Lower airway obstruction may result from Aspiration Bronchospasm Airspace filling disorders (eg, pneumonia, pulmonary edema, pulmonary hemorrhage) Drowning Decreased respiratory effort Decreased respiratory effort reflects central nervous system (CNS) impairment due to one of the following: Central nervous system disorder Adverse medication or illicit drug effect Metabolic disorder Central nervous system disorders that affect the brain stem (eg, stroke, infection, tumor) can cause hypoventilation.