Dynamic cardiorespiratory interaction during head-up tilt-mediated presyncope

In 28 healthy adults, we compared the dynamic interaction between respiration and cerebral autoregulation in 2 groups of subjects: those who did and did not develop presyncopal symptoms during 70° passive head-up tilt (HUT), i.e., nonpresyncopal (23 subjects) and presyncopal (5 subjects). Airflow, CO₂, cerebral blood flow velocity (CBF), ECG, and blood pressure (BP) were recorded. To determine whether influences of mean BP (MBP) and systolic SP (SBP) on CBF were altered in presyncopal subjects, coherencies and transfer functions between these variables and mean and peak CBF (CBF _m, and CBF_p) were estimated. To determine the influence of end-tidal CO₂ (ETCO₂) on CBF, the relative CO₂ reactivity (%change in CBF_m per mmHg change in ETCO₂) was calculated. We found that in presyncopal subjects before symptoms during HUT, coherence between SBP and CBF_p was higher (P = 0.02) and gains of transfer functions between BP (MBP and SBP) and CBF_m, were larger (MBP, P = 0.01; SBP, P = 0.01) in the respiratory frequency region. In the last 3 min before presyncope, presyncopals had a reduced relative CO₂ reactivity (P = 0.005), likely a consequence of the larger decrease in ETCO ₂. We hypothesize that the CO₂-mediated increase in resistance attenuates autoregulation such that the relationship between systemic and cerebral hemodynamics is enhanced. Our results suggest that an altered cardiorespiratory interaction involving cerebral hemodynamics may contribute in the cascade of events during tilt that culminate in unexplained syncope.