Interpretation
Numerous studies have shown that uterine contractions result in
compression of the uterine spiral arteries, decreased placental
perfusion and intermittent fetal hypoxia [5, 18-21]. Peebles et al.
additionally indicated that an interval of 2-3 minutes between
contractions resulted in stable fetal cerebral hemoglobin saturations,
while shorter intervals resulted in decreased
oxyhemoglobin:deoxyhemoglobin ratios [22]. Thus, the time between
contractions allows for maternal-fetal gas exchange, fetal metabolic
recovery, and maintenance of normal fetal acid base status. In cases of
uterine tachysystole, when this recovery time is decreased, accumulation
of cord blood lactate is seen [23]. Our data suggest that higher
intrauterine pressure during this recovery phase is associated with
increased risk of fetal acidemia (as evidenced by low pH and elevated
lactate in the umbilical artery). We suspect that the increased uterine
resting tone results in decreased recovery in utero-placental perfusion
between contractions, similar to the effect seen with tachysystole. In
our sub-group analysis, excluding women with evidence of tachysystole
prior to delivery, we observed the same relationship between
intrauterine resting tone and neonatal morbidity, suggesting an
independent effect.