The pleth is an extremely important graph to see. It tells you how good the pulsatile signal is. If the quality of the pulsatile signal is poor, then the calculation of the oxygen saturation may be wrong. The pulse oximeter uses very complicated calculations to work out oxygen saturation. A poor pleth tracing can easily fool the computer into wrongly calculating the oxygen saturation. As human beings, we like to believe what is good, so when we see a nice saturation like 99 % , we tend to believe it, when actually the patients actual saturation may be much lower. So always look at pleth first, before looking at oxygen saturation.