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Simultaneous cerebrovascular and cardiovascular responses during presyncope
R L Bondar1, M S Kassam, F Stein
1Centre for Advanced Technology Education, Ryerson Polytechnic University, Toronto, Ontario, Canada.
Stroke
|October 1, 1995
Summary
During presyncope, cerebral blood flow decreases while blood pressure is maintained, indicating increased cerebral vascular tone. This suggests a downward shift in the cerebral autoregulation curve in healthy individuals.
Area of Science:
- Cardiovascular Physiology
- Neuroscience
- Cerebrovascular Regulation
Background:
- Presyncope involves symptoms preceding syncope, with cerebral autoregulation plasticity unclear.
- Systemic hemodynamic instability is implicated in syncope, but classic models don't fully explain observed cerebral responses.
- Previous studies using lower body negative pressure (LBNP) showed falling mean flow velocities (MFVs) with stable mean arterial blood pressure (MABP).
Purpose of the Study:
- To investigate the cerebrovascular and cardiovascular responses during presyncope induced by LBNP in healthy volunteers.
- To evaluate the behavior of cerebral autoregulation under simulated central hypovolemia.
- To clarify the role of cerebral vascular tone in the transition to presyncope.
Main Methods:
- Healthy volunteers (4 male, 5 female) were subjected to presyncopal LBNP.
- Cerebrovascular and cardiovascular responses were monitored using continuous transcranial Doppler sonography for MFV.
- Mean arterial blood pressure (MABP) and heart rate were continuously recorded.
Main Results:
- At presyncope, MFV decreased by 27.3% ± 14% from baseline (P < .05).
- MABP increased by 2.0% ± 27% above baseline.
- Estimated cerebrovascular resistance increased, with MFV decreasing before MABP changes.
Conclusions:
- Increased cerebrovascular resistance, falling MFV, and stable MABP indicate heightened cerebral vascular tone and a downward shift in the cerebral autoregulation curve.
- Cerebral vessels may exhibit differential sensitivity to sympathetic drive.
- Further research on dynamic MABP changes during LBNP is needed to assess cerebral autoregulation plasticity.