Human responses to upright tilt: a window on central autonomic integration

1. We examined interactions between haemodynamic and autonomic neural oscil lations during passive upright tilt, to gain better insight into human auto nomic regulatory mechanisms. 2. We recorded the electrocardiogram, finger photoplethysmographic arterial pressure, respiration and peroneal nerve muscle sympathetic activity in ni ne healthy young adults. Subjects breathed in time with a metronome at 12 b reaths min(-1) (0.2 Hz) for 5 min each, in supine, and 20, 40, 60, 70 and 8 0 deg head-up positions. We performed fast Fourier transform (and autoregre ssive) power spectral analyses and integrated low-frequency (0.05-0.15 Hz) and respiratory-frequency (0.15-0.5 Hz) spectral powers. 3. Integrated areas of muscle sympathetic bursts and their low- and respira tory-frequency spectral powers increased directly and significantly with th e tilt angle. The centre frequency of low-frequency sympathetic oscillation s was constant before and during tilt. Sympathetic bursts occurred more com monly during expiration than inspiration at low tilt angles, but occurred e qually in expiration and inspiration at high tilt angles. 4. Systolic and diastolic pressures and their low- and respiratory-frequenc y spectral powers increased, and R-R intervals and their respiratory-freque ncy spectral power decreased progressively with the tilt angle. Low-frequen cy R-R interval spectral power did not change. 5. The cross-spectral phase angle between systolic pressures and R-R interv als remained constant and consistently negative at the low frequency, but s hifted progressively from positive to negative at the respiratory frequency during tilt. The arterial baroreflex modulus, calculated from low-frequenc y cross-spectra, decreased at high tilt angles. 6. Our results document changes of baroreflex responses during upright tilt , which may reflect leftward movement of subjects on their arterial pressur e sympathetic and vagal response relations. The intensity, but not the cent re frequency of low-frequency cardiovascular rhythms, is modulated by the l evel of arterial baroreceptor input. Tilt reduces respiratory gating of sym pathetic and vagal motoneurone responsiveness to stimulatory inputs for dif ferent reasons; during tilt, sympathetic stimulation increases to a level t hat overwhelms the respiratory gate, and vagal stimulation decreases to a l evel below that necessary for maximal respiratory gating to occur.