Association of prematurity, lung disease and body size with lung volume and ventilation inhomogeneity in unsedated neonates: a multicentre study

BACKGROUND

Previous studies have suggested that preterm birth with or without subsequent chronic lung disease is associated with reduced functional residual capacity (FRC) and increased ventilation inhomogeneity in the neonatal period. We aimed to establish whether such findings are associated with the degree of prematurity, neonatal respiratory illness and disproportionate somatic growth.

METHODS

Multiple breath washout measurements using an ultrasonic flowmeter were obtained from 219 infants on 306 test occasions during the first few months of life, at three neonatal units in the UK and Australia. Tests were performed during unsedated sleep in clinically stable infants (assigned to four exclusive diagnostic categories: term controls, preterm controls, respiratory distress syndrome and chronic lung disease). The determinants of neonatal lung function were assessed using multivariable, multilevel modelling.

RESULTS

After adjustment for age and body proportions, the factors gestation, intrauterine growth restriction and days of supplemental oxygen were all significantly associated with a reduced FRC. In contrast, increased ventilation inhomogeneity (elevated lung clearance index) was only significantly associated with duration of supplemental oxygen. After adjusting for continuous variables, diagnostic category made no further contribution to the models. Despite using identical techniques, unexpected inter-centre differences occurred, associated with the equipment used; these did not alter the negative association of preterm delivery and disease severity with lung function outcomes.

CONCLUSION

Reduction in FRC is independently associated with prematurity, intrauterine growth restriction and severity of neonatal lung disease. Determinants of lung function shortly after birth are highly complex in different disease groups.

Lung function and clinical risk factors for asthma in infants and young children with recurrent wheeze

BACKGROUND

Although several risk factors for asthma have been identified in infants and young children with recurrent wheeze, the relevance of assessing lung function in this group remains unclear. Whether lung function is reduced during the first 2 years in recurrently wheezy children, with and without clinical risk factors for developing subsequent asthma (ie, parental asthma, personal history of allergic rhinitis, wheezing without colds and/or eosinophil level >4%) compared with healthy controls was assessed in this study.

METHODS

Forced expiratory flows and volumes in steroid naïve young children with >or=3 episodes of physician confirmed wheeze and healthy controls, aged 8-20 months, were measured using the tidal and raised volume rapid thoracoabdominal compression manoeuvres.

RESULTS

Technically acceptable results were obtained in 50 wheezy children and 30 controls using tidal rapid thoracoabdominal compression, and 44 wheezy children and 29 controls with the raised volume technique. After adjustment for sex, age, body length at test and maternal smoking, significant reductions in z scores for forced expiratory volume at 0.5 s (mean difference (95% CI) -1.0 (-1.5 to -0.5)), forced expired flow after 75% forced vital capacity (FVC) has been exhaled (FEF(25)) (-0.6 (-1.0 to -0.2)) and average forced expired flow over the mid 50% of FVC (FEF(25-75)) (-0.8 (-1.2 to -0.4)) were observed in those with recurrent wheeze compared with controls. Wheezy children with risk factors for asthma (n = 15) had significantly lower z scores for FVC (-0.7 (-1.4 to -0.04)) and FEF(25-75) (-0.6 (-1.2 to -0.1)) than those without such risk factors (n = 29).

CONCLUSIONS

Compared with healthy controls, airway function is reduced in young children with recurrent wheeze, particularly those at risk for subsequent asthma. These findings provide further evidence for associations between clinical risk factors and impaired respiratory function in early life.

Growth and lung function in Asian patients with cystic fibrosis

BACKGROUND

The incidence of cystic fibrosis (CF) in Asians is rare. How these patients fare in terms of morbidity and mortality in the UK compared to their non-Asian peers is not well documented.

AIMS

To retrospectively study annual reviews of 31 Asian CF patients from three London paediatric CF centres.

METHODS

Disease severity was assessed by lung function, age at first infection with Pseudomonas aeruginosa, and body mass index (BMI). The Asian children were compared with 143 matched non-Asian patients with CF. Matching criteria used were same sex and treatment centre as the Asian index patient. In addition, the controls were matched so that their date of birth, date of diagnosis, and date at annual review were within 12 months of the index patient.

RESULTS

There was no significant difference in age at diagnosis or age at annual review between the Asian and non-Asian children. Mean Z-scores for FEV1 and FVC were significantly lower for the Asian girls. There was no significant difference in Z-scores for BMI between the Asian children and their controls. Age at first isolation of Pseudomonas aeruginosa in Asian girls was significantly later than for their controls (8.3 years compared to 5.6 years for non-Asian girls).

CONCLUSIONS

While the Asian boys' lung function seems comparable with that of their non-Asian peers, the Asian girls emerge as a potentially vulnerable group and more work is required to discover why this is the case.

Effect of raised lung volume technique on subsequent measures of V'maxFRC in infants

Partial and "full" forced expiratory maneuvers are both used to assess airway function in infants. Despite the increasing use of the raised volume technique, there is little information regarding the influence of lung inflations as are necessary for the raised volume technique on other measurements of lung function in infants. The aim of this study was to assess whether application of the raised volume technique influences subsequent tidal measurements of maximal expired flow at functional residual capacity (V'maxFRC). Paired measurements of V'maxFRC were obtained in 29 healthy infants (aged 6-65 weeks) before and after raised volume maneuvers, wherein a lung inflation pressure of 3 kPa was used. When compared with measurements prior to raising lung volume, there was a highly significant (P < 0.001) decrease in V'maxFRC by 40 ml.sec(-1) when measurements were repeated (95% CI, -59, -20 ml.sec(-1)), equivalent to a reduction of 20% or -0.6 SD scores in flows. There was no significant change in selected tidal breathing parameters, 95% CI of differences between the two sets of measurements being -1.5, 1.2 bpm for respiratory rate; -0.5, 0.2 ml.kg(-1) for weight corrected tidal volume, and -0.04, 0.01 for tidal breathing ratio (tPTEF:tE). In conclusion, although the mechanism remains unclear, raised volume maneuvers may influence subsequent measures of lung function in infants. Further research is needed to clarify the potential mechanisms. In the meantime, the potential impact of the order of lung function tests within any given study protocol should be considered carefully.

Low birth weight for gestation and airway function in infancy: exploring the fetal origins hypothesis

BACKGROUND

Poor fetal growth has been associated with impaired airway function in adult life, but evidence linking birth weight and airway function in early childhood is sparse. We examined the hypothesis that low birth weight for gestation is associated with impaired airway function shortly after birth and that this is independent of impaired postnatal somatic growth.

METHODS

Airway function was measured using the raised volume technique in healthy white infants of low (< or =10th centile) or appropriate (> or =20th centile) birth weight for gestation and was expressed as forced expiratory volume in 0.4 s (FEV0.4), forced vital capacity (FVC), and the maximal expired flow at 25% of forced vital capacity (MEF25). Infant length and weight, maternal height and weight, maternal report of smoking prenatally and postnatally, and parental occupation were recorded.

RESULTS

Mothers of low birth weight for gestation infants (n=98) were lighter, shorter, and more likely to smoke and have partners in manual occupations. At 6 weeks their infants remained lighter and shorter than those of appropriate birth weight (n=136). FEV0.4, FVC, and MEF25 were reduced in infants of low birth weight for gestation, in those whose mothers smoked in pregnancy, or who were in manual occupations. After adjusting for relevant maternal and infant characteristics, infants in the low birth weight for gestation group experienced a mean reduction of 11 ml in FEV0.4 (95% CI 4 to 18; p=0.002), of 12 ml in FVC (95% CI 4 to 19; p=0.004), and of 28 ml/s in MEF25 (95% CI 7 to 48; p=0.03).

CONCLUSIONS

Airway function is diminished in early postnatal life as a consequence of a complex causal pathway which includes social disadvantage as indicated by maternal social class, smoking and height, birth weight as a proximal and related consequence of these factors, and genetic predisposition to asthma. Further work is needed to establish the relevance of these findings to subsequent airway growth and development in later infancy and early childhood.

Assessment of tidal breathing parameters in infants with cystic fibrosis

Simple methods are needed to assess lung function in infants with cystic fibrosis (CF). This study determined the relationship between simple measurements obtained from tidal breathing with those from more complicated forced expiratory manoeuvres. Healthy infants and infants with CF were recruited from two maternity units and five specialist CF hospitals, respectively. Respiratory rate, tidal volume, minute ventilation and the tidal breathing ratio (TPTEF:TE) were measured in sedated infants and compared with forced expiratory volume in 0.4 seconds (FEV0.4) measured by the raised volume technique. Altogether, 95 healthy infants and 47 infants with CF of similar age, sex, ethnicity and proportion exposed to maternal smoking were recruited. There was no difference in TPTEF:TE and tidal volume between healthy infants and those with CF. Minute ventilation was significantly greater in infants with CF due to a mean (95% confidence interval) increase in respiratory rate of 5.8 (3.2-8.4) min(-1). Thirteen (28%) infants with CF had a respiratory rate elevated by >2 SD. However, no association between respiratory rate and FEV0.4 could be identified. Tidal breathing ratio was not useful in identifying diminished airway function in infants with cystic fibrosis. An elevated respiratory rate may be due in part to ventilation heterogeneity but is poorly predictive of diminished airway function measured by forced expiration.

Exploring the relationship between forced maximal flow at functional residual capacity and parameters of forced expiration from raised lung volume in healthy infants

The raised volume rapid thoraco-abdominal compression technique (RVRTC) is being increasingly used to assess airway function in infants, but as yet no consensus exists regarding the equipment, methods, or analysis of recorded data. The aim of this study was to explore the relationship between maximal flow at functional residual capacity (V'(maxFRC)) and parameters derived from raised lung volumes, and to address analytical aspects of the latter technique in an attempt to assist with future standardization initiatives. Forced vital capacity (FVC) from lung volume raised to 3 kPa, timed forced expiratory volumes (FEV(t)), and forced expiratory flow parameters at different percentages of expired FVC (FEF(%)) were measured in 98 healthy infants (1-69 weeks of age). V'(maxFRC) using the tidal rapid thoraco-abdominal compression (RTC) technique was also measured. The within-subject relationships and within-subject variability of the various parameters were assessed. Duration of forced expiration was < 0.5 sec in 5 infants, meaning that FEV(0.3) and FEV(0.4) were the only timed volume parameters that could be calculated in all infants during the first months of life, and even when it could be calculated, FEV(0.5) approached FVC in many of these infants. It is recommended that FEV(0.4) be routinely reported in infants less than 3 months of age. Contrary to previous reports, within subject variability of V'(maxFRC) was less than that of FEF(75) (mean CV = 6.3% and 8.9%, respectively).A more standardized protocol when analyzing data from the RVRTC would facilitate comparisons of results between centers in the future.

The association between birthweight, sex, and airway function in infants of nonsmoking mothers

The risk of respiratory illness and death is increased in infants of low birthweight for gestational age, but the underlying physiologic mechanisms remain unclear. We examined the hypothesis that airway function is diminished in infants of low birthweight for gestational age, independent of exposure to maternal smoking. Respiratory function was measured using partial and raised volume forced expiratory maneuvers in 103 infants (> 35 wk gestation; 56 boys) not exposed pre- or postnatally to maternal smoking who, according to birthweight, were either small (SGA; n = 38) or appropriate (AGA; n = 65) for gestational age. At testing, SGA infants were of similar postnatal age (mean [SD]: SGA 6.8 [2.4] wk, AGA 5.9 [2.3] wk), but remained shorter and lighter than AGA infants. In univariate analyses, FVC, forced expired volume in 0.4 s (FEV(0.4)), and FEF(75) were significantly diminished in SGA compared with AGA infants (mean [95% CI of difference]: FVC: 127 versus 143 ml [-29, -2]; FEV(0.4): 112 versus 125 ml [-24, -2]; and FEF(75): 173 versus 203 ml s(-1) [-57, -3], respectively), but these differences were no longer significant after allowing for sex and body size. Furthermore, FEF(75) was on average 35 ml s(-1) lower in boys than girls (95% CI: -61, -8). We conclude that diminished airway function in SGA infants shortly after birth appears to be primarily mediated through impaired somatic growth.

Assessment of passive respiratory mechanics in infants: double versus single occlusion?

The single breath or occlusion technique (SOT) is widely used to assess passive respiratory mechanics in infants, but depends on various underlying assumptions. Recently, it has been proposed that such measurements could be internally validated by performing two brief airway occlusions during the same expiration. The aim of this study was to evaluate the use of the double occlusion technique (DOT) using a new commercially available program (Jaeger MasterScreen BabyBody Erich Jaeger GmbH, Würzburg, Germany). Paired measurements of respiratory system compliance (Crs) and resistance (Rrs) using both SOT and DOT were obtained in 18 healthy sedated infants (age range 4-41 weeks, weight 2.7-9.9 kg). There was close agreement between both methods of assessing Crs in all infants, the mean within-subject difference (95% confidence interval (CI)) for DOT-SOT being -0.06 (-0.55- +0.42) mL x kPa(-1) x kg(-1). By contrast, estimates of Rrs,DO were on average 20% lower than those for Rrs,SO, (mean within-subject difference (95% CI) being -0.67 (-1.04- -0.31) kPa x L(-1) x s; p<0.01). The relatively lower values obtained for Rrs,DO may reflect the higher mean lung volume at which it was calculated. Further work is required to investigate the clinical and epidemiological relevance of this new approach, and whether there are any advantages of using both techniques when assessing passive mechanics in infants.

Influence of jacket placement on respiratory compliance during raised lung volume measurements in infants

SUMMARY. Recent introduction of the raised lung volume rapid thoraco-abdominal compression (RVRTC) technique for measuring forced expiratory maneuvers in infants provides the potential opportunity to assess respiratory mechanics simultaneously by using multiple linear regression (MLR) of the relaxed breaths preceding jacket inflation to force expiration. This study was undertaken to investigate whether data obtained from raised lung volume are influenced by placement of the rapid thoraco-abdominal compression (RTC) squeeze jacket. Paired measurements of tidal volume (V(T)) and respiratory rate (RR) during tidal breathing, and of inflation volume (V(inf)), respiratory system compliance (C(rs)), and resistance (R(rs)) during passive lung inflations were made in 60 (30 male) healthy term infants with and without a fastened, but uninflated RTC jacket in place. Jacket placement was associated with a significant reduction (P < 0.0001) in weight-corrected V(inf) [-1.86 (95% confidence interval, -2.46, -1.27) mL.kg(-1)] and C(rs) [-0.77 (-1.04, -0.49) mL.kPa(-1).kg(-1)]. This represented a reduction in weight-corrected C(rs) from 9.00 to 8.24 mL.kPa(-1).kg(-1), with the fall being >10% in 42% of infants studied. There was no significant change in R(rs) or weight-corrected V(T). If passive respiratory mechanics are to be measured during raised lung volume maneuvers, they should be performed prior to the jacket being fastened, unless considerable care is taken with each infant to ensure that the jacket does not restrict chest wall movement during maximum inflation.

Analysis of forced expiratory maneuvers from raised lung volumes in preterm infants

During recent years it has been suggested that forced expiratory measurements, derived from a lung volume set by a standardized inflation pressure, are more reproducible than those attained during tidal breathing when the rapid thoracoabdominal compression technique is used in infants. The aim of this study was to evaluate the feasibility of obtaining measurements from raised lung volumes in unsedated preterm infants. Measurements were made in 18 infants (gestational age 26-35 wk, postnatal age 1-10 wk, test weight 1.4-3. 5 kg). Several inflations [1.5-2.5 kPa (15-25 cmH2O)] were used to briefly inhibit respiratory effort before the rapid thoracoabdominal compression was performed. Conventional analysis of flows and volumes at fixed times and percentages of the forced expiration resulted in a relatively high variability in this population. However, by using the elastic equilibrium point (i.e., the passively determined lung volume, derived from passive expirations before the forced expiration) as a volume landmark, it was feasible to achieve reproducible results in unsedated preterm infants, despite their strong respiratory reflexes and rapid respiratory rates. Because this approach is independent of changes in expiratory time, expired volume, or applied pressures, it may facilitate investigation of the effects of growth, development, and disease on airway function in infants, particularly during the first weeks of life, when conventional analysis of forced expirations may be inappropriate.

Respiratory function among preterm infants whose mothers smoked during pregnancy

We examined whether the adverse effects of prenatal exposure to tobacco on lung development are limited to the last weeks of gestation by comparing respiratory function in preterm infants whose mothers had and had not smoked during pregnancy. Maximal forced expiratory flow (Vmax FRC) and time to peak tidal expiratory flow as a proportion of total expiratory time (TPTEF:TE) were measured prior to discharge from hospital in 108 preterm infants (mean [SD] gestational age, 33.5 [1.8] wk), 40 of whose mothers had smoked during pregnancy. Infant urinary cotinine was less than 4 ng/ml in those born to nonsmokers, but it was as high as 458 ng/ml in exposed infants (p < 0.0001). TPTEF:TE was significantly lower in infants exposed to tobacco in utero (mean [SD], 0.369 [0.109]) when compared with those who were not (0.426 [0.135]) (p <= 0.02). Vmax FRC was also reduced in exposed infants (mean [SD], 85.2 [41.7] ml/s versus 103.8 [49.7] ml/s) (p = 0.07). After allowing for sex, ethnic group, body size, postnatal age, and socioeconomic status, TPTEF:TE remained significantly diminished in infants exposed prenatally to tobacco (p < 0.05). Thus, impaired respiratory function is evident in infants born on average 7 wk prior to the expected delivery date, suggesting that the adverse effects of prenatal exposure to tobacco are not limited to the last weeks of pregnancy.

Influence of ethnicity and gender on airway function in preterm infants

While maximal expiratory flow at functional residual capacity, calculated from partial expiratory flow volume curves (V'maxFRC), is a valuable measure of peripheral airway function in infants, limited data are available in preterm infants despite their high prevalence of respiratory problems. To investigate the influence of gender and ethnic group, V'maxFRC and other indices of respiratory function were measured in 28 black and 28 white preterm infants (50% female in each group) at time of discharge from the neonatal unit (mean [SD] weight 2.36 [0.3] kg, postnatal age 19 [9] d). No infant had any history of cardiorespiratory disease and all were born to non-smoking mothers. V'maxFRC tended to be higher in girls than boys (115 versus 94 ml.s-1 [95% CI: -5; 47]) but there was no significant difference in this parameter between black and white infants (111 versus 98 ml.s-1 [95% CI of difference: -12; 40]). Respiratory resistance (Rrs) was significantly lower in black than white infants (95% CI: -2.9; -0.4 kPa.L-1.s) and tended to be lower in female than male infants (95% CI: -2.3; 0.2 kPa.L-1.s). Similarly, time to peak tidal expiratory flow as a proportion of total expiratory time (tPTEF:tE) was significantly longer in black than white (95% CI: 0.06, 0.20) and in female than male (95% CI: 0.02, 0.15) infants. These findings suggest that certain parameters of airway function may be influenced by both ethnic group and gender in preterm infants, both of which should therefore be taken mw account when investigating the effects of disease and/or therapeutic interventions in this group.

Delayed maturation of Hering-Breuer inflation reflex activity in preterm infants

We have previously shown that the strength of the Hering-Breuer inflation reflex (HBIR) diminishes between 2 and 12 mo of age in full-term babies. The purpose of this study was to determine whether the onset of this decline had commenced by 3 to 4 mo of age in healthy full-term infants and whether preterm delivery influences the pattern of maturation. Serial measurements of HBIR activity using the end-inspiratory occlusion technique were made in 25 preterm and 27 full-term infants at matched postnatal and postconceptional ages during the first 6 mo of life. Although similar levels of reflex activity were observed at birth (mean +/- SD of 101.2% +/- 42.4% in preterm, and 101.0% +/- 33.9% in full-term infants), by 40 wk postconceptional age (PCA) (i.e., term equivalent) HBIR activity (mean +/- SD) had increased to 121.7% +/- 51.2% in preterm infants, which was significantly greater than that in full-term infants of similar PCA (95% CI of difference: 0.2; 41.2%). By 15 wk postnatal age (PNA), HBIR activity had decreased to 68.8% +/- 26.6% in full-term infants, but remained significantly higher in those delivered prematurely (87.8% +/- 32.7%). However, when measurements were repeated at approximately 4 mo after the expected rather than actual date of delivery, these differences were no longer evident (95% CI difference preterm-full-term: -21.2; 3.8%). This study suggests that important transitions in respiratory control mechanisms occur between 8 and 15 wk PNA in full-term infants and that these changes are delayed in preterm infants.

Analysis of tidal breathing parameters in infancy: how variable is TPTEF:TE?

During recent years there has been increasing interest in the measurement of tidal breathing parameters, such as the time to reach peak tidal expiratory flow as a proportion of total expiratory time (TPTEF:TE), and their application to population-based studies of the determinants of early respiratory morbidity. However, little is known about factors influencing the within and between-subject variability of these parameters. This study examines the influence of sedation on TPTEF:TE, estimates the optimal number of breaths and breath epochs required to measure TPTEF:TE, and assesses short-term repeatability of this parameter during the first year of life, taking account of age-related differences. Measurements were made in 266 healthy infants and young children (1 d to 19 mo old). Mean (SD) TPTEF:TE fell from 0.49 (0.11) in the first 2 wk of life to 0.34 (0.09) by 5 to 8 wk, remaining similar thereafter. Sedation with triclofos sodium (75 mg/kg) had no significant effect on TPTEF:TE, which was 0.33 (0.10) in 23 unsedated 6-wk-old infants and 0.32 (0.08) in 49 sedated infants of similar age and weight (95% CI for the difference: -0.05, 0.04). At least 10 breaths in each of two separate epochs from each infant were required to provide a representative estimate of TPTEF:TE. The mean (SD) difference between repeat measurements made 5 to 108 min apart was 0.02 (0.08) in 34 infants younger than 6 wk of age (95% limits of agreement: -0.15, 0.18) and -0.01 (0.04) (95% limits of agreement -0.09, 0.08) in 30 infants 6 wk and older.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of respiratory function in Afro-Caribbean and Caucasian infants

Race is recognized as an important determinant of lung function in children and adults, but limited data exist for infants. Accurate interpretation of lung function tests during the neonatal period may depend on appropriate values for predicting normality. The aim of this study was to compare tidal breathing parameters, Hering-Breuer reflex (HBR) activity, and total respiratory compliance (Crs) in healthy newborn caucasian (white) and Afro-Caribbean (black) infants, to determine whether separate reference values were required for these two ethnic groups. Respiratory function was measured in 33 healthy black infants, 18 of whom were premature, and 33 healthy white infants matched for sex, gestational age, weight, postnatal age, and maternal smoking during pregnancy. There were no significant paired differences between black and white infants with respect to minute ventilation, respiratory frequency, the ratio of time to reach peak expiratory flow to total expiratory time, or HBR activity. Values of Crs were similar in black and white full-term infants (37.5 (SD 9.0) versus 35.0 (6.3) ml.kPa-1, respectively) suggesting that, in the immediate newborn period, separate reference values are not necessary for these parameters. However, Crs was somewhat lower in black than white preterm infants (26.0(5.2) versus 29.5(7.2) ml.kPa-1, this difference reaching statistical significance if results were expressed in relation to body weight (95% confidence interval of within-pair differences -4.0 to -0.02 ml.kPa-1 x kg-1; p < 0.05). We conclude that no separate reference values for tidal breathing, Hering-Breuer reflex activity or total respiratory compliance are required for white and black babies in the immediate newborn period.(ABSTRACT TRUNCATED AT 250 WORDS)