There is provided herein methods, apparatus and systems for evaluating carbon dioxide (CO2) concentration in a subject's breath, for example in subjects ventilated with High Frequency Ventilation (HFV), the method includes inserting to a trachea of a subject an endotrachial tube (ETT), sampling breath from an area in the trachea located in proximity to a distal end of the endotrachial tube (ETT) and evaluating one or more CO2 related parameters of the sampled breath.
