Cardiocirculatory coupling during sinusoidal baroreceptor stimulation and fixed-frequency breathing

The question of whether respiratory sinus arrhythmia (RSA) originates mainly from a central coupling between respiration and heart rate, or from baroreflex mechanisms, is a subject of controversy. If there is a major contribution of baroreflexes to RSA, cardiocirculatory coupling during breathing and during cyclic baroreflex stimulation should show similarities. We applied a sinusoidal stimulus to the carotid baroreceptors and generated heart rate fluctuations of the same magnitude as RSA with a frequency similar to, but different from, the breathing frequency (0.2 Hz, compared with 0.25 Hz), and at 0.1 Hz, in 17 supine healthy subjects (age 28-39 years). The data were analysed using discrete Fourier-transform and transfer function analysis. Respiratory fluctuations in systolic blood pressure preceded RSA with a time lag equal to that between baroreceptor stimulation and oscillations in RR interval (0.62+/-0.18 s compared with 0.57+/-0.28 s at 0.2 Hz neck suction). The response of systolic blood pressure to neck suction at 0.2 Hz was 5 times less than the respiratory blood pressure fluctuations. Neck suction at 0.1 Hz largely increased fluctuations in blood pressure and RR interval, whereas the spontaneous phase relationship between blood pressure and RR interval remained unchanged. Our results are not consistent with the hypothesis that the origin of RSA is predominantly a central phenomenon which secondarily generates fluctuations in blood pressure, but suggest that, under the condition of fixed-frequency breathing at 0.25 Hz, baroreflex mechanisms contribute to respiratory fluctuations in RR interval.