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abstract & commentary
Synopsis: The pulse oximeter more reliably predicted which patients really needed a cesarean section for true fetal distress.
Source: Garite TJ, et al. Am J Obstet Gynecol 2000; 183(5):1049-1058.
For the past 15 years, there has been an attempt to decrease the high cesarean section rate (CSR) in this country. Three of the most common reasons for performing cesarean section are dystocia, a previous cesarean operation, and fetal distress diagnosed by fetal heart rate patterns suggestive of hypoxia. Since 1990, the national CSR has dropped a few percentage points because of loosening definitions of "failure to progress" in labor, increased emphasis on vaginal birth after cesarean (VBAC), and, in some hospitals, heightened peer review. Recently, a technique has emerged which, hopefully, will allow clinicians to do fewer cesarean deliveries for fetal distress.
Garite and colleagues have reported on the results of a multi-center, randomized trial to evaluate a fetal pulse oximeter in labor. One thousand ten patients were recruited from nine centers. Entry was predicated upon an abnormal fetal heart rate pattern in labor. Five hundred eight patients were randomized to have a pulse oximeter placed through the vagina against their fetus’ cheeks. The 502 control patients were managed by fetal heart rate monitoring alone. The continuous oxygen saturation values were available to the managing clinicians in the study group. The CSR for fetal distress in the pulse oximeter group was less than half of the control group (4.5% vs 10.2%). Although there was no difference in major neonatal morbidity between groups, the neonates in the pulse oximeter groups whose mothers were sectioned for fetal distress had more legitimate evidence of metabolic acidemia than those in the control group where sections were done for the same diagnosis. Garite et al concluded that the pulse oximeter more reliably predicted which patients really needed a cesarean for true "fetal distress."
A surprising finding in the study was that the total CSR in both groups was similar (26% vs 29%) because of a statistically significant increase in cesarean sections done for dystocia in the pulse oximetry group. When Garite et al attempted to weed out confounding variables including clinician bias, predisposing factors to dystocia, labor augmentation, etc., they found the only difference between groups was a higher rate of arrest of labor in the pulse oximeter group.
Comment by John C. Hobbins, MD
The function of fetal heart rate monitoring has been to identify fetuses that are tolerating labor poorly because of hypoxia secondary to the nebulous term "uteroplacental insufficiency." Randomized trials, especially in low-risk patients, have failed to show an improvement in most perinatal outcomes when continuous fetal heart rate monitoring was compared with intermittent auscultation. However, no study yet has shown that a reassuring fetal heart rate pattern is an imprecise predictor of normal fetal oxygenation. In fact, electronic fetal monitoring (EFM) is a reliable predictor of fetal well being, but an imperfect predictor of fetal ill being. This is where the pulse oximeter can be of great value. External EFM can be used as a screening tool to pick out the fetuses at risk for hypoxia whose oxygenation can then be assessed accurately with the pulse oximeter. As Garite et al point out, this should halve the amount of cesarean sections performed for fetal distress.
Another spinoff benefit from the study, not discussed in this paper, was the anecdotal finding that variable decelerations, including deep, "scary" variables, were rarely associated with a decrease in fetal oxygen saturation levels. Hopefully, since even severe variables represent a baroreceptor response to umbilical cord impingement rather than a chemoreceptor response to hypoxia, this should put an end to applying an oxygen mask to every patient having the slightest deflection on the fetal heart rate monitor. This practice, which not only confines the patient, but also frightens the wits out of her, should be abandoned unless there is solid evidence of fetal hypoxia through pulse oximetry.
The finding of a higher number of cesarean sections being performed for dystocia in the pulse oximetry group is puzzling and concerning, especially when the CSR in this group was 29%, a figure that is much higher than the average CSR in the United States. As stated before, Garite et al tried to explain in various ways the higher rate of dystocia in the pulse oximetry group, but could not. One interesting finding was a higher rate of nonreassuring fetal heart rate patterns (mostly variables) in those sectioned for dystocia.
Why should fetuses of mothers displaying sluggish progress in labor have a higher rate of fetal heart rate abnormalities? Is this some sort of heralding sign of an imperfect fetal/pelvic fit, or are we dealing with a maternal response (stress) to having a gadget placed in her because her caregivers think her baby may not be getting enough oxygen? Obviously, this finding needs more investigation.