1. Apparatus for monitoring a subject to detect the occurrence of apnea, comprising: a plurality of electrodes to be applied to spaced points on the subject for detecting the respiration rate and respiration amplitude of the subject by the change in electrical impedance between said spaced points; computing means including an averaging circuit averaging the detected respiration rate and respiration amplitude over a predetermined time interval covering a plurality of breaths of the subject and computing therefrom the average respiration volume of the subject; storage means for storing, as a reference average volume, the average respiration volume of the subject during an initial time interval, and also for storing, as current average volumes, the average respiration volume of the subject during subsequent time intervals; comparing means for comparing each of said current average respiration volumes with said reference average respiration volume; and signalling means acuated to indicate the occurrence of apnea whenever a current average respiration volume falls below said reference average respiration volume by a predetermined percentage.