Assessing microcirculation condition in critical illness using the pulse oximeter´s concept

Sepsis patients normally suffer microcirculatory dysfunction, which results in organ failure and increased risk of death [1]. Importantly, microcirculatory distress is the only independent factor for predicting patient outcome if it is not treated within 48 hours [2]. Therefore, analyzing oxygen transport and utilization can potentially assess microcirculation function and metabolic condition of an individual. In this study, a pulse oximeter is used to extract additional information signals due to absorption of red and infrared light. The IR signal is related to the overall blood volume, (HbO₂ + Hb) and the R signal is related to the amount of reduced hemoglobin, (Hb). Differences between these two signals thus represent the amount of oxygenated hemoglobin, (HbO₂). Unlike the standard pulse oximeter, the pulse oximeter used in this study provided the changes in red and infrared signals separately for analysis. In this study, a moderate physical exercise test has been conducted to validate the pulse oximeter concept. This test was done on healthy individuals to induce changes in extraction. This study and the use of this data was approved by the University of Canterbury Human Ethics Committee, Christchurch, New Zealand. In this test, AC R and IR signals were relatively higher during post-exercise periods compared to baseline, due to increases in heart rate. Median heart rate increases from 51 during rest to 83 beats per minute during post-exercise 1. Further exercise yielded median heart rates of 90, 90, 91 and 99 beats per minute. In addition, oxygen extraction was also changing during the post-exercise period, indicated by the difference in the AC IR signal to the R signal. Median oxygen extraction increases from 37.4% during rest to 41.2% during following intense exercise. For further (repeated) physical exercise tests yielded extraction of 37.9%, 36.6%, 39.4% and 40.4%. However, the increased rate varies across subjects showing significant - nter-subject variability due to existing fitness levels. The pulse oximeter sensor concept used in this study is capable of extracting valuable information to assess metabolic condition. Thus, implementing this concept on ICU patients has the potential to aid sepsis diagnosis and provide more accurate tracking of patient state and sepsis status.