Critical limits of pressure-flow relation in the human brain

Area of Science:

• Neuroscience
• Cardiovascular Physiology
• Clinical Medicine

Background:

• Understanding the physiological determinants of consciousness is crucial for diagnosing and managing conditions like syncope.
• Previous research has not clearly defined the minimal cerebral blood flow velocity required to maintain consciousness.

Purpose of the Study:

• To determine the minimal mean flow velocity necessary to preserve human consciousness.
• To investigate the pressure-flow relationship critical for maintaining consciousness during orthostatic stress.

Main Methods:

• Eighty patients with a history of unexplained syncope underwent passive upright tilt provocation.
• Cerebral blood flow velocity, blood pressure, and heart rate were continuously monitored noninvasively using transcranial Doppler.
• Patients were classified as asymptomatic or symptomatic based on their response to tilt.

Main Results:

• Symptomatic patients experienced a significant drop in mean flow velocity (58%) and blood pressure (systolic 37%, diastolic 31%).
• Asymptomatic patients showed a smaller drop in mean flow velocity (23%) with no significant blood pressure changes.
• In 80% of symptomatic patients, the critical lower limit for mean flow velocity was 50% below baseline supine values.
• Impaired cerebral autoregulation was indicated by different pressure-flow regression slopes in symptomatic versus asymptomatic groups.

Conclusions:

• A 50% reduction in baseline supine mean flow velocity represents the critical threshold for cerebral perfusion leading to syncope.
• Cerebral autoregulation is impaired in patients who experience syncope compared to those who remain asymptomatic.