Monitoring: Equipment and Computers
Background and Goal of Study: Pulse oximetry is a reliable technique to determine functional oxygen saturation (SpO2) with high accuracy and precision in the arterial blood. Multiple factors can influence the measurement; especially low perfusion  or venous congestion may severely decrease reliability. Reliability of pulse oximetry is poorly investigated under the influence of changes in gravity and acceleration.
Materials and Methods: After informed consent, five volunteers were investigated on a tilting table. One pulse oximeter (FRED, Brucker Medical, sensor BCI) was attached at the left earlobe (EARSpO2) to evaluate gravity effects. Another pulse oximeter (DigitTM, BCI) at the middle finger of the right hand at heart level (FSpO2) served as reference (no gravity effect). Bias for oxygen saturation (FSpO2 - EARSpO2) and heart rate were calculated in upright standing position (simulation of + 1 Gz for 4 min, 20 values each), in lying position (0Gz, 4min, 20 values), and head down position (−1 Gz, 4min, 20 values). Wilcoxon-test was used for statistical analysis, a P < 0.05 was considered to be significant.
Results and Discussions: All investigated volunteers were male with an average age of 26 ± 5 years and a BMI of 22.5 ± 2.1 kgm−2. Bias during simulated + 1 Gz (−0.9 ± 0.4%, range −0.2% to −1.3%) and simulated 0Gz (−1.0 ± 0.7%, range +0.2% to −2.0%) changed not significantly (P = 0.67). With simulated −1 Gz in head down position bias increased to +1.7 ± 1.0% (range +0.3% to +3.7%, P < 0.001). Bias for the heart rate decreased from +3.3 ± 6.2 min−1 vs. −3.6 ± 5.6min−1 (+1 Gz vs. 0Gz) to −10.1 ± 3.7 min−1 during −1 Gz (P < 0.001).
Conclusion(s): Reliability of pulse oximetry determining functional oxygen saturation and heart rate at the earlobe is significantly lower during simulated −1 Gz compared to the reference measurement at heart level or during +1Gz and 0Gz. The reason may be venous congestion under negative Gz-gravity, which should be investigated further.
1 Hinkelbein J et al. Aviat Space Environ Med 2004; 74(4): B91.