title: Caffeine decreases exercise-induced myocardial flow reserve.
abstract: OBJECTIVE
  We studied the acute effect of caffeine on myocardial blood flow (MBF) at rest and exercise in healthy volunteers at normoxia and during acute exposure to simulated altitude.
  
  
  BACKGROUND
  Caffeine is a widely consumed stimulant, although its cardiovascular safety remains controversial and its effect on MBF is unknown.
  
  
  METHODS
  15O-labeled H2O and positron emission tomography (PET) were used to measure regional MBF at rest and immediately after supine bicycle exercise in healthy volunteers at normoxia (n = 10; mean workload, 175 W; 98% predicted; mean age, 27 +/- 6 years) as well as during hypoxia, simulating an altitude of 4,500 m by inhalation of a mixture of 12.5% oxygen (n = 8; 148 W; 78% predicted; mean age, 29 +/- 4 years). Measurements were repeated 50 min after oral ingestion of caffeine (200 mg). Myocardial flow reserve (MFR) was calculated as the ratio of hyperemic to resting MBF.
  
  
  RESULTS
  Resting MBF was not affected by caffeine at normoxia (1.05 +/- 0.36 ml/min/g vs. 1.17 +/- 0.27 ml/min/g; p = NS), although it was significantly increased at hypoxia (1.71 +/- 0.41 ml/min/g vs. 2.22 +/- 0.49 ml/min/g; p < 0.001). By contrast, exercise-induced hyperemic MBF decreased significantly at normoxia (2.51 +/- 0.58 ml/min/g vs. 2.15 +/- 0.47 ml/min/g; p < 0.05) and hypoxia (5.15 +/- 0.79 ml/min/g vs. 3.98 +/- 0.83 ml/min/g; p < 0.005 vs. baseline; p < 0.005 vs. normoxia). The MFR decreased by 22% at normoxia (2.53 +/- 0.69 to 1.90 +/- 0.49; p < 0.01) and by 39% at hypoxia (3.13 +/- 0.60 to 1.87 +/- 0.45, p < 0.005; p < 0.05 vs. normoxia).
  
  
  CONCLUSIONS
  In healthy volunteers, a caffeine dose corresponding to two cups of coffee (200 mg) significantly decreased exercise-induced MFR at normoxia and was even more pronounced during exposure to altitude.
authors.affiliation: Cardiovascular Center, Nuclear Cardiology, University Hospital, Zurich, Switzerland.
authors.forename: Mehdi
authors.initials: M
authors.lastname: Namdar
authors.forename: Pascal
authors.initials: P
authors.lastname: Koepfli
authors.forename: Renate
authors.initials: R
authors.lastname: Grathwohl
authors.forename: Patrick T
authors.initials: PT
authors.lastname: Siegrist
authors.forename: Michael
authors.initials: M
authors.lastname: Klainguti
authors.forename: Tiziano
authors.initials: T
authors.lastname: Schepis
authors.forename: Raphael
authors.initials: R
authors.lastname: Delaloye
authors.forename: Christophe A
authors.initials: CA
authors.lastname: Wyss
authors.forename: Samuel P
authors.initials: SP
authors.lastname: Fleischmann
authors.forename: Oliver
authors.initials: O
authors.lastname: Gaemperli
authors.forename: Philipp A
authors.initials: PA
authors.lastname: Kaufmann
mesh_terms: D000328:Adult; D000531:Altitude; D002110:Caffeine / Q000494:pharmacology*; D000697:Central Nervous System Stimulants / Q000494:pharmacology*; D003326:Coronary Circulation / Q000187:drug effects*; D015444:Exercise / Q000502:physiology*; D005260:Female; D006439:Hemodynamics / Q000187:drug effects; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008055:Lipids / Q000097:blood; D008297:Male; D012039:Regional Blood Flow / Q000187:drug effects
chemical_list: D000697:Central Nervous System Stimulants; D008055:Lipids; D002110:Caffeine