Caffeine decreases exercise-induced myocardial flow reserve.
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.
Namdar
Mehdi
M
Cardiovascular Center, Nuclear Cardiology, University Hospital, Zurich, Switzerland.
Koepfli
Pascal
P
Grathwohl
Renate
R
Siegrist
Patrick T
PT
Klainguti
Michael
M
Schepis
Tiziano
T
Delaloye
Raphael
R
Wyss
Christophe A
CA
Fleischmann
Samuel P
SP
Gaemperli
Oliver
O
Kaufmann
Philipp A
PA
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
D000697:Central Nervous System Stimulants; D008055:Lipids; D002110:Caffeine