Abstract: A ventilatory system for ventilating a patient includes a gas administration module for administering a predetermined amount of an additional gas, such as NO, to the patient, the gas administration module being provided with a predetermined number of parallel valve units each provided with a throttle valve and a controlled valve in series, the internal volume between the throttle valve and the controlled valve being so small that no interfering effects occur due to opening and closing of the controlled valve with a frequency below a maximum frequency. Preferably, the system is also provided with a tracheal gas insufflation (TGI) module and a measurement system for measuring the concentration of the additional gas.