Effect of apparatus dead space on breathing parameters in newborns: "flow-through" versus conventional techniques

Commercial devices for tidal breathing measurements in newborns allow only short-term measurements, due to the high apparatus dead space of the face mask and pneumotachometer. The flow-through technique (FTT) minimizes the dead space by a background flow, thereby allowing long-term measurements. The aim of this study was to investigate the comparability of tidal breathing parameters using both techniques. Paired measurements of tidal breathing were performed in 86 sleeping infants (median (range) body weight 2.8 kg (1.9-5.3 kg), age 65 days (3-150 days)), using the FTT and SensorMedics 2600 (SM 2600). There was a significant bias (p <0.001) in all tidal breathing parameters. Compared with the FTT, increases (95% confidence interval (CI)) in tidal volume (VT), respiratory frequency (fR), and minute ventilation (V'E) were 0.74 (0.5-1.0) mL.kg(-1), 9.0 (6.9-11.2).min(-1) and 92 (74-109) mL.min(-1).kg(-1) when measured with the SM 2600, representing average increases of 13, 17 and 30%, respectively, in response to the added dead space. By contrast, time to peak tidal expiratory flow as a proportion of expiratory time (tPTEF/tE) was changed by -0.09 (-0.11-0.08). The mean (95% CI) change in tPTEF/tE of -54 (-62-45)%, when measured in infants by the SM 2600, was remarkably similar to that observed during in vitro validation studies (-59 (-73-44)%), suggesting that the discrepancies in timing parameters may be largely attributable to differences in signal processing. In conclusion, differences in measurement technique and precision of the devices used can result in significant differences in tidal breathing parameters. This may impede the comparison of results within and between infants and the clinical interpretation of tidal breathing measurements in newborns.