Objective Analyse the influence of breathing activity on cerebrovascular dynamics during presyncope.
Design Retrospective study
Setting University hospital
Patients Thirty-eight subjects developing neurocardiogenic syncope (syncope group), and 61 age-matched control subjects with negative tilt.
Interventions We measured middle cerebral artery mean blood flow velocity (MCFV), continuous noninvasive blood pressure (BP), end-tidal CO2 (CO2-et) and minute ventilation (VE) before and during 45' 60° tilting.
Main outcome measures Respiratory and cerebrovascular variability, cerebrovascular resistance, absolute (CVR) and corrected for CO2-et at 40 mmHg (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 CVR decrease in controls (baseline = 1.20±0.04; late-presyncope = 1.12±0.06 mmHg x s/cm, p<0.05), but not in syncope group (baseline = 1.09±0.04; late-presyncope = 1.09±0.06 mmHg x s/cm, p: ns). However, CVR-40 revealed similar reduction in both groups (controls: from 1.15±0.04 to 0.96±0.04 mmHg x s/cm; syncope group: from 1.01±0.04 to 0.83±0.04 mmHg x s/cm, p: ns). CVR-dyn of the two groups was also similar (p: ns). Respiratory variability increased in syncope group, from early tilt to late-presyncope (p<0.05 or better), preceding hyperventilation and being significantly correlated to an increase in MCFV and BP variability (p<0.01).
Conclusions During presyncope, the development of respiratory instability and hypocapnia impairs MCFV, thus facilitating the onset of syncope despite preserved cerebrovascular regulation.
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