Objective: The purpose of this study was to examine the hypothesis that fetuses from multiple pregnancies who exhibited spontaneous, stable hypoxia would show more rapid development of metabolic acidosis and hypotension during short repeated episodes of umbilical cord occlusion than normoxic singleton or twin fetuses and, if this proved to be true, to determine whether this deterioration could be identified by changes in T/QRS height or ST waveform shape.
Study design: Chronically instrumented near-term sheep fetuses (124 +/- 1 day) were subjected to 1-minute umbilical cord occlusions every 5 minutes (normoxic group, 8 fetuses; hypoxic group, 10 fetuses) for a total of 4 hours or until mean arterial blood pressure fell below 20 mm Hg for 2 successive occlusions.
Results: The spontaneous hypoxic, but not normoxic, fetuses had progressive compromise during repeated umbilical cord occlusions, with severe, progressive metabolic acidosis (pH, 7.07 +/- 0.14; base deficit,13 +/- 1.5 mmol/L vs 7.34 +/- 0.07 mmol/L; base deficit after the final occlusion, 1.1 +/- 1.4 mmol/L; P < .001), and hypotension (24 +/- 2 mm Hg vs 45.5 +/- 3 mm Hg; P < .0001); 4 hypoxic fetuses were unable to complete the 4-hour series of occlusions. The hypoxic group showed a much greater increase in T/QRS ratio during (P < .001) and between occlusions. Biphasic ST segment waveforms were not seen in either group either during or between occlusions.
Conclusion: Exaggerated elevation of the T/QRS ratio during decelerations in early labor may be a useful marker of greater myocardial anaerobic stress (eg, because of pre-existing spontaneous hypoxia), but not of the development of fetal acidemia or hypotension.