Headache, confusion, lethargy, seizure, coma
Extreme<unk>  (acute elevation >100) can result in cardiovascular collapse
Hypercapnia
Depressed central respiratory drive
Metabolic alkalosis
Structural CNS disease: brainstem lesions
Hypoxemia
Drug depression of respiratory center: opioids, sedatives, anesthetics
Metabolic acidosis
Endogenous toxins: tetanus