Continuous noninvasive ABP recording was performed via a servo-controlled finger plethysmograph (Finapres© 2300, Ohmeda, Englewood, Colorado) with the subject’s hand positioned at heart level. End-tidal partial pressure was measured in kPa with an infrared capnometer (Normocap©, Datex, Finland) during nasal expiration. NIRS measurements were performed using a 52-channel near-infrared spectrometer with a sampling rate of 10 Hz (ETG 4000©, Hitachi Medical Co., Tokyo, Japan). The basic principle of NIRS is that near-infrared light easily penetrates the skull and brain, but is differentially absorbed by the chromophores oxyhemoglobin ([oxyHb]) and deoxyhemoglobin ([deoxyHb]). Assuming constant scattering, the changes in the different absorption spectra of these chromophores can be converted into changes in their concentrations using the modified Lambert—Beer law.10 The NIRS probes were placed in a standardized manner on the anterior circumference of the brain (aligning the center probes with the sagittal midline, positioning the lower center probe at a distance of 1.5 cm above the nasion). The resulting 52 NIRS channels evenly covered the bilateral frontal cortex (partly extending into superior temporal areas) that is usually supplied by the MCA and anterior cerebral artery (ACA). The measurement setup is illustrated in Fig. 1(d). Each pair of horizontally or vertically adjacent emitting and receiving probes constituted a channel, with each channel located equidistantly between its constituent probes (i.e., 1.5 cm distance to each probe). This resulted in diagonal distances between channels of , whereas the horizontal and vertical distances between the emitting and the receiving probes were 3 cm. In 11 patients, multichannel NIRS measurements were complemented by simultaneous TCD measurements of the bilateral MCA through the temporal bone window using 2 MHz probes (Multi-Dop-X, DWL, Singen, Germany). Due to TCD artefacts, only seven of these patients were eligible for further analysis.