An automatic cycling machine used to assist or control alveolar respiration that exerts a negative pressure on the external surface of the chest wall, expanding the chest and moving air into the lungs. The two versions of the device are: 1) a rigid container enclosing all of a patient's body below the neck (i.e., an iron lung); or 2) a wearable shell (a cuirass) where only the thoracoabdominal region is enclosed. In both variants, the pressure inside the container is reduced relative to the atmospheric pressure at the patient's mouth and/or nose.