A negative pressure ventilator, often referred to colloquially as an iron lung, is a form of medical ventilator that enables a person to breathe when normal muscle control has been lost or the work of breathing exceeds the persons ability. Examples of the device include the Drinker respirator and the Both respirator. The negative form of pressure ventilation has been almost entirely superseded by positive pressure ventilation or biphasic cuirass ventilation.
Humans, like most other animals, breathe by negative pressure breathing: the rib cage expands and the diaphragm contracts, expanding the chest cavity. This causes the pressure in the chest cavity to decrease, and the lungs expand to fill the space. This, in turn, causes the pressure of the air inside the lungs to decrease (it becomes negative, relative to the atmosphere), and air flows into the lungs from the atmosphere: inhalation. When the diaphragm relaxes, the reverse happens and the person exhales. If a person loses part or all of the ability to control the muscles involved, breathing becomes difficult or impossible.