Effects of posture and breathing frequency on baroreflex measurements

The aim of this study was to assess the influences of posture and breathing frequency on baroreflex measurements. Six normal healthy volunteers breathed regularly at 6, 8, 10, and 12 breaths per minute each in a supine, sitting and standing position. Beat-to-beat RR-intervals and systolic pressure levels were calculated from raw ECG and Finapres signals. The time offset between positive RR-interval changes and subsequent negative systolic pressure level changes was calculated by cross correlation. Baroreflex sensitivity was calculated from the frequency domain of RR-interval and systolic pressure levels changes, using the ratio of the height of the peaks at each breathing frequency. Time offset significantly decreased by 1.83±0.69 s (mean ±SD) (p<0.001) when breathing rate increased from 6 to 12 breaths per minute. Posture had no influence. Baroreflex sensitivity decreased by 20.1 18.5 ms/mmHg (mean SD) (p<0.01) as posture changed from supine to standing, but was independent of breathing frequency. Posture and breathing frequency are two factors of many that can influence baroreflex measures. These preliminary results show that a better understanding of their effects is needed to allow repeatable measurement protocols to be developed.