Abstract
During neuronal activation, a local decrease of deoxygenated hemoglobin concentration (deoxy‐Hb) occurs which is the basis of functional brain imaging with blood oxygenation level dependent functional magnetic resonance imaging (BOLD‐fMRI). Elevated intracranial pressure (eICP) has been shown to impair functional deoxy‐Hb changes. This study investigated this effect and its relation to the underlying neuronal activity in the human primary somatosensory cortex (SI). Functional near‐infrared spectroscopy (fNIRS) during somatosensory evoked potentials (SEP) monitoring was performed on 75 subjects during conditions of median nerve stimulation (MNS) and resting state, combined with normal breathing (NB) and eICP by escalating breathing maneuvers (breath holding [BH], Valsalva maneuver with 15 mmHg [V15] and 35 mmHg expiratory pressure [V35]). During NB, fNIRS revealed a typical oxygenated hemoglobin concentration (oxy‐Hb) increase with deoxy‐Hb decrease during MNS enabling SI brain mapping. Breathing maneuvers associated eICP produced a known global change of oxy‐Hb and deoxy‐Hb with and without MNS. When subtracting measurements during resting state from measurements during MNS, neither functional oxy‐Hb nor deoxy‐Hb changes could be recovered while SEPs remained unchanged. In conclusion, Valsalva‐induced eICP prevents oxy‐Hb and deoxy‐Hb changes during neuronal activation in SI. This finding raises questions on the validity of oxy‐Hb‐ and deoxy‐Hb‐based brain imaging (e.g., BOLD‐fMRI) during eICP.
Highlights
Functional near-infrared spectroscopy during somatosensory evoked potentials (SEP) monitoring was performed on 75 subjects during conditions of median nerve stimulation (MNS) and resting state, combined with normal breathing (NB) and Elevated intracranial pressure (eICP) by escalating breathing maneuvers
We recently reproduced this finding in human primary motor cortex (MI) when applying Valsalva-induced eICP during MI activation via finger tapping and finding an amplitude decrease of functional deoxy-Hb changes leading to a complete abolishment of functional brain imaging with Functional near-infrared spectroscopy (fNIRS) when based on deoxy-Hb (Knauth, Heldmann, Münte, & Royl, 2017)
In order to reproduce this effect in a different cortical area, and at the same time enabling the obtainment of a direct, that is, electrophysiological, measurement of concurrent neuronal activation we transferred this study to human primary somatosensory cortex (SI) by investigating neuronal activation and neurovascular coupling (NVC) responses during median nerve stimulation (MNS) with different breathing maneuvers leading to changes in intracranial pressure (ICP)
Summary
Abbreviations: BH, breath holding without Valsalva maneuver; BOLD, blood oxygenation level dependent; CBV, cerebral blood volume; CSF, cerebrospinal fluid; deoxy-Hb, deoxygenated hemoglobin concentration; EEG, electroencephalography; eICP, elevated intracranial pressure; fMRI, functional magnetic resonance imaging; fNIRS, functional near-infrared spectroscopy; Hb, hemoglobin; ICP, intracranial pressure; MI, primary motor cortex; MNS, median nerve stimulation; NB, normal breathing; NVC, neurovascular coupling; oxy-Hb, oxygenated hemoglobin concentration; PetCO2, partial pressure of end-tidal carbon dioxide; SEP, somatosensory evoked potential; SI, primary somatosensory cortex; SNR, signal to noise ratio; total-Hb, total hemoglobin concentration; V15, Valsalva maneuver with 15 mmHg forced expiratory pressure against resistance; V35, Valsalva maneuver with 35 mmHg forced expiratory pressure against resistance; VM, Valsalva maneuver.
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