Fetal pulse oximetry: 8 vital questions

The study was underpowered to detect a significant difference in acidemia, and did not allow sufficient observation time to detect the natural progression of hypoxia to metabolic acidosis, a better indicator of fetal compromise. Additional research is needed.

Question 4Does oximetry correlate with acid-base status?

Many of the studies mentioned here assumed a correlation. Whenever oxygen saturation in the umbilical artery is 30% or more, acidosis (pH below 7.13) in the same blood is rare—only 1%.²¹ However, the correlation between fetal pulse oximetry values and acid-base status is much weaker.⁸.

Leszczynska-Gorzelak et al²² found no relationship between FSpO₂ levels in the first or second stage of labor and pH or partial pressure of oxygen in umbilical vein blood at delivery. Other investigators concluded similarly, considering intrapartum FSpO₂ of limited use for predicting acidosis at birth, irrespective of FSpO₂ cutoff.^23,24

Rijnders et al²⁴ found no significant correlation between fetal scalp or umbilical artery blood pH and mean FSpO₂ for the last 30 minutes before sampling (r = 0.02, P = .9). Even the lowest FSpO₂ level did not correlate with arterial pH (r = .04, P = .84). None of the study’s 3 cases of umbilical pH below 7.05 would have been detected using the mean FSpO₂ before delivery, and only 1 would have been detected using the lowest FSpO₂.

In another multicenter study involving the Nellcor system in 164 cases with abnormal FHR, a correlation between oximetry and FSB sampling (r = 0.29, P < .01) was noted in the first stage of labor, but second-stage FSpO₂ readings did not correlate with oxygen saturation, partial pressure of oxygen, pH, or bicarbonate level in the umbilical artery at birth.²⁵

An observational series²⁶ of 128 fetuses with nonreassuring FHR patterns concluded that fetal distress was insufficiently identified by oximetry. Only 2 of the 11 cases with umbilical artery pH below 7.20 were detected by pulse oximetry recordings below 30% during the last 30 minutes of the second stage, and out of 5 cases with hypoxic readings in the second stage, only 2 were acidotic at birth. The calculated sensitivity was 18%, specificity 92%, positive predictive value (PPV) 40%, and negative predictive value (NPV) 80%. A low Apgar score was never predicted by fetal pulse oximetry.

Others used the same Nellcor system over the final 30 minutes of labor and a cutoff for umbilical blood acidemia of pH below 7.13 and reported similar numbers: sensitivity 28%, specificity 94%, PPV 40%, and NPV 80%.²³

Vitoratos et al²⁷ analyzed FSpO₂ readings in active labor (not limited to the last 30 minutes before delivery) and obtained somewhat better values: sensitivity 72%, specificity 93%, PPV 61.5%, and NPV 95.8% for an umbilical artery blood pH below 7.15.

The impression that the validity of fetal pulse oximetry is higher in earlier labor than in the second stage is supported by data from Stiller et al. ²⁸ Leszczynska-Gorzelak et al²⁹ found a significant decrease in mean FSpO₂ from the first stage to the second stage of normal labor (51.9% versus 43.8%, P < .001), and Dildy et al¹⁴ noted a similar difference upon analyzing 160 normal labors (59% versus 53%), but other studies failed to verify such differences.^25,30

Observational studies had unrealistic pH cutoff. All the evidence presented thus far on the validity of fetal pulse oximetry in predicting acidemia is based on observational data. A common deficiency is the unrealistic cutoff for pathologic fetal acidemia—a pH of less than 7.13 to 7.20—when it is widely accepted that “pathologic fetal acidemia” reflects an umbilical artery blood pH below 7.³¹ Even in this group, two thirds of neonates are unaffected by morbidity.

Need to identify metabolic acidosis. It also is accepted that the presence of a metabolic component to fetal acidemia may be as important—if not more important—than a single pH cutoff.³¹ Only a few human studies of pulse oximetry have distinguished between respiratory and metabolic acidemia. When they did, intrapartum fetal pulse oximetry was unable to predict umbilical artery base excess.^23,25

The only randomized study failed to determine whether clinical decisions can be based solely on fetal pulse oximetry. ³ The investigators did suggest that sensitivity and specificity for metabolic acidemia was improved in the intervention group—a promising appraisal, in contrast with previous observational data.

In the study, 7 neonates (3 in the intervention group and 4 controls) had umbilical artery blood pH below 7. All 4 controls had vaginal delivery. There also were 6 cases of elevated base excess (ie, -16 mEq/L or below) among controls. None were recorded in the intervention group, and the 3 cases of acidemia were recognized antepartum and led to cesareans.