title: Intracranial pressure waves: characterization of a pulsation absorber with notch filter properties using systems analysis: laboratory investigation.
abstract: OBJECTIVE
  The relationship between the waveform of intracranial pressure (ICP) and arterial blood pressure can be quantitatively characterized using a newly developed technique in systems analysis, the time-varying transfer function. This technique considers the arterial blood pressure as an input signal composed of multiple frequencies represented in the output ICP according to the transfer function imposed by the intracranial system on the input signal. The transfer function can change with time and with physiological manipulations. The authors examined data obtained from canine experiments involving manipulations of ICP.
  
  
  METHODS
  The authors analyzed 11 experiments from 3 normal mongrel dogs under conditions of normal ICP and with changes in ICP made by bolus injection, infusion, or withdrawal of cerebrospinal fluid by using time-varying transfer function.
  
  
  RESULTS
  During normal ICP periods, the gain of the transfer function displayed a deep notch (> or = 1 log unit) centered at or near the cardiac frequency. In systems terms, the intracranial compartment under normal conditions appears to act as a notch filter attenuating the cardiac frequency input relative to other frequencies. Epochs of ICP elevation showed suppression of the notch, and the notch was restored when ICP returned to normal.
  
  
  CONCLUSIONS
  The intracranial system in these animals could be considered to include a pulsation absorber for which the target frequency appears to be close to the cardiac frequency. One possible source for such an absorber mechanism might be the free movement of cerebrospinal fluid, implying that impairment of this motion may have important clinical implications in various neurological conditions such as hydrocephalus.
authors.affiliation: Neurosurgery Department, Children's Hospital Boston, Harvard Medical School, Boston, MA 02115, USA.
authors.forename: Rui
authors.initials: R
authors.lastname: Zou
authors.forename: Eun-Hyoung
authors.initials: EH
authors.lastname: Park
authors.forename: Erin McCormack
authors.initials: EM
authors.lastname: Kelly
authors.forename: Michael
authors.initials: M
authors.lastname: Egnor
authors.forename: Mark E
authors.initials: ME
authors.lastname: Wagshul
authors.forename: Joseph R
authors.initials: JR
authors.lastname: Madsen
mesh_terms: D000818:Animals; D001794:Blood Pressure; D002555:Cerebrospinal Fluid / Q000502:physiology; D004285:Dogs; D007267:Injections; D007427:Intracranial Pressure / Q000502:physiology*; D013597:Systems Analysis