title: An assessment of dynamic autoregulation from spontaneous fluctuations of cerebral blood flow velocity: a comparison of two models, index of autoregulation and mean flow index.
abstract: BACKGROUND
  Various methods of assessment of cerebral autoregulation, using spontaneous slow fluctuations of blood flow velocity (FV), arterial blood pressure, and cerebral perfusion pressure, have been used in clinical practice. We studied the association between the dynamic index of autoregulation (ARI) and time correlation index (mean flow index, Mx) in a group of patients after head injury.
  
  
  METHODS
  Fifty head-injured patients of an average age of 31 yr, sedated, paralyzed, and ventilated (mild hypocapnia) with continuous monitoring of arterial blood pressure and intracranial pressure were studied. Cerebral blood FV was monitored daily for 3 days after injury during periods that were free from interventions (e.g., suctioning). Digitally recorded data were analyzed retrospectively. ARI was calculated as a coefficient graded from 0 (absence of autoregulation) to 9 (strongest autoregulation), describing a dynamic model of autoregulation. Mx was calculated as the correlation coefficient between 40 consecutive 6-s averages of FV and cerebral perfusion pressure and then averaged over the whole recording period. ARI and Mx values, assessed during the first 3 days after injury, were averaged for each patient.
  
  
  RESULTS
  ARI and Mx showed moderately strong mutual linear relationship with correlation r = -0.62; P = 0.0001. Both indices correlated with outcome, indicating worse autoregulation in patients achieving unfavorable outcome.
  
  
  CONCLUSIONS
  ARI and Mx agree relatively well in head-injured patients. Autoregulation affects outcome after head injury.
authors.affiliation: Academic Neurosurgical Unit, Addenbrooke's Hospital, Cambridge, UK. mc141@medschl.cam.ac.uk
authors.forename: Marek
authors.initials: M
authors.lastname: Czosnyka
authors.forename: Piotr
authors.initials: P
authors.lastname: Smielewski
authors.forename: Andrea
authors.initials: A
authors.lastname: Lavinio
authors.forename: John D
authors.initials: JD
authors.lastname: Pickard
authors.forename: Ronney
authors.initials: R
authors.lastname: Panerai
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001783:Blood Flow Velocity; D002560:Cerebrovascular Circulation; D006259:Craniocerebral Trauma / Q000503:physiopathology*; D005260:Female; D015600:Glasgow Coma Scale; D006706:Homeostasis; D006801:Humans; D008297:Male; D008875:Middle Aged; D008955:Models, Cardiovascular; D008991:Monitoring, Physiologic / Q000379:methods*; D012107:Research Design; D012189:Retrospective Studies; D012815:Signal Processing, Computer-Assisted; D013997:Time Factors