An apparatus 1 for estimating the energy expenditure of a patient 2 comprises means 3 for receiving a set of measurements 6 from a ventilator 4, wherein the set of measurements 6 comprises at least one gas concentration measurement. The apparatus 1 further comprises means 7 for estimating the energy expenditure of the patient 2 based on the set of measurements including respiratory volume and frequency 6. It also includes means to correct for temperature and humidity.
