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CLINICAL RESEARCH

Caffeine Decreases Exercise-Induced Myocardial Flow Reserve

Mehdi Namdar, MD^_*, Pascal Koepfli, MD^_*, Renate Grathwohl, MD^_*, Patrick T. Siegrist, MD^_*, Michael Klainguti, MD^_*, Tiziano Schepis, MD^_*, Raphael Delaloye, MD^_*, Christophe A. Wyss, MD^_*, Samuel P. Fleischmann, MD^_*, Oliver Gaemperli, MD^_* and Philipp A. Kaufmann, MD^_*,,_*

^_* Cardiovascular Center, Nuclear Cardiology, University Hospital
Center for Integrative Human Physiology (CIHP), University of Zurich, Zurich, Switzerland

Manuscript received July 3, 2005; revised manuscript received August 2, 2005, accepted August 16, 2005.

^_* Reprint requests and correspondence: Dr. Philipp A. Kaufmann, Nuclear Cardiology, University Hospital Zurich, Ramistr. 100 NUK C 32, CH-8091 Zurich, Switzerland (Email: pak{at}usz.ch).

OBJECTIVES: 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.

Abbreviations and Acronyms   CAD = coronary artery disease   MBF = myocardial blood flow   MFR = myocardial flow reserve   PET = positron emission tomography   RPP = rate-pressure product

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