Increasing mean airway pressure reduces functional MRI (fMRI) signal in the primary visual cortex

Changes in both blood flow and blood oxygenation determine the functional M RI (fMRI) signal. In the present study factors responsible for blood oxygen ation (e.g,, FiO(2)) were held constant so that changes in pixel count woul d above all reflect changes in regional cerebral blood flow (rCBF). Continu ous positive airway pressure (CPAP) breathing at 12 cm H2O, which was previ ously shown to influence rCBF, was applied in human volunteers (n = 19) to investigate the sensitivity of fMRI for changes in rCBF caused by increased mean airway pressure. Increasing the mean airway pressure decreased the pi xel count in the primary visual cortex (median (range)): baseline: 219 (58- 425) pixels vs. CPAP (12 cm H2O): 92 (0-262) pixels). These findings indica te that fMRI is sensitive to detect a reduced rCBF-response in the primary visual cortex. The underlying mechanism is likely to be a reduced basal rCB F due to constriction and/or compression of postcapillary venoles during CP AP breathing. These findings are important for interpreting fMRI results in awake and in artificially respirated patients, in whom positive airway pre ssure is used to improve pulmonary function during the diagnostic procedure . (C) 2001 Elsevier Science Inc. All rights reserved.