Response of regional oxygen saturation technologies during hypoxia

Introduction

The purpose of this study was to determine the rate and magnitude of response to hypoxia for three different regional oxygen saturation (rSO₂) devices. rSO₂ technologies are focused on absolute accuracy without consideration of response characteristics. Current rSO₂ technologies assume that the oxygen saturation is a fixed ratio of arterial and venous blood. Cerebral arteries have an oxygenation-related vasoactivity that may change the arterial/venous ratio during hypoxia. Thus, absolute rSO₂ accuracy may be less important compared with sensitivity to changes in cerebral rSO₂.

Methods

Ten subjects completed the study and are included in the analysis. One INVOS sensor (SAFB-SM) was placed on the left side and one Equanox (8000CA) or Foresight (1 July 2007 or 1 July 2005) sensor (alternated between subjects) was placed on the right side of the forehead for bilateral monitoring. Desaturation was induced by adjusting the inspiratory gas mixture of O₂/N₂. Desaturation was titrated from room air to achieve a plateau of 70% arterial oxygen saturation (SpO₂). Resaturation was induced by rapid change in FiO₂ to 1.0. After 5 minutes of SpO₂ 100%, the process was repeated by desaturation to SpO₂ 70% and rapid return to SpO₂ 100%. Cerebral and pulse oximetry data were recorded during the study and the time of each FiO₂ change and plateau was recorded. rSO₂ levels at 10, 20, 40, 60 and 80% of the total SpO₂ response were calculated for each device to assess the rate of rSO₂ change. The rate of rSO₂ change in seconds and total rSO₂ change were compared.

Results

The rate of rSO₂ change during desaturation was similar for all devices with an average slope factor of 0.17 for Foresight, 0.16 for Equanox and 0.21 for INVOS. The rate of rSO₂ change in seconds during resaturation from SaO₂ 70% to SaO₂ 100% was significantly faster for INVOS (42 ± 16) compared with Foresight (57 ± 20) (P < 0.05). There was significant difference in total rSO₂ change between INVOS (23 ± 4%) and Equanox (15 ± 4%) during desaturation and resaturation (P < 0.005) and between INVOS compared with Foresight (20 ± 5%) (P < 0.05).

Conclusion

All rSO₂ devices followed the SpO₂ slope during desaturation as expected. The differences between the devices in terms of total rSO₂ change reached statistical significance. There were also significant differences in the rate of rSO₂ change in seconds between INVOS and Foresight during resaturation. The rate of absolute change in seconds and the magnitude of absolute change may result in better resolution to detect physiological changes. Clinical studies are required to elucidate the clinical relevance of the differences.