APNOEA or APNEA, is a suspension of breathing. A person may voluntarily hold his breath for a limited length of time, as in swimming under water. Respiration stops during swallowing, by a protective reflex which prevents food from entering the wind pipe. Other stimuli, such as severe pain, may cause a reflex in hibition of respiration. True apnoea, however, as defined by physiologists, is a standstill of respiration brought about by over ventilation of the lungs. The carbon dioxide content of the blood thereby becomes lower than usual and there is no impulse to breathe until it returns to normal. In certain diseases respiration may be irregular, intervals of apnoea alternating with intervals of over-breathing.