One of the reasons that ratios do not work is that some patients can start out with a TCOM on air of 10 mmHg and if they quadruple they are still up to only 40 mmHg. There is therefore, a relationship between where you START and where you end up with sea level oxygen. There is a way to combine them in predicting benefit from HBOT but it is complicated and it is far easier and reliable to use in chamber Tcom