Bispectral analysis as a tool to investigate dynamics of cardiorespiratory physiology

Introduction: Recent studies show that cardiorespiratory variables are nonlinearly coupled. In the present study, we tested the feasibility of using bispectral analysis to estimate changes in nonlinear phase coupling. Methods: We stimulated chemo- and baro-reflexes by switching inspired gas between air and air with 5% CO₂ added during supine and 70° head-up tilt. Bispectra were estimated using Fourier transform of the triple cumulant and were integrated between 0.04 and 0.3 Hz. Variables analyzed included cardiac RR intervals, systolic BP (SBP), tidal volume (VT), end tidal pressure of CO ₂ (PETCO₂) and mean cerebral blood flow velocity (CBFM). Data were collected in 30 healthy adults who were then grouped according to whether they become presyncopal (PS) during tilt or remained normal with no signs or symptoms of presyncope (NPS). Results: Six subjects (20%) developed presyncope. During tilt with room air breathing, phase coupling in RR, SBP, and between RR-SBP and RR-VT was lower in PS than in NPS, while phase coupling between CBFM-SBP was higher. In contrast, during tilt with 5% inhaled CO ₂, phase coupling between CBFM-PETCO₂ was lower for PS than for NPS. Discussion: Perturbations to baro- and chemo-reflexes led to changes in nonlinear phase coupling that were altered in presyncope. Although physiological interpretation of changes in phase coupling are less than clear at this time, our results show that such nonlinear indexes may provide a helpful perspective in understanding the complex phenomenon of orthostatically mediated syncope.