Influence of respiratory instability during neurocardiogenic presyncope on cerebrovascular and cardiovascular dynamics

Objective: To analyse the influence of breathing activity on cerebrovascular dynamics during presyncope. Design: Retrospective study. Setting: University hospital. Patients: 38 subjects developing neurocardiogenic syncope (syncope group), and 61 age-matched control subjects with negative tilt. Interventions: Middle cerebral artery mean blood flow velocity (MCFV), continuous non-invasive blood pressure (BP), end-tidal CO₂ (CO ₂-et) and minute ventilation were measured before and during 45' 60° tilting. Main outcome measures: Respiratory and cerebrovascular variability, cerebrovascular resistance (CVR) - absolute and corrected for CO₂-et at 40 mm Hg (CVR-40) - and dynamic cerebrovascular regulation (CVR-dyn: transfer function phase analysis between MCFV and BP), obtained during supine rest (baseline), first 5 minutes of tilt (early tilt), early- and late presyncope (first and second half, respectively, of 4 minutes preceding syncope in syncope group, and equivalent time in controls). Results: Tilting induced a mean (SE) CVR decrease in controls (baseline 1.20 (0.04); late presyncope 1.12 (0.06) mm Hg x s/cm, p