Time of measurement influences the variability of tidal breathing parameters in healthy and sick infants

Abstract

The aim of this study was to investigate the influence of the time, when measuring tidal breathing parameters 1 min (epoch 1) and 5 min (epoch 2) after application of the facemask in healthy infants and infants with bronchopulmonary dysplasia (BPD), using the dead space free flow-through technique. In both patient groups, there were no statistically significant differences between epoch 1 and 2, in most of the tidal breathing parameters, except an increased V̇e and increased correlation dimension of the respiratory signal in the BPD infants in epoch 1. However, in nearly all parameters the coefficient of variation (CV) was significantly higher in epoch 1 compared with epoch 2, and in some infants, we found very high CVs (>50%) in epoch 1, which disappeared in epoch 2. The study shows that after having applied the facemask, a sufficient amount of adaptation time is necessary in order to reduce the within-subject variability and improve the reproducibility and interpretation of tidal breathing measurements in infants.

Introduction

Standardization of the measuring technique and conditions is a crucial issue for pulmonary function testing in newborns and an essential prerequisite to get accurate and reproducible results (Gaultier et al., 1995, Stocks et al., 1994). This is particularly evident in tidal breathing measurements in newborns, due to the high within-subject and inter-subject variability of the breathing pattern in this age. Main factors affecting the measurement of tidal breathing include technical properties of the devices, interaction between the equipment and the respiratory system, the time after application of the equipment, and the duration of the measurement (Bates et al., 2000).

Airflow is usually obtained using transducers connected to a mask. Although, their application is convenient, the disadvantage is that the measurements are affected by nerval irritation caused by the facemask (Dolfin et al., 1983, Fleming et al., 1982), airway resistance of the flowmeter and, above all, the dead space of the apparatus (Marsh et al., 1993). The latter clearly limits the duration of the measurement. The flow-through technique (FTT) was developed especially for measurements in newborns. This technique eliminates the apparatus dead space virtually by a background flow and enables long-term measurements (Foitzik et al., 1998, Schmalisch et al., 2001).

Despite the elimination of the apparatus dead space by FTT and minimization of the additional airway resistance, the stimulatory effects caused by the application of the facemask remain. This may influence the tidal breathing parameters and their variability, as well as the whole control of breathing. Thus, if stimulatory effects are present, the result of tidal breathing measurements will depend more or less on the time of the measurement. However, these effects could not be investigated in the past, due to the high apparatus dead space of the conventional technique, which did not allow long-term measurements in newborns.

Beside several tidal breathing parameters to describe infant breathing (Bates et al., 2000), lately, data-analysis techniques derived from dynamic system theory are used to investigate respiration. In recent studies, respiration was described as the output of a non-linear system showing chaotic behavior (Donaldson, 1992, Pilgram et al., 1995, Small et al., 1999). Such systems can be characterized by a quantity known as the correlation dimension (D2), which distinguishes them from systems that are purely stochastic. The usefulness of the application of the correlation dimension on respiratory signals has been shown in infants (Pilgram et al., 1995) and animals (Sammon and Bruce, 1991). It is hypothesized that the application of the facemask and associated sensory stimulation of the skin of the face result in functionally induced changes in D2.

Therefore, the aim of this study was to measure both conventional tidal breathing parameters, their within-subject variability, and the correlation dimension D2, immediately after application of the facemask, as well as after an adaptation time of 5 min. The measurements were performed in healthy infants and infants with bronchopulmonary dysplasia (BPD), representing one of the most important patient group for infant respiratory function testing.