Identification of open-loop transfer functions in closed-loop baroreflex system using random breathing in humans

The existence of feedback loop in the baroreflex system makes it difficult to determine the open-loop transfer function characteristic of the central arc by which blood pressure (BP) modulates sympathetic nerve activity (SNA) and the peripheral arc by which SNA modulates BP. Random aortic pressure perturbation and electrical stimulation of aortic depressor nerve have been proposed to identify these open-loop characteristics under closed loop conditions in animals, but are limited of application in humans. We explored the random breathing technique to identify open-loop characteristics under closed-loop condition in 14 healthy subjects (10 m/4f 27±2years, mean S.E.). Random interval breathing (RB) can perturb BP with white noise characteristics over the physiologically relevant frequency range of 0.01-0.3 Hz. We measured the muscle SNA of the peroneal nerve and the continuous BP. The peripheral arc transfer function (SNA→4BP) could be approximated by a second-order low-pass filter and the fitted parameters of dynamic gain, natural frequency, damping ratio, and lag time were 1.12±0.35, 0.06±0.01 Hz, 0.49±0.14, and 1.05±0.26 s, respectively. The central arc transfer function (BP→NA) could be approximated by a first-order high-pass filter and the fitted parameters of dynamic gain, corner frequency, and lag time were 1.75±0.28, 0.1±0.04 Hz, and 0.57±0.18 s, respectively. In conclusion, the proposed random breathing technique is an useful tool of assessment in human autonomic nervous system for identification of open-loop transfer functions in the closed-loop baroreflex system.