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 widel...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)signif icantly decreased exercise-induced MFR at normoxia and was even more pronounced during exposure to altitude.展开更多
文摘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)signif icantly decreased exercise-induced MFR at normoxia and was even more pronounced during exposure to altitude.
