title: Principles of cerebral hemodynamics when intracranial pressure is raised: lessons from the peripheral circulation.
abstract: BACKGROUND
  The brain is highly vascular and richly perfused, and dependent on continuous flow for normal function. Although confined within the skull, pressure within the brain is usually less than 15 mmHg, and shows small pulsations related to arterial pulse under normal circumstances. Pulsatile arterial hemodynamics in the brain have been studied before, but are still inadequately understood, especially during changes of intracranial pressure (ICP) after head injury.
  
  
  METHODS
  In seeking cohesive explanations, we measured ICP and radial artery pressure (RAP) invasively with high-fidelity manometer systems, together with middle cerebral artery flow velocity (MCAFV) (transcranial Doppler) and central aortic pressure (CAP) generated from RAP, using a generalized transfer function technique, in eight young unconscious, ventilated adults following closed head trauma. We focused on vascular effects of spontaneous rises of ICP ('plateau waves').
  
  
  RESULTS
  A rise in mean ICP from 29 to 53 mmHg caused no consistent change in pressure outside the cranium, or in heart rate, but ICP pulsations increased in amplitude from 8 to 20 mmHg, and ICP waveform came to resemble that in the aorta. Cerebral perfusion pressure (=central aortic pressure - ICP), which equates with transmural pressure, fell from 61 to 36 mmHg. Mean MCAFV fell from 53 to 40 cm/s, whereas pulsatile MCAFV increased from 77 to 98 cm/s. These significant changes (all P < 0.01) may be explained using the Monro-Kellie doctrine, because of compression of the brain, as occurs in a limb when external pressure is applied.
  
  
  CONCLUSIONS
  The findings emphasize importance of reducing ICP, when raised, and on the additional benefits of reducing wave reflection from the lower body.
authors.affiliation: aAustralian School of Advanced Medicine, Macquarie University bSt Vincent's Clinic cUniversity of New South Wales dVictor Chang Cardiac Research Institute, Sydney, Australia eAcademic Neurosurgical Unit, Addenbrooke's Hospital, Cambridge University, Cambridge, UK.
authors.forename: Mi Ok
authors.initials: MO
authors.lastname: Kim
authors.forename: Audrey
authors.initials: A
authors.lastname: Adji
authors.forename: Michael F
authors.initials: MF
authors.lastname: O'Rourke
authors.forename: Alberto P
authors.initials: AP
authors.lastname: Avolio
authors.forename: Peter
authors.initials: P
authors.lastname: Smielewski
authors.forename: John D
authors.initials: JD
authors.lastname: Pickard
authors.forename: Marek
authors.initials: M
authors.lastname: Czosnyka
mesh_terms: D000328:Adult; D062186:Arterial Pressure; D001794:Blood Pressure / Q000502:physiology; D001921:Brain / Q000098:blood supply*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005260:Female; D006439:Hemodynamics; D006801:Humans; D007427:Intracranial Pressure / Q000502:physiology*; D008297:Male; D020768:Middle Cerebral Artery / Q000502:physiology; D011673:Pulsatile Flow; D055815:Young Adult