CAFFEINE ENHANCES MODULATION OF PARASYMPATHETIC NERVE ACTIVITY IN HUMANS - QUANTIFICATION USING POWER SPECTRAL-ANALYSIS

We investigated changes in autonomic nerve activity following caffeine intake by power spectral analysis of R-R intervals of heartbeats in h umans. A beverage containing 240 mg of caffeine or a control beverage was given to 10 healthy volunteers, and R-R intervals were measured wh ile subjects were sitting and controlling their respiration at a const ant rate. After consumption of the caffeine-containing beverage, a tra nsient and significant increase (P < 0.001) in spectral integrated val ues (areas under the curve) of high frequency power (high component, H C) was observed, and at 30 min the value was significantly greater tha n in controls (P < 0.02), suggesting an increase in vagal autonomic ne rve activity. The effect of caffeine was also examined using decaffein ated coffee supplemented with exogenous caffeine (2 mg/kg body wt). A transient and significant increase (P = 0.0001) in HG was observed, an d the value was significantly greater (P < 0.02) than when subjects co nsumed decaffeinated coffee without supplemental caffeine. The ratio o f HC to total integrated value (which is also used as a selective indi cator of vagal activity) was also significantly higher (P < 0.04) afte r caffeine consumption. Physiological variables accompanying the chang e in autonomic nerve activity (i,e., blood pressure, surface body temp erature and heart rate) were not significantly affected by caffeine in take. These results suggest that power spectral analysis of heartbeat R-R intervals is an effective and noninvasive method for detecting sub tle changes in autonomic nerve activity in response to food intake.