Using technology similar to that of adult pulse oximetry, this instrumentation allows assessment offetal oxyhemoglobin saturation once membranes are ruptured. A unique padlike sensor is inserted through the cervix and positioned against the fetal face. he transcervical device reliably registers fetal oxygen saturation in 70 to 95 percent ofwomen throughout 50 to 88 percent of their labors (Yam, 2000). Using fetal pulse oximetry, the lower limit for normal fetal oxygen saturation is generally considered to be 30 percent (Gorenberg, 2003; Stiller, 2002). However, when measured in umbilical arterial blood, fetal oxygen saturation normally varies greatly, as shown in Figure 24-23. Bloom u:
J:Jr� and associates (1999) reported that brief, transient fetal oxygen saturations <30 percent were common during labor because such values were observed in 53 percent offetuses with normal outcomes. When persistent for 2 minutes or longer, however, saturation values <30 percent were associated with a greater risk of potential fetal compromise.
women with term pregnancies and in whom predeined abnor mal fetal heart rate patterns developed. Patients received either continuous fetal pulse oximetry. he use of fetal pulse oxim etrysigniicantlyreduced the cesarean delivery rate for nonreas suring fetal status from 10.2 to 4.5 percent. Alternatively, the cesarean delivery rate for dystocia rose significantly from 9 to 19 percent when pulse oximetry was used. No neonatal ben eits or adverse efects were associated with fetal pulse oximetry.
Based on these observations, the Food and Drug Administra tion approved marketing of the Nellcor N-400 Fetal Oxygen
Monitoring System.
Since then, three other randomized trials have compared fetal pulse oximetry with standard care. In all three trials, neo natal outcomes were similar between the two study arms. East and coworkers (2006) reported that the addition of oximetry fetal heart rate pattern. However, Bloom (2006) and lauser (2005), each with their colleagues, found no diference in cesar ean delivery rates between the two study groups. Because of these indings, in 2005, the manufacturer discontinued sale of the fetal oximeter system in the United States.
As fetal hypoxia worsens, the fetal ECG changes. Namely, the mature fetus exposed to hypoxemia develops an elevated ST segment and a progressive rise in the T-wave height that can be expressed as a T:QRS ratio (Fig. 24-24). Increasing T:QRS ratios are thought to relect the fetal cardiac ability to adapt to hypoxia and appear before neurological damage. Further worsening of hypoxia then leads to progressively negative ST-segment delection that takes on a biphasic form (Fig. 24-25). It is reasonable to consider that ST-segment abnormalities might occur late in the course of fetal compromise. Indeed, it has been hypothesized that ST-segment changes relect myocardial tissue hypoxia.
Because of these indings, several investigators have assessed the value of analyzing these parameters as an adjunct to conventional fetal monitoring. The technique requires internal fetal heart monitoring and special equipment to process the fetal ECG. In 2005, the manufacturer-Neoventa Medicalreceived Food and Drug Administration approval for their ST analysis program named the STAN system.