The Measurement of Tidal Volumes in Spontaneously Breathing Children during General Anaesthesia using a Haloscale Infant Wright Respirometer

Abstract

The accuracy of tidal volume measurements made with a Wright Haloscale infant respirometer in children breathing spontaneously during general anaesthesia was assessed by a bench test. The tidal volumes and peak flow rates of 20 spontaneously breathing, anaesthetised children were measured with a pneumotachograph before and during surgery, and similar volumes, at the same flow rates, were delivered by a calibrated syringe simultaneously to the respirometer and a pneumotachograph. The results reveal that the mean (+/- s.d.) peak gas flow rates of children aged 6 years and less, 7.5 (+/- 1.6) and 9.3 (+/- 0.1) l/min before surgery and during surgery respectively, are significantly less than the peak flow rates, 11.3 (+/- 1.0) and 11.9 (+/- 1.5) before and during surgery, respectively, of children aged more than 6 years; and that the respirometer underestimates tidal volume by 10% when the peak flow rate is 11 l/min, and the percentage error in tidal volume estimation by the respirometer increases as the peak gas flow declines below 10 l/min.

The Haloscale “Infanta” Wright Respirometer

“…The capability of any turbine type of instrument to measure tidal volumes accurately cannot be deduced from the continuous flow calibration without knowledge of the wave-form for any particular tidal volume. This explains the dis-crepancies between Infanta and pneumotachograph volumes put forward by Meeke and coworkers [16] when a small range of volumes was delivered from a calibrated syringe at different flow rates. These authors completely ignored the influence of wave-form, although Bushman [17] clearly demonstrated that the relationship between adult Wright respirometer readings and wave-form was far more complex than could be predicted from measurement of peak flow.…”