Incongruous effect of phenylephrine on changes in cerebral blood volume measured by near-infrared spectroscopy (NIRS) indicating extracranial contamination.

We assessed extracranial contamination of the near-infrared spectroscopy (NIRS) signal during administration of phenylephrine. The study was performed with NIRO 200NX which employs both the Modified Beer-Lambert (MBL) method to measure total hemoglobin (tHb, expressed in µM), and Spatially Resolved Spectroscopy (SRS) to measure total hemoglobin content (nTHI, expressed in arbitrary units (a.u.)). SRS tends to not be affected by extracranial blood flow. As vasoconstriction with phenylephrine mainly occurs in the extracranial area, we hypothesized that if NIRS measurements are indeed prone to extracranial contamination, tHb will be more affected by the administration of phenylephrine than nTHI. After ethical committee approval, 20 consenting cardiac surgery patients were included. Phenylephrine was administered whenever clinically indicated and its effect on nTHI and tHb was evaluated. To adjust for the difference in raw scale units, Z-scores were calculated. Data were analyzed with Wilcoxon Signed Ranks Test and the Hodges-Lehmann method. A total of 191 data sets were obtained in 20 patients (10 male, 65 ± 15 years, 77 ± 16 kg, 166 ± 11 cm). The median difference before and after administration of phenylephrine was - 0.006 a.u. [95%CI - 0.010 to - 0.002] (p < 0.001) and - 0.415 µM [95%CI - 0.665 to - 0.205] (p < 0.001) for nTHI and tHb, respectively. The median difference between the Z-scores of nTHI and tHb was - 0.02 [95%CI - 0.04 to - 0.003] (p = 0.03), with a higher variability in the Z-scores of tHb. Phenylephrine induced significant larger changes in MBL values compared to SRS values, indicating that the MBL method might be more prone to extracranial contamination. Trial and clinical registry: Trial registration number: B670201939459, ethical committee number: 2019/0265, date of approval: March 19, 2019.

Effect of posture and extracranial contamination on results of cerebral oximetry by near-infrared spectroscopy.

BACKGROUND: Near-infrared spectroscopy (NIRS)-based cerebral oximetry is a noninvasive technology used to estimate regional cerebral oxygen saturation (rSO2). Extracranial blood flow is known to substantially affect rSO2 values measured by most clinically available devices. Several studies have also reported that the Trendelenburg position and upright position have a larger effect on rSO2 measurements than the supine position. Therefore, we investigated the effect of these two positions (the Trendelenburg position versus the upright position) and extracranial contamination on rSO2 measurements obtained using two commercially available devices and one prototype device. METHODS: Twelve healthy volunteers were enrolled in the study. They each had three cerebral oximetry devices applied to their forehead (FORE-SIGHT ELITE™, CAS Medical Systems Inc., Branford, CT, USA; INVOS 5100c™, Medtronic, Minneapolis, MN, USA; and NIRO-TRS, Hamamatsu Photonics, Hamamatsu, Japan). A circumferential pneumatic head cuff was positioned proximal to the NIRS cerebral oximetry sensors. We measured rSO2, heart rate (HR), and blood pressure (BP) in six conditions (supine, Trendelenburg and upright positions, with and without scalp ischemia induced by head cuff inflation) every 5 min with each oximetry device. Total hemoglobin (tHb), which is associated with cerebral blood volume (CBV) as measured by positron emission tomography, was measured using the NIRO-TRS device to determine extracranial blood volume in each position. RESULTS: Measurements of rSO2 with all the devices were affected by extracranial contamination. The percentage of extracranial contamination was highest with INVOS 5100c™ in the upright position (INVOS, 21.3%; FORE-SIGHT, 14.3%; NIRO-TRS, 3.6%). Measurements of rSO2 obtained in the upright position were significantly lower than those obtained in the supine position, using INVOS-5100c™ and FORE-SIGHT ELITE™ (71 vs. 74% and 67 vs. 72%, respectively), but not using NIRO-TRS (62 vs. 64%). A significant decrease in tHb was observed after head cuff inflation in the supine and Trendelenburg positions (supine, 0.132-0.123 µmol/l; Trendelenburg, 0.133-0.125 µmol/l). CONCLUSIONS: Except when using NIRO-TRS, measurements of rSO2 in the forehead are significantly lower when measured in the upright position than in the supine position. All devices in this study were affected by extracranial contamination.