Cardiocirculatory coupling during sinusoidal baroreceptor stimulation and fixed-frequency breathing

The question of whether respiratory sinus arrhythmia (RSA) originates mainl y from a central coupling between respiration and heart rate, or from baror eflex mechanisms, is a subject of controversy. If there is a major contribu tion of baroreflexes to RSA, cardiocirculatory coupling during breathing an d during cyclic baroreflex stimulation should show similarities. We applied a sinusoidal stimulus to the carotid baroreceptors and generated heart rat e fluctuations of the same magnitude as RSA with a frequency similar to, bu t 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 we re analysed using discrete Fourier-transform and transfer function analysis . Respiratory fluctuations in systolic blood pressure preceded RSA with a t ime 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 sucti on). 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 hypoth esis that the origin of RSA is predominantly a central phenomenon which sec ondarily generates fluctuations in blood pressure, but suggest that, under the condition of fixed-frequency breathing at 0.25 Hz, baroreflex mechanism s contribute to respiratory fluctuations in RR interval.