Normobaric variation in ppO2

     In this study, the partial pressure of oxygen (ppO2) in the breathing gas was varied using a commercial "altitude simulation" system. The subject was monitored by pulse oximetry (SpO2) and Cellular Energy Index (CEi), producing the above graphs. The simulated altitude values in the bottom graph were generated by the apparatus that used pressure swing adsorption (PSA) technology to separate atmospheric air (ppO2 ~158 mmHg at 20.8% of 760 mmHg) into higher and lower oxygen levels. The study was similar to tests performed on aircraft pilots to evaluate their tolerance of hypoxic stress and was performed in a laboratory located near sea level in altitude. During the study, the subject wore a tight-fitting face mask that supplied the breathing gas, and repeatedly performed the task of writing his signature to assess his fine motor coordination and mental status.

     Per the test procedure, the subject started by breathing 60% oxygen (ppO2 ~ 460 mmHg). During the next period of about 5 minutes, the ppO2 was decreased to 83 mmHg, simulating altitude of about 15,000 feet above mean sea level (AMSL), then brought back to 460 mmHg for about 3 minutes. The second segment of the study simulated 20,000 feet AMSL by the subject breathing a ppO2 of 72 mmHg, then returned to ppO2 of 460 mmHg.

     Blood oxygen was measured by pulse oximetry, showing the SpO2 trend from 100% down to about 70% hemoglobin-oxygen saturation. By comparison in this study, CEi covers the full range of cellular oxygen utilization from deeply hypoxic to hyperoxic, with a numeric range from -250 up to +120. This study recorded overshoot hyperoxia at the end that exceeded the initial CEi value, which appears to indicate that the subject's skin cells adapted to the imposed hypoxic stress. This adaptation is not evident with pulse oximetry. Also notable in this study is the ~45-second time delay between the onset of the CEi variation and the onset of change of the pulse oximeter data.

     The subject felt only slightly impaired during the first portion of the study, but reported decreased mental acuity and had distorted handwriting during the second portion. Remarkably, he also reported that his subjective return to his normal mental acuity took over 30 minutes following the end of the study.

     The above data graphs also show evidence of progressive cellular adaptation to the repeated episodes of hypoxic stress during the study. Where SpO2 simply returned to 100% upon the first return to breathing 60% oxygen, the CEi trend moved into the cellular hyperoxia range. A similar, but more pronounced CEi rebound into cellular hyperoxia is seen following the second cycle, which is, again, not evident in the pulse oximetry data. Based on the pulse oximeter's blood oxygen trend, one would conclude that there was no correlation of the wide swings in oxygen supply to the subject's subjective and objective loss of cognitive function. The CEi trend, on the other hand, provides objective evidence that there was a cellular response to this form of physiological and biochemical stress.