Limitations of forehead pulse oximetry

During initial clinical tests to calibrate our reflectance pulse oximetry system, we observed serious physiologic limitations to the use of pulse oximetry in the forehead region. We present a case of simultaneous reflectance and transmission mode pulse oximetry monitoring in a child undergoing cardiac surgery for congenital cyanotic heart disease with a large intracardiac shunt. During general anesthesia, when the patient was endotracheally intubated and mechanically ventilated, the transmission mode saturation agreed well with arterial oxygen saturation measurements; but, our reflectance pulse oximeter, with the sensor applied to the forehead, displayed spuriously lower (−18%) oxygen saturations. Before and after anesthesia and surgery, there was fine agreement between reflectance and transmission mode saturation values. We suggest that the difference was caused by vasodilatation and pooling of venous blood due to compromised venous return to the heart, and a combination of arterial and venous pulsations in the forehead region. This means that the reflectance pulse oximeter measured a mixed arterial-venous oxygen saturation.