Uteroplacental O2 uptake: continuous measurements during uterine quiescence and contractions

To determine the constancy of uteroplacental O2 uptake (VO2) during uterine quiescence and both spontaneous and oxytocin-induced uterine contractions, we have developed a method to measure VO2 continuously and reproducibly. In seven ewes during uterine quiescence, total uterine blood flow (Qut) averaged 200 ml X min-1 X kg uterine contents-1, with intra-animal SD of 18 min X min-1 X kg-1 and interanimal SD of 86 ml X min-1 X kg-1. Uterine arteriovenous O2 content difference averaged 4.2 +/- 0.5 ml X dl-1. VO2 averaged 8.2 +/- 1.2 ml X min-1 X kg-1, with interanimal SD of 3.6 ml X min-1 X kg-1. During 21 spontaneous prelabor contractions, VO2 decreased 3.8% to 8.7 +/- 0.3 ml X min-1 X kg-1 (NS) during the contraction and subsequently increased 12.3% to 10.1 +/- 0.3 ml X min-1 X kg-1 (P less than 0.05) near the end of the contraction. With oxytocin-induced contractions, VO2 fell 10% to 6.4 +/- 0.2 ml X min-1 X kg-1 at the contraction onset (P less than 0.05) but by the end of the contraction had increased 6.1% to 7.6 +/- 0.2 ml X min-1 X kg-1 (NS). In conclusion, uteroplacental VO2 was measured continuously and reproducibly using relatively straightforward methodology; uteroplacental blood flow, arteriovenous O2 content difference, and uteroplacental O2 uptake were normally distributed with coefficients of variation less than 15%; and during both spontaneous and oxytocin-induced uterine contractions Qut decreased significantly, while VO2 initially decreased moderately and then showed an overshoot, during the contraction.