Abstract: A device and method for applying positive airway pressure to a patient includes an indirect calorimetry system for measuring a patient's resting energy expenditure (REE). The device includes a breathing circuit, one end of which includes a patient interface device such as a nasal face mask. A bi-directional pump is positioned within the breathing circuit. The bi-directional pump pumps gas in a first direction during the application of positive airway pressure. The bi-directional pump reverses direction when the device is in the indirect calorimetry mode. In this IC mode, patient's expired gases are diluted and passed through an IC module containing an oxygen sensor, a carbon dioxide sensor, and a flow rate sensor. The device includes a microprocessor for calculating the patient's REE based on the measured oxygen concentration, carbon dioxide concentration, and flow rate.