Abstract: A device for ventilating a patient by means of a lung ventilator which comprises a gas source and a controlled valve arrangement for intermittent feeding of breathing gas from the gas source. A connecting piece is mounted on an endotracheal tube or a tracheal cannula inserted into the trachea of the patient. Said piece comprises a substantially straight main tube and a branch tube and the junction between these tubes is so designed that it has a pneumatic valve function, whereby essentially the entire quantity of the breathing gas supplied from the lung ventilator to the branch tube is conducted towards said one end of the main tube, whereas gas flow, uneffected by breathing gas that may be blown into the branch tube, is allowed in both directions through the main tube past the orifice of the branch tube into the main tube.