Relationship between maldistribution of ventilation and airways obstruction in children with asthma

Peripheral airway involvement in CF and asthma compared by inert gas washout

Multiple-breath N2 washouts were performed before and after bronchodilation in 15 patients with moderately severe asthma (mean age 13 years), and in 11 patients with cystic fibrosis (CF; mean age 16 years) matched for FEV1. Eighteen healthy subjects (mean age 15 years) were studied before bronchodilation. The lung clearance index (LCI) was determined from the washout curve and gas trapping was assessed from five subsequent large breaths. Analysis of the progression of the concentration normalized phase III slopes (Sn(III)) over the washout was used to determine inhomogeneity in the conducting airways (S(cond)) and inhomogeneity close to or within the gas exchange zone (S(acin)). Before bronchodilation all washout indexes were abnormal in the CF group, and all but S(acin) in the asthma group. Two indexes were higher in CF than in asthma: LCI (11.5 (3.3) vs. 8.7 (1.3); P < 0.01), and S(acin) (0.307 (0.207) vs. 0.142 (0.071); P < 0.01), while gas trapping indexes and S(cond) (0.151 (0.071) vs. 0.127 (0.041)) did not differ significantly. After bronchodilation, all indexes improved in asthma and only S(cond) remained abnormally elevated. In CF, only a modest but statistically significant S(acin) improvement was seen and all indexes stayed abnormal. The study shows that overall ventilation inhomogeneity and particularly inhomogeneity in or close to the gas exchange zone are more pronounced in CF than in asthma matched for FEV1, while inhomogeneity in the conducting airway zone is similar. After bronchodilation, residual abnormalities of airway function are seen all through the airway tree in CF but only in the conducting airways in asthma.

Gas mixing efficiency from birth to adulthood measured by multiple-breath washout

Efficient mixing of inspired gas with the resident gas of the lung is an essential requirement of effective respiration. This review focuses on one method for quantifying ventilation inhomogeneity: the multiple-breath inert gas washout (MBW). MBW has been employed as a research tool in adults and school age children for more than 50 years. Modifications allowing data collection in infants and preschoolers have been described recently. Indices of overall ventilation inhomogeneity, such as the lung clearance index and moment ratios, are raised in many infants with lung disease of prematurity, and in young children with cystic fibrosis. These indices may be more sensitive than other lung function measures for the early detection of airway disease. We describe, for the first time, a development of the MBW analysis that allows calculation of acinar and conductive zone inhomogeneity indices in spontaneously breathing children. Although methodological and analytical issues remain, the future clinical and research applications of MBW justify accelerated research in this field.

Moment analysis of multibreath nitrogen washout in healthy preterm infants

Moment analysis (MA) of multibreath nitrogen washout (MBNW) has not previously been applied to newborn infants. The aim of the present study was to adapt this method to healthy preterm infants using an improved technique suitable for small infants, and to determine reference values of MA. Twenty healthy preterm infants with a mean birth weight (+/-SD) of 1,666+/-402 g and a mean gestational age of 31.3+/-2.1 weeks were studied during their first 7-28 days of life. Computerized bedside equipment with very low dead space was constructed. The limits of normal variability were determined from results of duplicate studies. Outcome variables included functional residual capacity (FRC), the first-to-zeroth moment ratio (M1/M0), the second-to-zeroth moment ratio (M2/M0), and the lung clearance index (LCI). The starting point for MA had considerable impact on the results. Mean M1/M0 and M2/M0 were 2.18+/-0.18 and 8.71+/-1.24, respectively. No significant relation between moment ratios and weight, gender or age was found. Intrasubject coefficients of variation (CV) for M1/M0 (7.9+/-5.9%) and for M2/M0 (12.1+/-9.1%) and intersubject CV for M1/M0 (8%) and M2/M0 (14%) were similar to those reported in children and adults. Mean lung clearance index was 10.8+/-1.4 and intra- and intersubject CVs were 11.3+/-8.9% and 13%, respectively. Mean functional residual capacity (FRC) was 22.5+/-2.1 ml/kg. Mean intra- and intersubject CVs for FRC were 8.3% and 9%, respectively. We conclude that the MBNW test can be performed by a simple bedside method and that MA appears to be a suitable method for measuring gas mixing in preterm infants.

Moment analysis of a multibreath nitrogen washout based on an alveolar gas dilution number

A common method for analyzing a multibreath nitrogen washout (MBNW) is to perform moment analysis and derive the mean dilution number (MDN). A homogeneously mixed alveolar space with zero series dead space (VD = 0) will always result in a MDN = 1, regardless of breathing pattern. A higher MDN implies more inhomogeneity. But, if VD greater than 0, the MDN can become sensitive (artificially high) to VD/VT ratios. We present an alveolar-based mean dilution number (AMDN) that uses the cumulative expired alveolar volume. Unlike the MDN, the AMDN for a homogeneously mixed alveolar space is unity, regardless of VD or VT, and hence should be a more appropriate index of inhomogeneity at the alveolar level. Two sets of experiments were used to compare the AMDN with the MDN. First, a MBNW was performed by five healthy subjects at spontaneous VD/VT and at a low VD/VT achieved by a controlled increase in VT. Here, the MDN decreased from 1.98 +/- 0.1 to 1.79 +/- 0.06, whereas the AMDN was essentially unchanged (1.42 +/- 0.04 to 1.38 +/- 0.06). Second, MBNW values from seven healthy subjects, five with cystic fibrosis, and 10 asthmatic subjects (before and after bronchodilation) were analyzed. Compared with the MDN, the AMDN showed a significantly wider separation between clinical groups. Also, the AMDN demonstrated an increased variability within both sick groups versus a decrease in the healthy group. We conclude that the AMDN is superior to the MDN because of its decreased sensitivity to breathing pattern but increased sensitivity to degree of disease.