hypoventilation

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Reduced or deficient ventilation of the lungs, resulting in reduced aeration of blood in the lungs and an increased level of carbon dioxide in the blood.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun medicine Respiratory depression, occurring when ventilation is inadequate to perform the necessary gas exchange, which may be caused by certain medical conditions, by holding one's breath, or by drug overdose.