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

¹ Dept of Neonatology (Charité), Humboldt University, Berlin, Germany, ² Portex Anaesthesia, Intensive Therapy and Respiratory Medicine Unit, Institute of Child Health, London, UK

CORRESPONDENCE: G. Schmalisch, Dept of Neonatology (Charité), Humboldt University Berlin, Schumannstraße 20/21, D-10098, Berlin, Germany. Fax: 49 3028025824

This study was supported by the German Ministry for Research and Technology, project "Perinatal Lung" (grant 01 ZZ 9511) and Deutsche Forschungsgemeinschaft (Schm 1160/1-2).

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 (V_T), respiratory frequency (f_R), 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 (t_PTEF/t_E) was changed by –0.09 (–0.11––0.08). The mean (95% CI) change in t_PTEF/t_E 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.

Keywords: apparatus dead space, flow-through technique, infants, pneumotachography, tidal breathing, ventilation

This article has been cited by other articles:

				G. Schmalisch, B. Foitzik, R.R. Wauer, and J. Stocks In vitro assessment of equipment and software to assess tidal breathing parameters in infants Eur. Respir. J., January 1, 2001; 17(1): 100 - 107. [Abstract] [Full Text] [PDF]