If not for these responses, venous return to the heart would drop, cardiac output would fall, and cerebral perfusion to the reticular activating system would be inadequate to maintain consciousness. Prodromal symptoms occur once mean blood pressure falls below approximately 60 mm Hg. [21] The threshold for unconsciousness is a mean blood pressure below about 50 mm Hg at heart level, which corresponds to 30 mm Hg of cerebral arterial pressure. [21] Below that pressure, unconsciousness occurs usually within 7 seconds. [21]