Changes in the ST waveform of the fetal lamb electrocardiogram with hypoxemia.

The ST waveform of the fetal electrocardiogram (ECG) was examined in 10 chronically instrumented fetal lambs from 115 days to term. Averaged ST waveforms were plotted at 5-minute intervals in six fetuses for 2 to 22 days. No diurnal or other rhythms were seen. To correct for changes in signal gain the amplitude of the T wave was measured relative to the amplitude of the QRS complex. The T/QRS ratio was normally less than 0.30. Persistently elevated ST waveforms with a T/QRS range 0.32 to 0.65 preceded fetal death by some days in three fetuses and were associated with anemia and/or hypotension in a further three. In these animals hypoxia produced a further rise in the ST waveform (mean T/QRS, from 0.48 to 0.81) and all died during labor. In lambs with a normal ST waveform there were differences in the response to hypoxia over 1 hour for a similar fall in PO2. In eight experiments the ST segment and T wave increased (mean T/QRS, from 0.17 to 0.59) and promptly reverted to normal with normoxia. There was a significant rise in mean arterial blood pressure, plasma lactate, and glucose and a fall in pH. In four experiments there was little change (mean T/QRS, from 0.19 to 0.25), with a small rise in plasma lactate suggesting that these lambs were able to maintain aerobic myocardial metabolism. Overall there was a strong correlation of the T/QRS ratio to the rate of rise of lactate. These findings complement previous results in the acute fetal lamb preparation and suggest that ST waveform elevation expressed as the T/QRS ratio identifies a change to anerobic myocardial metabolism.