Reflectance pulse oximetry at the forehead of newborns: The influence of varying pressure on the probe

Objective: Transmission pulse oximetry (TPO) is not a practical method of intrapartum fetal monitoring of arterial oxygen saturation. Reflectance pulse oximetry (RPO) requires a sensor applied to the skin of the fetal head and may be a useful technique. During labor, various degrees of pressure will be exerted on the RPO sensor. Previous studies have shown that moderate pressure on the sensor can improve the RPO signal. At increasing pressure, however, blood flow underneath the sensor will be occluded. This study examines the influence of pressure applied to the RPO sensor on the signal from the forehead of healthy newborns as a model for the fetal situation.Methods: After institutional approval, 12 healthy newborns were studied. The RPO probe was placed at the forehead. Pressure on the probe was increased stepwise from 0 to 80 mmHg, and the effect on the ratio between the relative changes of the red and infrared light intensities (R/IR, inversely related to oxygen saturation) and pulse sizes was evaluated. Additionally, the effect of firm pressure (>150 mmHg) on the probe was evaluated.Results: R/IR values remained virtually unchanged when pressure onto the probe was increased from 0 to 80 mmHg, although the standard deviation slightly decreased. The pulse size increased as pressure on the probe increased. During firm pressure on the probe (> 150 mmHg), plethysmographic signals remained detectable, but R/IR values markedly increased.Conclusions: In newborns, mild to moderate pressure on the probe has little influence on the RPO signal at the forehead. Even during firm pressure, RPO can be used to obtain pulsatile signals, that presumably derive from tissue underneath the skull, such as the cerebral circulation.