The Collaborative International Dictionary of English v.0.48:

iron lung \i"ron lung"\, n.
   A steel chamber, usually cylindrical, enclosing the entire
   body of a person except for the head, used to assist
   respiration for people suffering from disease, especially
   poliomyelitis. A reciprocating piston at the end causes
   alternating high and low pressure inside, which forces or
   assists the breathing of the patient within.
   [PJC]

WordNet (r) 3.0 (2006):

iron lung
    n 1: respirator that produces alternations in air pressure in a
         chamber surrounding a patient's chest to force air into and
         out of the lungs thus providing artificial respiration

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "iron lung":
   Aqua-Lung, artificial respiration, aspiration, asthmatic wheeze,
   breath, breath of air, breathing, broken wind, cough, exhalation,
   expiration, exsufflation, gasp, gulp, hack, hiccup, inhalation,
   inhalator, inspiration, insufflation, mouth-to-mouth resuscitation,
   oxygen mask, oxygen tent, pant, puff, respiration, resuscitator,
   scuba, sigh, sneeze, sniff, sniffle, snore, snoring, snuff,
   snuffle, sternutation, stertor, suspiration, wheeze, wind