title: A continuous correlation between intracranial pressure and cerebral blood flow velocity reflects cerebral autoregulation impairment during intracranial pressure plateau waves.
abstract: BACKGROUND
  In the healthy brain, small oscillations in intracranial pressure (ICP) occur synchronously with those in cerebral blood volume (CBV), cerebrovascular resistance, and consequently cerebral blood flow velocity (CBFV). Previous work has shown that the usual synchrony between ICP and CBFV is lost during intracranial hypertension. Moreover, a continuously computed measure of the ICP/CBFV association (Fix index) was a more sensitive predictor of outcome after traumatic brain injury (TBI) than a measure of autoregulation (Mx index). In the current study we computed Fix during ICP plateau waves, to observe its behavior during a defined period of cerebrovascular vasodilatation.
  
  
  METHODS
  Twenty-nine recordings of arterial blood pressure (ABP), ICP, and CBFV taken during ICP plateau waves were obtained from the Addenbrooke's hospital TBI database. Raw data was filtered prior to computing Mx and Fix according to previously published methods. Analyzed data was segmented into three phases (pre, peak, and post), and a median value of each parameter was stored for analysis.
  
  
  RESULTS
  ICP increased from a median of 22-44 mmHg before falling to 19 mmHg. Both Mx and Fix responded to the increase in ICP, with Mx trending toward +1, while Fix trended toward -1. Mx and Fix correlated significantly (Spearman's R = -0.89, p < 0.000001), however, Fix spanned a greater range than Mx. A plot of Mx and Fix against CPP showed a plateau (Mx) or trough (Fix) consistent with a zone of "optimal CPP".
  
  
  CONCLUSIONS
  The Fix index can identify complete loss of cerebral autoregulation as the point at which the normally positive CBF/CBV correlation is reversed. Both CBF and CBV can be monitored noninvasively using near-infrared spectroscopy (NIRS), suggesting that a noninvasive method of monitoring autoregulation using only NIRS may be possible.
authors.affiliation: Department of Neurosurgery, Alfred Hospital, 1st Floor, Old Baker Building, Commercial Road, Melbourne, VIC, 3003, Australia, p.lewis@alfred.org.au.
authors.forename: Philip M
authors.initials: PM
authors.lastname: Lewis
authors.forename: Peter
authors.initials: P
authors.lastname: Smielewski
authors.forename: Jeffrey V
authors.initials: JV
authors.lastname: Rosenfeld
authors.forename: John D
authors.initials: JD
authors.lastname: Pickard
authors.forename: Marek
authors.initials: M
authors.lastname: Czosnyka
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D062186:Arterial Pressure / Q000502:physiology*; D001783:Blood Flow Velocity; D001930:Brain Injuries / Q000503:physiopathology*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D016208:Databases, Factual; D005260:Female; D006706:Homeostasis / Q000502:physiology*; D006801:Humans; D019586:Intracranial Hypertension / Q000503:physiopathology*; D007427:Intracranial Pressure / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic; D012189:Retrospective Studies; D019265:Spectroscopy, Near-Infrared; D017585:Ultrasonography, Doppler, Transcranial; D014655:Vascular Resistance / Q000502:physiology; D055815:Young Adult