title: The limitations of near-infrared spectroscopy to assess cerebrovascular reactivity: the role of slow frequency oscillations.
abstract: BACKGROUND
  A total hemoglobin reactivity index (THx) derived from near-infrared spectroscopy (NIRS) has recently been introduced to assess cerebrovascular reactivity noninvasively. Analogously to the pressure reactivity index (PRx), THx is calculated as correlation coefficient with arterial blood pressure (ABP). However, the reliability of THx in the injured brain is uncertain. Although slow oscillations have been described in NIRS signals, their significance for assessment of autoregulation remains unclear. In the current study, we investigated the role of slow oscillations of total hemoglobin for NIRS-based cerebrovascular reactivity monitoring.
  
  
  METHODS
  This study was based on a retrospective analysis of data that were consecutively recorded for a different project published previously. Thirty-seven patients with traumatic brain injury and admitted to Addenbrooke's Neurosciences Critical Care Unit between June 2008 and June 2009 were included. After artifact removal, we performed spectral analysis of the tissue hemoglobin index (THI, a measure of oxy- and deoxygenated hemoglobin) and intracranial pressure (ICP) signal. PRx and THx were calculated as moving correlations between ICP and ABP, and THI and ABP, respectively. The agreement between PRx and THx as a function of normalized power of slow oscillations (0.015-0.055 Hz) contained in the input signals was assessed performing between-subject and within-subject correlation analyses. Furthermore, the correlation between the THx values derived from the right and left sides was analyzed.
  
  
  RESULTS
  The agreement between PRx and THx depended on the power of slow oscillations in the input signals. Between-subject comparisons revealed a significant correlation between THx and PRx (r = 0.80, 95% confidence interval 0.53-0.92, P < 0.01) for patients with normalized slow wave activity >0.4 in the THI signal, compared with r = 0.07 (95% confidence interval -0.40 to 0.51, P = 0.79) in the remaining files. Furthermore, within-subject comparisons suggested that THx may be used as a substitute for PRx only when there is an at least moderate agreement (r = 0.36) between the THx values derived from the right and left sides.
  
  
  CONCLUSIONS
  Our results suggest that the NIRS-based cerebrovascular reactivity index THx can be used as a noninvasive substitute for PRx, but only during phases with sufficient slow wave power in the input signal. Furthermore, a good agreement between the THx measures on both sides seems to be a prerequisite for comparison of a global (PRx) versus the more local (THx) index. Nevertheless, noninvasive assessment of cerebrovascular reactivity may be desirable in patients without ICP monitoring and help to guide ABP management in these patients.
authors.affiliation: Academic Neurosurgical Unit, University of Cambridge Clinical School, Cambridge, United Kingdom. jennifer.diedler@med.uni-heidelberg.de
authors.forename: Jennifer
authors.initials: J
authors.lastname: Diedler
authors.forename: Christian
authors.initials: C
authors.lastname: Zweifel
authors.forename: Karol P
authors.initials: KP
authors.lastname: Budohoski
authors.forename: Magdalena
authors.initials: M
authors.lastname: Kasprowicz
authors.forename: Enrico
authors.initials: E
authors.lastname: Sorrentino
authors.forename: Christina
authors.initials: C
authors.lastname: Haubrich
authors.forename: Kenneth M
authors.initials: KM
authors.lastname: Brady
authors.forename: Marek
authors.initials: M
authors.lastname: Czosnyka
authors.forename: John D
authors.initials: JD
authors.lastname: Pickard
authors.forename: Peter
authors.initials: P
authors.lastname: Smielewski
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D015415:Biomarkers / Q000097:blood; D001930:Brain Injuries / Q000097:blood / Q000175:diagnosis* / Q000503:physiopathology; D002560:Cerebrovascular Circulation; D004739:England; D005583:Fourier Analysis; D006454:Hemoglobins / Q000378:metabolism*; D006706:Homeostasis; D006801:Humans; D007427:Intracranial Pressure; D008875:Middle Aged; D008991:Monitoring, Physiologic / Q000379:methods*; D010108:Oxyhemoglobins / Q000378:metabolism*; D011237:Predictive Value of Tests; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012720:Severity of Illness Index; D019265:Spectroscopy, Near-Infrared; D013997:Time Factors; D055815:Young Adult
chemical_list: D015415:Biomarkers; D006454:Hemoglobins; D010108:Oxyhemoglobins; C100003:deoxyhemoglobin