Interestingly, the duration of uterine contractions-60 to 80 seconds-does not lengthen appreciably from early active labor through the second stage (Bakker, 2007; Pontonnier, 1975). Presumably, this duration constancy serves fetal respiratory gas exchange. During a uterine contraction, as the intrauterine pressure exceeds that of the intervillous space, respiratory gas exchange is halted. This leads to functional fetal "breath holding," which has a 60-to 80-second limit that remains relatively constant.
readily be depressed by the inger. At greater intensities, the uterine wall then becomes so hard that it resists easy depression. Uterine contractions usually are not associated with pain until their strength exceeds 15 mm Hg. Presumably, this is the minimum pressure required to distend the lower uterine segment and cervix. It follows that Braxton Hicks contractions exceeding 15 mm Hg may be perceived as uncomfortable because distention of the uterus, cervix, and birth canal is generally thought to produce discomfort.
Hendricks (1968) observed that "the clinician makes great demands upon the uterus." The uterus is expected to remain well relaxed during pregnancy, to contract efectively but intermittently during labor, and then to remain in a state of almost constant contraction for several hours postpartum. Figure 24-28 demonstrates an example of normal uterine activity during labor. Uterine activity progressively and gradually rises from early through late labor. Interestingly, uterine contractions after birth are identical to those resulting in delivery of the newborn. Logically, the uterus that performs poorly before delivery is also prone to atony and puerperal hemorrhage.
J.A. 5-26-60
Intrauterine pressure (mm Hg) Delivery 2:54 PM.
8:45 A.M.
10:20 A.M.
1:10ePM. 2:30 PM.
3:30 PM.
5:20 P.M.
Caldeyro-Barcia and Poseiro (1960) also observed empirically that uterine contractions are clinically palpable only after their intensity exceeds 10 mm Hg. Moreover, until the intensity of contractions reaches 40 mm Hg, the uterine wall can
FIGURE 24-28 Intrauterine pressure recorded through a single catheter.
A. Prelabor. B. Early labor. C. Active labor. D. Late labor. E. Spontaneous activity Y2 hour postpartum. F. Spontaneous activity 2Y2 hours postpartum. (Redrawn from Hendricks (H: Uterine contractility changes in the early puerperium, (lin Obstet Gynecol. 1968 Mar;11 (1 ):125-144.) , I of the --contraction
Amnionic pressure[1 0 Tonus -.A
FIGURE 24-29 Schematic representation of the normal contractile wave of labor. Large uterus on the let shows the four points at which intramyometrial pressure was recorded with microballoons. Four corresponding pressure tracings are shown in relation to each other by shading on the small uteri at top. (Adapted with permission from (aldeyro-Barcia R, Poseiro JJ: Physiology ofthe uterine contraction. (lin Obstet Gynecol 1960 3:386.) • Origin and Propagation of Contractions