[Effect of background flow on the accuracy of respiratory flow and respiratory volume measurement in newborn infants].

Measurement of ventilation, in particular in preterm infants, is greatly impaired by equipment dead space, with its significant effect on the ventilatory pattern and gas exchange. For patients of this age, therefore, dead-spacefree methods are needed for long-term measurements. Rebreathing can be avoided if the pneumotachograph (PNT) and face mask are flushed with a continuous background flow. The effect of this on the measurements has not yet been investigated in detail. A measuring system comprising two identical baby PNTs (Jaeger/Germany) permitting a background flow of between 0 and 7 l per min was used. Spontaneous breathing was simulated with a 100 ml calibration syringe employing volumes of 20, 40, 60 and 100 ml (Rudolph/USA) and a frequency of 30 min-1. The measurements were carried out with a T-piece from a respirator circuit, a hand mask (50 ml) and a face chamber having a volume of 850 ml (Siemens-Elema/Sweden). To investigate the dynamic properties of the equipment, we employed flow jumps generated with a magnetic valve (response time < 2 ms) and analysed the responses using Fourier analysis. We were unable to find any significant relationship between the accuracy of volume measurement and tidal volume for any of the measured volumes. An increase in background flow resulted in an underestimation of the volume with an error < 3%. We found no influence of the background flow or type of face mask on the frequency response.(ABSTRACT TRUNCATED AT 250 WORDS)