Journal
JOURNAL OF APPLIED PHYSIOLOGY
Volume 110, Issue 6, Pages 1646-1655Publisher
AMER PHYSIOLOGICAL SOC
DOI: 10.1152/japplphysiol.00003.2011
Keywords
cerebrovascular reactivity; hyperventilation; Valsalva maneuver; head-up tilt
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Funding
- Istituto Di Ricovero e Cura a Carattere Scientifico, C. Mondino Foundation, Pavia, Italy
- Istituto Nazionale Ricerche Cardiovascolari-Consorzio Interuniversitario
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The attractive possibility of near infrared spectroscopy (NIRS) to noninvasively assess cerebral blood volume and oxygenation is challenged by the possible interference from extracranial tissues. However, to what extent this may affect cerebral NIRS monitoring during standard clinical tests is ignored. To address this issue, 29 healthy subjects underwent a randomized sequence of three maneuvers that differently affect intra-and extracranial circulation: Valsalva maneuver (VM), hyperventilation (HV), and head-up tilt (HUT). Putative intracranial (i) and extracranial (e) NIRS signals were collected from the forehead and from the cheek, respectively, and acquired together with cutaneous plethysmography at the forehead (PPG), cerebral blood velocity from the middle cerebral artery, and arterial blood pressure. Extracranial contribution to cerebral NIRS monitoring was investigated by comparing Beer-Lambert (BL) and spatially resolved spectroscopy (SRS) blood volume indicators [the total hemoglobin concentration (tHb) and the total hemoglobin index, (THI)] and by correlating their changes with changes in extracranial circulation. While THIe and tHbe generally provided concordant indications, tHbi and THIi exhibited opposite-sign changes in a high percentage of cases (VM: 46%; HV: 31%; HUT: 40%). Moreover, tHbi was correlated with THIi only during HV (P < 0.05), not during VM and HUT, while it correlated with PPG in all three maneuvers (P < 0.01). These results evidence that extracranial circulation may markedly affect BL parameters in a high percentage of cases, even during standard clinical tests. Surface plethysmography at the forehead is suggested as complementary monitoring helpful in the interpretation of cerebral NIRS parameters.
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