia. The T/QRS ratio also became raised in fetal lambs dying acutely from haemorrhage, or more slowly over several days from infection.During hypoxia, an elevation of the ST waveform only occurred when the myocardium was hypoxic as evidenced by an accumulation of blood lactate. There was a highly significant correlation (r = 0.815) of the T/QRS ratio to the rate of rise of blood lactate. The ST waveform rapidly reverted to normal with resumption of normoxia, which further confirmed the association with anaerobic myocardial glycogenolysis. The ST waveform change with hypoxia could not be reproduced by the fetal infusion of adrenaline at a rate known to produce a similar blood level as that found during hypoxia.No consistent relationship was found between changes in heart rate, heart rate variation, or the ST waveform, either before or during hypoxia. Heart rate and heart rate variation were influenced by underlying rhythms, particularly one of 30 to 60 minutes periodicity. Acute hypoxia generally increased heart rate variation but to a level no greater than might occur normally. Observations of the human fetal ECG, from scalp electrodes in labour, revealed similar ST waveform changes as in the fetal lamb, in association with situations suggestive of hypoxaemia. The T/QRS ratio, measured within the last 30 minutes of labour, was found to be correlated with cord lactate levels, but not pH. A microprocessor based system was developed, which automatically sampled and averaged the ECG waveform to provide quantitative measurements of the T/QRS ratio and ST waveform area.

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