LIMITATIONS OF FOREHEAD PULSE OXIMETRY

During initial clinical tests to calibrate our reflectance pulse oxime try system, we observed serious physiologic limitations to the use of pulse oximetry in the forehead region. We present a case of simultaneo us reflectance and transmission mode pulse oximetry monitoring in a ch ild undergoing cardiac surgery for congenital cyanotic heart disease w ith a large intracardiac shunt. During general anesthesia, when the pa tient was endotracheally intubated and mechanically ventilated, the tr ansmission mode saturation agreed well with arterial oxygen saturation measurements; but, our reflectance pulse oximeter, with the sensor ap plied to the forehead, displayed spuriously lower (-18%) oxygen satura tions. Before and after anesthesia and surgery, there was fine agreeme nt between reflectance and transmission mode saturation values. We sug gest that the difference was caused by vasodilatation and pooling of v enous blood due to compromised venous return to the heart, and a combi nation of arterial and venous pulsations in the forehead region. This means that the reflectance pulse oximeter measured a mixed arterial-ve nous oxygen saturation.