Characterizing near‐infrared spectroscopy signal under hypercapnia

Abstract

Vasoactive stress tests (i.e. hypercapnia, elevated partial pressure of arterial CO 2 [PaCO 2 ]) are commonly used in functional MRI (fMRI), to induce cerebral blood flow changes and expose hidden perfusion deficits in the brain. Compared with fMRI, near-infrared spectroscopy (NIRS) is an alternative low-cost, real-time, and non-invasive tool, which can be applied in out-of-hospital settings. To develop and optimize vasoactive stress tests for NIRS, several hypercapniainduced tasks were tested using concurrent-NIRS/fMRI on healthy subjects. The results indicated that the cerebral and extracerebral reactivity to elevated PaCO 2 depended on the rate of the CO 2 increase. A steep increase resulted in different cerebral and extracerebral reactivities, leading to unpredictable NIRS measurements compared with fMRI. However, a ramped increase, induced by ramped-CO 2 inhalation or breath-holding tasks, induced synchronized cerebral, and extracerebral reactivities, resulting in consistent NIRS and fMRI measurements. These results demonstrate that only tasks that increase PaCO 2 gradually can produce reliable NIRS results.

Evaluation of fNIRS signal components elicited by cognitive and hypercapnic stimuli

“…This statement is further supported by the fact that during cognitive tasks HbR has higher correlation with BOLD-fMRI than HbO or HbT 108 . Higher correlations between HbR and BOLD-fMRI during CO 2 tasks have also been reported 71 , 99 . However, due to its low dynamic range, HbR is more prone to motion artifacts and therefore results in an underestimation of hemodynamic activity associated with NVC or CVR 30 .…”

“…However, presence of SBF during the hypercapnic task is surprising, since studies have reported that PaCO 2 does not induce SBF changes 23 , 97 . A possible explanation underlying such findings may be that SBF changes in extracerebral layers are temporally delayed, rather than minimal or not present 98 , 99 . In cognitive tasks, systemic activities such as myogenic, respiratory and cardiac oscillations were found to have higher contributions in the short-SDS measurements than the long-SDS measurements (see Post-hoc SDS columns of Table 1 ).…”

Evaluation of fNIRS Signal Components Elicited by Cognitive and Hypercapnic Stimuli

“…This statement is further supported by the fact that during cognitive tasks HbR has higher correlation with BOLD-fMRI than HbO or HbT 108 . Higher correlations between HbR and BOLD-fMRI during CO2 tasks have also been reported 71,99 . However, due to its low dynamic range, HbR is more prone to motion artifacts and therefore results in an underestimation of hemodynamic activity associated with NVC or CVR 30 .…”

“…However, presence of SBF during the hypercapnic task is surprising, since studies have reported that PaCO2 doesn't induce SBF changes 23,97 . A possible explanation behind such findings may be that SBF changes in extracerebral layers are temporally delayed, rather than minimal or not present 98,99 . In cognitive tasks, systemic activities such as myogenic, respiratory and cardiac oscillations were found to have higher contributions in the short-SDS measurements than the long-SDS measurements (see Post-hoc SDS columns of Table 1).…”

Hypnotizability-Related Cerebral Blood Flow

“…This possibility remains largely overlooked but is particularly insidious when analyzing tasks associated with autonomic activations, that may profoundly affect the circulation in cutaneous tissues of the head. This confounding factor has been held responsible for inconsistent effects of CO2 challenges on B-L NIRS variables [63]. Conversely, the SRS variables were shown to be quite independent of extracranial circulation, providing consistent changes in response to hyperventilation and rebreathing [64,65].…”