DO FETAL ELECTROCARDIOGRAM PR-RR CHANGES REFLECT PROGRESSIVE ASPHYXIAAFTER REPEATED UMBILICAL-CORD OCCLUSION IN FETAL SHEEP

The aim of this study was to determine whether there is a relationship between changes in PR-RR correlation of the fetal ECG and progressive changes in fetal acid-base status and blood pressure (BP) during repe ated umbilical occlusion. Chronically instrumented fetal sheep at 126. 8 +/- 0.6 d (mean +/- SEM) were randomized to receive 1 min of total u mbilical cord occlusion either every 5 min for 4 h (1:5 group; n = 8), or every 2.5 min until BP fell <2.7 kPa (20 mm Hg) on two successive occlusions (1:2.5 group; n = 8). The PR-RR correlation was determined in 5- or 2.5-min intervals. Umbilical cord occlusion caused variable d ecelerations with initial sustained hypertension. In the 1:5 group BP remained elevated throughout, and there was little change in acid-base status (pH = 7.34 +/- 0.07, base deficit = 1.3 +/- 3.9 after 4 h). Tn contrast, after the third occlusion the 1:2.5 group showed progressiv e hypotension during occlusions, and severe progressive metabolic acid emia (pH 6.92 +/- 0.1, base deficit 17.0 +/- 4.7 mmol/L after the last occlusion). In both groups, the PR-RR relationship switched from posi tive to negative with the onset of occlusions, then reverted to positi ve after a variable interval. In the 1:2.5 group later reversion of th e PR-RR to positive was associated with earlier and more prolonged hyp otension during the middle and end of the occlusion series (p < 0.001) . We conclude that the initial switch to a negative PR-RR relationship during repetitive umbilical occlusion was due to a reflex-mediated re sponse unrelated to fetal acidosis or hypotension. Both stable well co mpensated fetuses and severely hypoxic, hypotensive fetuses subsequent ly showed a positive PR-RR correlation.