Characterizing near‐infrared spectroscopy signal under hypercapnia

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Abstract
Vasoactive stress tests (i.e. hypercapnia, elevated partial pressure of arterial CO2 [PaCO2]) 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 hypercapnia‐induced tasks were tested using concurrent‐NIRS/fMRI on healthy subjects. The results indicated that the cerebral and extracerebral reactivity to elevated PaCO2 depended on the rate of the CO2 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‐CO2 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 PaCO2 gradually can produce reliable NIRS results.
Funding Information
  • National Science Foundation (Graduate Research Fellowship (NSF GRFP DGE‐1333468))
  • Indiana Clinical and Translational Sciences Institute (the Pilot Funding for Research Use of Core Facilit)
  • National Institutes of Health (K25 DA031769 (YT))

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