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[degrees] passive head-up tilt (HUT), i.e., nonpresyncopal (23 subjects) and presyncopal (5 subjects). Airflow, C[O.sub.2], 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 (CB[F.sub.m] and CB[F.sub.p]) were estimated. To determine the influence of end-tidal C[O.sub.2] (ETC[O.sub.2]) on CBF, the relative C[O.sub.2] reactivity (%change in CB[F.sub.m] per mmHg change in ETC[O.sub.2]) was calculated. We found that in presyncopal subjects before symptoms during HUT, coherence between SBP and CB[F.sub.p] was higher (P = 0.02) and gains of transfer functions between BP (MBP and SBP) and CB[F.sub.m] were larger (MBP, P = 0.01; SBP, P = 0.01) in the respiratory frequency region. In the last 3 min before presyncope, presyncopals bad a reduced relative C[O.sub.2] reactivity (P = 0.005), likely a consequence of the larger decrease in ETC[O.sub.2]. We hypothesize that the C[O.sub.2]-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. cerebral autoregulation; respiratory frequency region