Abstract 105: Changes in NIRS-Measured Cerebral Tissue Oxygen Saturation During Hyperventilation: A Comparison Between NIRO™ and INVOS™

Introduction: Cerebral tissue oxygen saturation measured with transcranial near-infrared spectroscopy (NIRS) has been reported to predict neurological outcome of cardiac arrest patients. Because NIRS values are confounded by extracranial tissues, there can be considerable inter-device variation in the measured values. We compared hyperventilation-induced changes in NIRS values measured with 2 commercially available devices in healthy volunteers. Methods: After obtaining an approval from the ethics committee at Iseikai hospital and written informed consents, 29 healthy volunteers joined the study. Probes of NIRO™ 200NX (Hamamatsu Photonics, Japan) and INVOS™ 5100C (Covidien, USA) were applied respectively on the right and left side of the volunteers’ forehead. After obtaining the baseline values, the volunteers were asked to hyperventilate for 40 seconds attempting to reduce the end-tidal CO 2 by ~20 mmHg. Measurements were repeated, after 20 minutes of resting interval, with the probes applied on the contralateral sides. Hyperventilation-induced response of the NIRS value was categorized as “correct” when the average value over the 5 second period toward the end of the hyperventilation decreased by >15% the baseline or “erroneous” otherwise. Results: The mean (± SD) end-tidal CO 2 decreased from the baseline value of 36 ± 5.1 mmHg to 15.2 ± 4.4 mmHg during hyperventilation. Out of 58 measurements with each of the devices, NIRO values were “correct” in 54 (93.1%) measurements, whereas INVOS values were “correct” in 32 (52.2%) measurements. NIRO presented “erroneous” values on both sides of the forehead in no volunteers whereas INVOS presented “erroneous” values in 7 (35%) of 29 volunteers. Discussion: Hyperventilation consistently decreases cerebral blood flow in healthy subjects. Thus, the results of the present study suggest that NIRS devices may fail to reflect correct changes in cerebral tissue oxygen saturation in ~7% of measurements with NIRO and in ~48% with INVOS. The bilateral “erroneous” measurements with INVOS in 7 (35%) volunteers may suggest that INVOS is subject to confounding by extracranial tissues. A potential source of confounding may include the scalp where blood flow increases during hyperventilation.