If the pCO 2 is allowed to rise high enough, mental status changes will ensue. Acute elevations of pCO 2 to more than 60 mmHg cause confusion and headache. When CO 2 exceeds 70 mmHg, loss of consciousness and seizures can occur. However, hypercapnic neurologic changes are reversible. The effect of hypercapnia on the autonomic nervous system is one of the most important.
Hypercapnia is associated with elevated plasma levels of epinephrine and norepinephrine.
In the critical care setting, some clinicians advocate reducing ventilation volumes to prevent lung injury and allowing the patient's pCO 2 to rise to 60-70 mmHg. A degree of respiratory acidosis may be acceptable as long as adequate oxygenation is maintained. Permissive hypercapnia is contraindicated in patients with brain injury or renal disease who could be harmed by the rising pCO 2 . Therapeutic uses for inhaled carbon dioxide pCO 2 can be maintained above 40 mmHg by inhaling CO 2 gas. This proves useful when the goals of therapy are to decrease pulmonary blood flow.