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

Abstract

1. We examined interactions between haemodynamic and autonomic neural oscillations during passive upright tilt, to gain better insight into human autonomic regulatory mechanisms. 2. We recorded the electrocardiogram, finger photoplethysmographic arterial pressure, respiration and peroneal nerve muscle sympathetic activity in nine healthy young adults. Subjects breathed in time with a metronome at 12 breaths min-1 (0.2 Hz) for 5 min each, in supine, and 20, 40, 60, 70 and 80 deg head-up positions. We performed fast Fourier transform (and autoregressive) 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 respiratory-frequency spectral powers increased directly and significantly with the tilt angle. The centre frequency of low-frequency sympathetic oscillations was constant before and during tilt. Sympathetic bursts occurred more commonly during expiration than inspiration at low tilt angles, but occurred equally in expiration and inspiration at high tilt angles. 4. Systolic and diastolic pressures and their low- and respiratory-frequency spectral powers increased, and R-R intervals and their respiratory-frequency spectral power decreased progressively with the tilt angle. Low-frequency R-R interval spectral power did not change. 5. The cross-spectral phase angle between systolic pressures and R-R intervals remained constant and consistently negative at the low frequency, but shifted progressively from positive to negative at the respiratory frequency during tilt. The arterial baroreflex modulus, calculated from low-frequency 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 pressure sympathetic and vagal response relations. The intensity, but not the centre frequency of low-frequency cardiovascular rhythms, is modulated by the level of arterial baroreceptor input. Tilt reduces respiratory gating of sympathetic and vagal motoneurone responsiveness to stimulatory inputs for different reasons; during tilt, sympathetic stimulation increases to a level that overwhelms the respiratory gate, and vagal stimulation decreases to a level below that necessary for maximal respiratory gating to occur.

Figure 1. Experimental record from one subject

Representative arterial pressure and muscle sympathetic nerve tracings from one subject during supine and 60 deg passive upright tilt.

Figure 2. Average muscle sympathetic nerve activity for all subjects

Mean ±s.e.m. changes of muscle sympathetic nerve activity as a function of the sine of the tilt angle. * Significantly different from supine.

Figure 3. Muscle sympathetic nerve area spectral power

Average muscle sympathetic nerve area spectral power at each tilt angle (top panel), and integrated spectral power in low- and respiratory-frequency bands, plotted as functions of the sine of tilt angle (bottom panel). a.u., arbitrary units. Bottom panel: thick lines denote significant regressions. r, correlation coefficient derived from least-squares linear regression. * Significantly different from supine.

Figure 5. Systolic pressure spectral power

Average systolic pressure spectral power at each tilt angle (top panel), and at low- and respiratory-frequency bands, plotted as functions of the sine of tilt angle (bottom panel). Bottom panel: thick lines denote significant regression. * Significantly different from supine.

Figure 6. Diastolic pressure spectral power

Average diastolic pressure spectral power at each tilt angle (top panel), and at low- and respiratory-frequency bands, plotted as functions of the sine of tilt angle (bottom panel). Bottom panel: thick lines denote significant regression. * Significantly different from supine.{

Figure 7. R-R interval spectral power

Average R-R interval spectral power at each tilt angle (top panel), and at low- and respiratory-frequency bands, plotted as functions of the sine of tilt angle (bottom panel). Bottom panel: thick line, significant regression; thin line, insignificant regression. * Significantly different from supine.

Figure 8. Systolic pressure and R-R interval phase angles

Average phase angles between systolic pressures and R-R intervals, derived from cross-spectral analysis in subjects whose squared coherence was > = 0.50. Numbers in parentheses indicate the number of subjects at higher tilt angles. Thick line, significant regression; thin line, insignificant regression.

Figure 4. Inspiratory/expiratory distribution of muscle sympathetic nerve activity

Respiratory gating of muscle sympathetic nerve activity during inspiration and expiration for all subjects. * Significant difference between breath phases.

Figure 9. Cross-spectral baroreflex modulus

Average spontaneous baroreflex gain derived from cross-spectral analysis in subjects with significant squared coherence. * Significantly different from 0, 20 and 40 deg.