Caffeine Decreases Exercise-Induced Myocardial Flow Reserve

Caffeine Decreases Exercise-Induced Myocardial Flow Reserve Mehdi Namdar, Pascal Koepfli, Renate Grathwohl, Patrick T. Siegrist, Michael Klainguti, Tiziano Schepis, Raphael Delaloye, Christophe A. Wyss, Samuel P. Fleischmann, Oliver Gaemperli, Philipp A. Kaufmann Caffeine is a widely consumed stimulant, although its cardiovascular safety remains controversial and its effect on myocardial blood flow (MBF) is unknown. We studied the acute effect of caffeine on resting and exercise-induced MBF as well as on myocardial flow reserve (MFR) in healthy volunteers at normoxia and during acute exposure to simulated altitude, mimicking comparable states of oxygen deprivation in ischemic coronary artery disease. 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 hypoxia. 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. Caffeine is a widely consumed stimulant, although its cardiovascular safety remains controversial and its effect on MBF is unknown. 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. 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). In healthy volunteers, a caffeine dose corresponding to two cups of coffee (200 mg) significantly