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.