An apparatus and method for affording a re-breathing capability to a breathing circuit. The apparatus includes a valve that permits air flow through or re-breathing in the breathing circuit. A CO2 sensor and an air flow sensor detects CO2 a rate of air flow in the breathing circuit, respectively. A processor computes a parameter based on the CO2 waveform feature and a volumetric CO2 waveform feature based on signals from the air flow sensor and/or the CO2 sensor. An alarm is triggered when the parameter deviates from a selected magnitude. In a further embodiment, at least one parameter associated with an energy source for actuating the valve is monitored. Deviation of the value from at least one selected value of the at least one parameter is corrected.