title: Approximate entropy: a regularity statistic for assessment of intracranial pressure.
abstract: BACKGROUND
  Waveform analysis of ICP and mABP provideqs important information about cerebrovascular reactivity and intracranial compliance. Traditionally spectral analysis and correlation statistics have been used despite certain limitations. Approximate entropy (ApEn) is an established measure of system regularity; which can change with pathology. This study defines a novel method for application of ApEn to the ICP/mABP waveform, and reports changing ApEn with pathophysiology in the rodent brain.
  
  
  METHODS
  Nine Sprague-Dawley rats were anesthetized for mABP, and ICP recording. Rats were exposed to 15 minutes hypotension (45-50 mmHg), followed by resuscitation, equilibration and infusion of a 500 ul subdural hematoma. Raw data, ApEn(RAW), and wave-period, ApEn(PERIOD), were assessed for stochastic fluctuations using ApEn (m = 2, r = 0.2 SD). Results were compared with the established parameters: standard deviation, harmonic transfer function, Pressure Reactivity Index and Correlation Coefficient.
  
  
  RESULTS
  At baseline, ApEn(ABP-PERIOD) and ApEn(ICP-PERIOD) were 1.464 +/- 0.003, and 0.690 +/- 0.020 respectively, suggesting that random heart rate fluctuations are damped during transfer to the ICP waveform. ApEn(ICP-PERIOD) consistently rose during hypotension (ICP 7 +/- 2: ApEn(ICP-PERIOD) 1.086 +/- 0.074, p < 0.05) and after SDH infusion (ICP 35 +/- 12: ApEn(ICP-PERIOD) 1.24 +/- 0.03, p < 0.01). These values were closer to mABP and suggest enhanced transfer of random fluctuations. Significant changes in ApEn were seen in the absence of significant changes in other parameters.
  
  
  CONCLUSIONS
  This study defines a system for analyzing the transfer of random fluctuations in mABP waveform to the ICP waveform. ApEn appears to be responsive to changes in intracranial compliance and/or cerebrovascular resistance, therefore more formal studies of the sensitivity and specificity of this novel measure are warranted. These initial findings suggest that ApEn may be a useful adjunct measure of the ICP waveform.
authors.affiliation: Department of Physiology, Medical College of Virginia, Virginia Commonwealth University, Richmond, VA, USA.
authors.forename: A
authors.initials: A
authors.lastname: Beaumont
authors.forename: A
authors.initials: A
authors.lastname: Marmarou
mesh_terms: D000818:Animals; D001794:Blood Pressure / Q000502:physiology*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D004195:Disease Models, Animal; D019277:Entropy; D006408:Hematoma, Subdural / Q000503:physiopathology; D007022:Hypotension / Q000503:physiopathology; D007427:Intracranial Pressure / Q000502:physiology*; D008297:Male; D020768:Middle Cerebral Artery / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley