Figure 14. Low- and respiratory-frequency phase angles derived from cross spectral analysis of systolic pressures and R–R intervals during passive upright tilt (upper panels) and spontaneous breathing or mechanical ventilation (lower panels).

Nearly constant low frequency phase angles are consistent with baroreflex physiology. Conversely, highly variable respiratory-frequency phase angles during tilt and mechanical breathing point toward respiratory influences, and away from baroreflex mechanisms. Adapted from Koh et al. (1998) and Cooke et al. (1999).