A modified Pitot tube for the accurate measurement of tidal volume in children

A device using modification of a Pitot tube has been designed for measurement of tidal volume in infants and small children. Its accuracy was compared both n vitro and n vivo to that of a calibrated pneumotachograph (Fleish #1) designed for a similar flow range. In vitro measurement of air flow with the modified Pitot tube (MPT) was within 5% of the pneumotachograph readings over a range of 1-60 l/min. Similar accuracy was found with measurement of tidal volumes from 20 ml to 1 l, delivered by a calibrated volume-cycled ventilator using a variety of inspiratory flow rates. Tidal volume measurements with the MPT were compared to the pneumotachograph using helium, oxygen, carbon dioxide, and a range of nitrous oxide/oxygen mixtures. A manual control was incorporated into the MPT electronics to allow direct measurements of tidal volume with different nitrous oxide/oxygen concentrations. In vivo, the insertion of the MPT into the patient circuit caused no apparent changes in ventilatory parameters in children under 20 kg. Measurement of tidal volumes with the MPT agreed to within 8% of pneumotachograph readings. The low dead space (1.5 cc) and light weight (12 gm) of the MPT confer advantages over the pneumotachograph (15 ml dead space and a weight of 90 gm) for routine use in pediatric anesthesia.