1. An apparatus for detecting the inhalation and exhalation phases of a respiratory cycle having associated respiratory gas flow, said apparatus comprising: signal production means for producing first and second signals representative of the respiratory gas flow with said signal production means further including means for delaying one of said signals in time relative to the other of said signals; and said signals having respective amplitudes so that one of said signals presents the greater amplitude during at least a portion of one of the phases and so that the other of said signals presents the greater amplitude during at least a portion of the other of said phases; and processing means for processing said signals for determining therefrom the occurrence of said respective phases and for producing outputs representative of said phases.