EFFECT OF A BACKGROUND FLOW ON THE ACCURA CY OF VENTILATORY MEASUREMENTS IN NEONATES

Measurement of ventilation, in particular in preterm infants, is great ly impaired by equipment dead space, with its significant effect on th e ventilatory pattern and gas exchange. For patients of this age, ther efore, dead-spacefree methods are needed for long-term measurements. R ebreathing can be avoided if the pneumotachograph (PNT) and face mask are flushed with a continuous background flow. The effect of this on t he measurements has not yet been investigated in detail. A measuring s ystem comprising two identical baby PNTs (Jaeger/Germany) permitting a background flow of between 0 and 7 1 per min was used. Spontaneous br eathing was simulated with a 100 ml calibration syringe employing volu mes 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 85 0 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 analysi s. We were unable to find any significant relationship between the acc uracy of volume measurement and tidal volume for any of the measured v olumes. An increase in background flow resulted in an underestimation of the volume with an error <3 %. We found no influence of the backgro und flow or type of face mask on the frequency response. However, the length of the connecting tubing of the PNT has a major influence on th e frequency response (response frequency 68 Hz for 0.5 m, 48 Hz for 1 m, and 31 Hz for 1.7 m). These investigations show that the effect of background flow on the accuracy of ventilatory measurements is negligi ble, provided that two identical PNTs with a wide range of linearity a re used. The length of connecting tubes should be kept as short as pos sible.