An incompetent inspiratory valve will allow exhaled CO2- containing gas to enter the inspiratory limb of the circuit during expiration. During the next inspiration, the CO2-containing gas in the inspiratory limb enters the patient, extending the expiratory alveolar plateau (phase III) of the time capnogram (Fig. above). A decrease in CO2 follows the extended alveolar plateau and represents the appearance of the CO2-free gas from the machine end of the inspiratory limb. During this latter phase of inhalation, CO2 concentration may reach zero. The capnogram thus created may be indistinguishable from the normal capnogram, at least during the initial phase of rebreathing. The time capnograph is unable to reveal rebreathing because it is unaware of the beginning of actual inspiration.1

References:

Kumar et al. Canadian Journal of Anaesthesia 1992;39:997-9.