Systems and methods are provided for decreasing intracranial pressure and enhancing circulation in a breathing person, as well as for increasing the respiratory rate and encouraging spontaneous respiration. According to such methods, a valve system is interfaced to a person's airway. The valve system has a threshold valve and an airway member that interfaces with the person's airway. The threshold valve is configured to prevent respiratory gas flow to the person's lungs during a portion of an inhalation event until the inspiration equals or exceeds an opening pressure of the threshold valve. Also, a small level of external vacuum is constantly supplied at a juncture between the threshold valve and the airway member.