An experiment was performed in vitro to determine the effect of an airway-to-patient circuit on the breathing pattern actually delivered to the patient by a neonatal pulmonary ventilator. To this end, an automatic measuring system was implemented to record: (a) the ventilation parameter analog output signals provided by the ventilator and (b) the pressure value directly measured at the patient delivery site. The experimental analysis was conducted by examining various circuit configurations (tubes that differ in dimensions and rigidity, with and without humidifier, etc) and pressure waveform changes. Noticeable differences between the respiratory pattern set by physicians and that actually delivered to the patient were observed and the measured data provide indication for correct air circuit design. Finally, the results contributed to increase the attention of the medical therapist, who is the only one able to estimate the actual patient ventilatory needs and their ranges of variation, to a more accurate ventilator setting in order to assure the correct breathing pattern for neonate health.