title: Cerebrovascular reactivity measured by near-infrared spectroscopy.
abstract: OBJECTIVE
  The pressure reactivity index (PRx) describes cerebral vessel reactivity by correlation of slow waves of intracranial pressure (ICP) and arterial blood pressure. In theory, slow changes in the relative total hemoglobin (rTHb) measured by near-infrared spectroscopy are caused by the same blood volume changes that cause slow waves of ICP. Our objective was to develop a new index of vascular reactivity, the hemoglobin volume index (HVx), which is a low-frequency correlation of arterial blood pressure and rTHb measured with near-infrared spectroscopy.
  
  
  METHODS
  Gradual hypotension was induced in piglets while cortical laser-Doppler flux was monitored. ICP was monitored, and rTHb was measured continuously using reflectance near-infrared spectroscopy. The HVx was recorded as a moving linear correlation between slow waves (20 to 300 seconds) of arterial blood pressure and rTHb. Autoregulation curves were constructed by averaging values of the PRx or HVx in 5-mm Hg bins of cerebral perfusion pressure.
  
  
  RESULTS
  The laser-Doppler flux-determined lower limit of autoregulation was 29.4+/-6.7 mm Hg (+/-SD). Coherence between rTHb and ICP was high at low frequencies. HVx was linearly correlated with PRx. The PRx and HVx both showed higher values below the lower limit of autoregulation and lower values above the lower limit of autoregulation. Areas under the receiver operator characteristic curves were 0.88 and 0.85 for the PRx and HVx, respectively.
  
  
  CONCLUSIONS
  Coherence between the rTHb and ICP waveforms at the frequency of slow waves suggests that slow waves of ICP are related to blood volume changes. The HVx has potential for further development as a noninvasive alternative to the PRx.
authors.affiliation: Department of Anesthesiology and Critical Care Medicine, Johns Hopkins University School of Medicine, Baltimore, MD 21287, USA.
authors.forename: Jennifer K
authors.initials: JK
authors.lastname: Lee
authors.forename: Kathleen K
authors.initials: KK
authors.lastname: Kibler
authors.forename: Paul B
authors.initials: PB
authors.lastname: Benni
authors.forename: R Blaine
authors.initials: RB
authors.lastname: Easley
authors.forename: Marek
authors.initials: M
authors.lastname: Czosnyka
authors.forename: Peter
authors.initials: P
authors.lastname: Smielewski
authors.forename: Raymond C
authors.initials: RC
authors.lastname: Koehler
authors.forename: Donald H
authors.initials: DH
authors.lastname: Shaffner
authors.forename: Ken M
authors.initials: KM
authors.lastname: Brady
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D001784:Blood Gas Analysis; D001794:Blood Pressure / Q000502:physiology; D001831:Body Temperature / Q000502:physiology; D002560:Cerebrovascular Circulation / Q000502:physiology*; D006454:Hemoglobins / Q000378:metabolism; D006706:Homeostasis; D007427:Intracranial Pressure / Q000502:physiology; D012372:ROC Curve; D019265:Spectroscopy, Near-Infrared / Q000379:methods*; D013552:Swine
chemical_list: D006454:Hemoglobins