A monitor that measures Apnea (Greek: απνοια, from α-, privative, πνεειν, to breathe) which is a term for suspension of external breathing. During apnea there is no movement of the muscles of respiration and the volume of the lungs initially remains unchanged. Depending on the patency (openness) of the airways there may or may not be a flow of gas between the lungs and the environment; gas exchange within the lungs and cellular respiration is not affected. Apnea can be voluntarily achieved (e.g., "holding one's breath"), drug-induced (e.g., opiate toxicity), mechanically induced (e.g., strangulation or choking), or it can occur as a consequence of neurological disease or trauma.