Measurement of tracheal airflow in newborns by a differential flow system

The differential arrangement of two pneumotachographs in a gas blow-by system, upstream and downstream from the patient connection port, permits accurate measurement of the tracheal air flow without introduction of flowmeter dead-space. The method does not require but will accept a constant background gas flow rate and permits humidification and easy switching of the inhalant gases. The admissible background flow rates limiting measurement errors below 5% in the range of tidal volumes relevant for newborns for two sizes of commercially available pneumotachographs were found to be -3 L/min and 0-12 L/min, respectively. The lack of restrictions imposed on the properties of the background flow source makes the differential system especially suitable for measurement of ventilation and pulmonary mechanics in spontaneously breathing or mechanically ventilated newborn infants.