Baroreflex measurements are clinically useful but are not in widespread use due to their poor repeatability. 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. In each case a single lead ECG and non-invasive continuous blood pressure (Finapres) signals were recorded to computer. Beat-to-beat RR intervals and systolic pressure levels were calculated off-line, and linear interpolation was used to convert from the beat domain to the time domain. 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 could be calculated for 89% of the recordings and the results showed that it significantly decreased by 1.83 ± 0.69 s (mean ± SD) (p<0.001) when breathing frequency increased from 6 to 12 breaths per minute. Posture had no influence.
Baroreflex sensitivity could be calculated for 94% of the recordings and the results showed that it significantly 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. This was in contrast to the time offset. 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.
Bowers EJ, Murray A. Effects of posture and breathing frequency on baroreflex measurements. Computers in Cardiology 2002; 29: 469-72.
Ms Emma Bowers, Freeman Unit, Newcastle upon Tyne. Tel: +44 (0)191-223-1370.
Regional Medical Physics Department, Newcastle General Hospital, Newcastle upon Tyne, NE4 6BE, UK.
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