Unfortunately, the study design did not guarantee that patient management was based exclusively on EFM with or without fetal pulse oximetry. Vibro-acoustic stimulation or FSB sampling was required before proceeding to cesarean delivery in both groups.
It appears that the negative predictive value of fetal oximetry is of greater practical value than other attributes.
When FSpO2 was less than 30% for the entire interval between 2 contractions, or was unobtainable, the physician was supposed to revert to interpretation of EFM. When that was persistently nonreassuring, the physician was given the option of scalp stimulation or FSB sampling. Thus, it was not determined whether clinical decisions can be based exclusively on fetal pulse oximetry. Schmidt et al26 suggested that such exclusive application of fetal pulse oximetry might actually jeopardize fetal health.
Question 5 Does the combination of oximetry and EFM improve accuracy?
Fetal pulse oximetry was not used independently in any of the studies discussed here, but in association with EFM, which has a sensitivity for fetal acidosis of 93%, specificity of 29%, PPV of 2.6%, and NPV of 99.5%.32
From a statistical point of view, whenever 2 evaluation methods with the same endpoint (fetal acidosis) are combined, sensitivity decreases while specificity increases, theoretically resulting in less unnecessary intervention. That is exactly what investigators have reported: sensitivity as low as 18%26 for fetal oximetry, and specificity as high as 94%.23 However, the value of this new technology might not be so much the prediction of acidosis but identification of the well-oxygenated fetus so that labor may be safely continued in the presence of a concerning—but not ominous—FHR tracing.
How fetal pulse oximetry works
Fetal pulse oximetry employs principles of optical spectrophotometry and plethysmography to provide information on the percentage of oxygen bound to hemoglobin. Oxyhemoglobin (oxygenated hemoglobin) and deoxyhemoglobin (hemoglobin without oxygen) absorb red and infrared light differently: more red absorption by deoxyhemoglobin, and more infrared absorption by oxyhemoglobin.
By measuring the relative absorption at each wavelength, the fraction of hemoglobin that carries oxygen can be determined. The arterial oxygen saturation is expressed as a percentage. The technology has been refined to measure fetal arterial oxyhemoglobin saturation during labor.
Pulse oximetry sensors must be calibrated for fetal biological values. In the fetus, normal oxygen saturation is much lower than in the adult or neonate; hemoglobin has a higher affinity for oxygen and is in higher concentration; and there are more capillaries per unit of tissue, higher cardiac output, and a higher heart rate.
In the adult or neonate, pulse oximetry sensors can be attached to fingers, toes, ears, or the bridge of the nose, but such stable placement is not feasible in utero. Further, good contact between sensor and fetal skin is a prerequisite for avoidance of artifacts. This last aspect has presented a sizeable challenge.
Fetal sensors measure reflected light. There is disagreement about the merits of the 2 sensor types, reflectance and transmission. Both include 2 light emitters (for red and infrared light) and a detector. In the transmission sensor (the adult or neonatal type), the light produced by the light-emitting diodes (LED) is picked up by the detector after traversing the interposed tissues. Since tissue interposition is not possible in the fetus, most fetal studies have used reflectance sensors, in which the LED and detector are placed side by side, and the light to be analyzed is reflected by the tissues. This design adds variance depending on the light’s depth of tissue penetration and device position changes.
Placement of the sensor. The Nellcor N-400 includes a reflectance sensor housed in a smooth, pliable head that is advanced through the cervix with the aid of a handle. The handle has a removable stylet to stiffen it during placement.
The sensor is placed against the fetal temple, cheek, or forehead and is held in place by the uterine wall. Placement is similar to that of an internal pressure catheter. Once the stylet is removed, it should not be reinserted.
Because the sensor usually descends and rotates with the fetal head, displacements are frequent and adjustments in sensor placement may be necessary. Placement adjustments can be attempted without the stylet and, if unsuccessful, a new device can be inserted. The Nellcor sensor is not reusable.
The prerequisites for insertion are dilatation of at least 2 cm, ruptured membranes, cephalic presentation, single fetus, gestational age of at least 36 weeks, and no placenta previa.
The manufacturer reports that active genital herpes, HIV, and hepatitis B or E seropositivity preclude fetal pulse oximetry monitoring.
Placement may be impossible when the presenting part is at high station (-3 or above) or low station (+2 or below).
The Nellcor N-400 system has been commercially available in many European countries since 1995, and in Canada since 1998. It was approved for sale in the United States in early 2003.