title: Nonlinear dynamics of blood pressure variability after caffeine consumption.
abstract: OBJECTIVE
  The occurrence of continuous blood pressure (BP) fluctuations is an intrinsic feature of the cardiovascular system and is related to complex cardiovascular mechanisms and environmental stimulations or daily habits such as coffee drinking. The aim of this study was to investigate the effect of caffeine on indices expressing the complex and "chaotic" nonlinear characteristics of BP variability.
  
  
  METHODS
  Fourteen healthy subjects consumed 240 mg of caffeine or placebo according to a randomized, double-blind, crossover design. Continuous radial pressure waveforms were recorded by applanation tonometry at baseline and at 30, 60, 90 and 120 minutes after ingestion of caffeine or placebo.
  
  
  METHODS
  Approximate entropy (ApEn) was determined to assess BP irregularity. Detrended fluctuation analysis was also used to quantify the fractal correlation properties of the BP data by estimating the scaling (self-similarity) exponent alpha. Analysis of variance for repeated measurements was used to detect significant changes in the measured variables at 30, 60, 90 and 120 minutes after ingestion of caffeine or placebo.
  
  
  RESULTS
  BP fluctuations demonstrated high regularity and predictability as indicated by the low values of ApEn (0.2 +/- 0.3), and they remained unchanged after caffeine ingestion. In contrast, the long-term scaling exponent alpha of the BP time series was significantly increased from 0.99 to 1.04 (p=0.01) after caffeine ingestion, while the placebo induced no significant change.
  
  
  CONCLUSIONS
  Caffeine induced greater "randomness" in BP fluctuations, possibly by triggering cardiovascular mechanisms that could not be directly specified but should be investigated in future studies.
authors.affiliation: First Department of Cardiology, Hippocration Hospital, National and Kapodistrian University of Athens, Greece. theopap@hippocratio.gr
authors.forename: Theodore G
authors.initials: TG
authors.lastname: Papaioannou
authors.forename: Charalambos
authors.initials: C
authors.lastname: Vlachopoulos
authors.forename: Nikolaos
authors.initials: N
authors.lastname: Ioakeimidis
authors.forename: Nikolaos
authors.initials: N
authors.lastname: Alexopoulos
authors.forename: Christodoulos
authors.initials: C
authors.lastname: Stefanadis
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D001794:Blood Pressure / Q000187:drug effects* / Q000502:physiology; D002110:Caffeine / Q000494:pharmacology*; D000697:Central Nervous System Stimulants / Q000494:pharmacology*; D018592:Cross-Over Studies; D004311:Double-Blind Method; D006801:Humans; D008297:Male; D017711:Nonlinear Dynamics; D013997:Time Factors
chemical_list: D000697:Central Nervous System Stimulants; D002110:Caffeine