Near-Infrared Spectroscopy Usefulness in Validation of Hyperventilation Test.

Background : The hyperventilation test is used in clinical practice for diagnosis and therapeutic purposes; however, in the absence of a standardized protocol, the procedure varies significantly, predisposing tested subjects to risks such as cerebral hypoxia and ischemia. Near-infrared spectroscopy (NIRS), a noninvasive technique performed for cerebral oximetry monitoring, was used in the present study to identify the minimum decrease in the end-tidal CO 2 (ETCO 2 ) during hyperventilation necessary to induce changes on NIRS. Materials and Methods : We recruited 46 volunteers with no preexisting medical conditions. Each subject was asked to breathe at a baseline rate (8-14 breaths/min) for 2 min and then to hyperventilate at a double respiratory rate for the next 4 min. The parameters recorded during the procedure were the regional cerebral oxyhemoglobin and deoxyhemoglobin concentrations via NIRS, ETCO 2 , and the respiratory rate. Results : During hyperventilation, ETCO 2 values dropped (31.4 ± 12.2%) vs. baseline in all subjects. Changes in cerebral oximetry were observed only in those subjects ( n = 30) who registered a decrease (%) in ETCO 2 of 37.58 ± 10.34%, but not in the subjects ( n = 16) for which the decrease in ETCO 2 was 20.31 ± 5.6%. According to AUC-ROC analysis, a cutoff value of ETCO 2 decrease >26% was found to predict changes in oximetry (AUC-ROC = 0.93, p < 0.0001). Seven subjects reported symptoms, such as dizziness, vertigo, and numbness, throughout the procedure. Conclusions : The rise in the respiratory rate alone cannot effectively predict the occurrence of a cerebral vasoconstrictor response induced by hyperventilation, and synchronous ETCO 2 and cerebral oximetry monitoring could be used to validate this clinical test. NIRS seems to be a useful tool in predicting vasoconstriction following hyperventilation.