The normal contractile wave of labor originates near the uterine end of one of the fallopian tubes. hus, these areas act as "pacemakers" (Fig. 24-29). he right pacemaker usually predominates over the left and starts most contractile waves. Contractions spread from the pacemaker area throughout the uterus at 2 em/sec, and the whole organ is depolarized within 15 seconds. his depolarization wave propagates downward toward the cervix. Intensity is greatest in the fundus, and it diminishes in the lower uterus. his phenomenon is thought to relect the reduced myometrial thickness from the fundus to the cervix. Presumably, this descending pressure gradient serves to direct fetal descent toward the cervix and to eface the cer vix. Importantly, all parts of the uterus are synchronized and reach their peak pressure almost simultaneously, giving rise to the curvilinear waveform shown in Figure 24-29. Young and Zhang (2004) have shown that the initiation of each contraction is triggered by a tissue-level bioelectrical event.
The pacemaker theory also serves to explain the varying intensity of adjacent coupled contractions shown in panels A and B of Figure 24-28. Such coupling was termed incoordination by Caldeyro-Barcia and Poseiro (1960). A contractile wave begins in one cornual-region pacemaker but does not synchronously depolarize the entire uterus. As a result, another contraction begins in the contralateral pacemaker and produces the second contractile wave ofthe couplet. hese small contractions alternating with larger ones appear to be typical of early labor. Indeed, labor may progress with such uterine activity, albeit at a slower pace. hese authors also observed that labor would progress slowly ifregular contractions were hypotonicthat is, contractions with intensity <25 mm Hg or frequency <2 per 10 minutes.
Terms for the description and quantiication of uterine contractions have been recommended by the American College of Obstetricians and Gynecologists (2017b). Normal uterine activiy is defined as five or fewer contractions in 10 minutes, averaged during a 30-minute span. Tachysystole is more than ive contractions in 10 minutes, averaged over 30 minutes. Tachysystole can be applied to spontaneous or induced labor. he term hyperstimulation was abandoned.
Stewart and associates (2012) prospectively studied uterine tachysystole in 584 women undergoing labor induction with misoprostol at Parkland Hospital. A higher rate of adverse neonatal outcomes was not associated with an increasing number of contractions per 10 minutes or per 30 minutes. Counts of six or more contractions in 10 minutes, however, were significantly associated with fetal heart rate decelerations.
Electrodes for fetal heart rate evaluation and catheters for uterine contraction measurement are both associated with infrequent but potentially serious complications. Rarely, an intrauterine pressure catheter during placement may lacerate a fetal vessel in the placenta. Also with insertion, placental and possibly uterine perforation can cause hemorrhage, abruption, serious morbidity, and spurious recordings that have resulted in inappropriate management. Severe cord compression has been described from entanglement with the pressure catheter. Injury to the fetal scalp or breech by a heart rate electrode is rarely severe. However, application at some other site-such as the eye in face presentations-can be serious.