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

FEV manoeuvre induced changes in breath VOC compositions: an unconventional view on lung function tests

Breath volatile organic compound (VOC) analysis can open a non-invasive window onto pathological and metabolic processes in the body. Decades of clinical breath-gas analysis have revealed that changes in exhaled VOC concentrations are important rather than disease specific biomarkers. As physiological parameters, such as respiratory rate or cardiac output, have profound effects on exhaled VOCs, here we investigated VOC exhalation under respiratory manoeuvres. Breath VOCs were monitored by means of real-time mass-spectrometry during conventional FEV manoeuvres in 50 healthy humans. Simultaneously, we measured respiratory and hemodynamic parameters noninvasively. Tidal volume and minute ventilation increased by 292 and 171% during the manoeuvre. FEV manoeuvre induced substance specific changes in VOC concentrations. pET-CO2 and alveolar isoprene increased by 6 and 21% during maximum exhalation. Then they decreased by 18 and 37% at forced expiration mirroring cardiac output. Acetone concentrations rose by 4.5% despite increasing minute ventilation. Blood-borne furan and dimethyl-sulphide mimicked isoprene profile. Exogenous acetonitrile, sulphides, and most aliphatic and aromatic VOCs changed minimally. Reliable breath tests must avoid forced breathing. As isoprene exhalations mirrored FEV performances, endogenous VOCs might assure quality of lung function tests. Analysis of exhaled VOC concentrations can provide additional information on physiology of respiration and gas exchange.

Lung function at 6 and 18 months after preterm birth in relation to severity of bronchopulmonary dysplasia

Many preterm infants with bronchopulmonary dysplasia (BPD) demonstrate impaired lung function and respiratory symptoms during infancy. The relationships between initial BPD severity, lung function and respiratory morbidity are not fully understood. We aimed to investigate the association between BPD severity and subsequent lung function and whether lung function impairment is related to respiratory morbidity.

STUDY DESIGN AND METHODS

In this longitudinal cohort study, 55 infants born preterm (23-30 weeks of gestation) with mild or moderate/severe BPD, based on oxygen requirement at 36 gestational weeks, were followed up at 6 and 18 months postnatal age. Respiratory symptoms, such as recurrent or chronic chough and wheeze, were noted and patient records were scrutinized. Lung function was assessed by passive lung mechanics, whole body plethysmography, and tidal and raised volume rapid thoraco-abdominal compression techniques. Results were related to published normative values.

RESULTS

Besides residual functional capacity (FRC) and respiratory system compliance (Cso ) assessed at 18 months, all measures of lung function were significantly below normative values. Moderate/severe BPD differed significantly from mild BPD only with respect to reduced Cso . At follow-up at 6 and 18 months, participants with respiratory symptoms showed lower; maximal forced expiratory flow at FRC (V'maxFRC) (P = 0.006, P = 0.001), forced mid-expiratory flows (MEF50 ) (P = 0.006, P = 0.048), and Cso (P = 0.004, P = 0.015) as compared to participants without symptoms.

CONCLUSIONS

In the present study BPD severity did not predict lung function, but may be associated with impaired alveolarization, indicated by reduced Cso . Respiratory morbidity was associated with reduced airway function and respiratory compliance in infancy after preterm birth.

Influence of sighs in the raised volume rapid thoracic compression technique (RVRTC) in infants

The raised volume rapid thoracic compression (RVRTC) technique has shown to be very promising in the evaluation of infant's lung function. In this technique lungs are inflated several times to a preset pressure prior to the thoracic compression. Many infants made a spontaneous inspiration (sigh) at the end of these inflations. Our hypothesis was that such sighs could change the major variables derived by this technique and the objective of this study was to evaluate the influence of these sighs during lung inflation in the RVRTC technique in infants. Pairs of maneuvers with and without sighs during lung inflation were obtained in 33 of 48 consecutive tests. Curves with sighs showed significantly higher values of FVC (median: 456 x 437 ml; P < 0.001) and FEV0.5 compared to those without, whereas FEF75 and FEF85 were significantly lower (median: 417 x 439 ml/sec, P = 0.008 and 251 x 273 ml/sec, P = 0.01; respectively). The mean percent change between maneuvers for FVC, FEV0.5, FEF75, and FEF85 was respectively: 6.4%, 3.8%, -3.1%, and -3.5%. These differences represent a mean change of 0.38 z score for FVC and of 0.12 z score for FEF75 and FEF85. In conclusion, the presence of sighs during lung inflation significantly changes RVRTC values in infants. We suggest that the presence or the absence of sighs should be registered for each maneuver and that it should be considered for within and between subject comparisons.

How does the changing profile of infants who are referred for extracorporeal membrane oxygenation affect their overall respiratory outcome?

OBJECTIVE

Extracorporeal membrane oxygenation has been shown to be effective in term neonates with severe but reversible lung disease within the context of randomized, controlled trials. Extracorporeal membrane oxygenation now has been open to a wider population of infants in the United Kingdom, and other treatments have become available. The population referred for extracorporeal membrane oxygenation, therefore, has changed. The aims of this study were to (1) compare respiratory outcomes of infants who received extracorporeal membrane oxygenation in recent years with those from 10 years ago and (2) determine whether respiratory outcome varied with diagnostic group.

METHODS

All infants who were referred to a single extracorporeal membrane oxygenation center and were <12 months old during a 7-year period were eligible. One year after extracorporeal membrane oxygenation, lung volume, airway conductance, maximum expiratory flow, and indices of tidal breathing were measured.

RESULTS

A total of 106 infants (77% of those eligible) were tested, and results were compared with those of 51 infants referred for extracorporeal membrane oxygenation as part of the original United Kingdom extracorporeal membrane oxygenation trial. Lung volume was not different, but there was a strong trend for the infants who were seen in more recent years to have better forced expiratory flow and specific airway conductance. Restricting analysis to the major subgroup (meconium aspiration) confirmed these findings. When divided into diagnostic subgroups, infants who required extracorporeal membrane oxygenation for respiratory distress syndrome or who were >2 weeks old when extracorporeal membrane oxygenation was commenced had a poorer respiratory outcome than others.

CONCLUSIONS

The respiratory outcome of infants who were treated beyond the tightly regulated criteria of the United Kingdom trial remains good and even shows a trend toward improvement. Certain subgroups require extracorporeal membrane oxygenation for longer and have poorer pulmonary function when followed up.

Lung function tests in neonates and infants with chronic lung disease: forced expiratory maneuvers

This fourth paper in a review series on the role of lung function testing in infants and young children with acute neonatal disorders and chronic lung disease of infancy (CLDI) addresses measurements of forced expiration using rapid thoraco-abdominal compression (RTC) techniques and the forced deflation technique. Following orientation of the reader to the subject area, we focus our comments on the areas of inquiry proposed in the introductory paper to this series. The quality of the published literature is reviewed critically, and recommendations are provided to guide future investigation in this field. All studies on infants and young children with CLDI using forced expiratory or deflation maneuvers demonstrated that forced flows at low lung volume remain persistently low through the first 3 years of life. Measurement of maximal flow at functional residual capacity (V'maxFRC) is the most commonly used method for assessing airway function in infants, but is highly dependent on lung volume and airway tone. Recent studies suggested that the raised volume RTC technique, which assesses lung function over an extended volume range as in older children, may be a more sensitive means of discriminating changes in airway function in infants with respiratory disease. The forced deflation technique allows investigation of pulmonary function during the early development of CLDI in intubated subjects, but its invasive nature precludes its use in the routine setting. For all techniques, there is an urgent need to establish suitable reference data and evaluate within- and between-occasion repeatability, prior to establishing the clinical usefulness of these techniques in assessing baseline airway function and/or response to interventions in subjects with CLDI.

The response to β-agonists in wheezy infants: three methods compared

BACKGROUND

Studies into the effects of salbutamol in the treatment of wheeze in infancy have been conflicting, possibly due to differences in outcome variables. We aimed to assess the response to salbutamol using indices derived from passive and forced expiration.

METHODS

We recruited 39 infants who had a history of wheezing (mean age 43 weeks) and measured maximum flow at functional residual capacity (V'(max FRC)) by rapid thoracoabdominal compression (RTC), and forced expired volume at 0.4s (FEV0.4) using the raised-volume RTC technique (RV-RTC). We calculated passive compliance (C(rs)), resistance (R(rs)) and time constant (tau) from relaxed expirations that followed the augmented inspirations delivered during RV-RTC. Measurements were repeated after aerosol salbutamol (800 mcg).

RESULTS

Data were obtained in 32 infants for V'(max FRC), 22 for FEV0.4 and 19 for passive mechanics. There were no mean changes in any index of forced expiration after salbutamol. Some individuals showed significant changes (improvement or worsening) in one or other index. Overall, there was a small increase in C(rs) after salbutamol but no change in R(rs) or tau.

CONCLUSIONS

We found no consistent pattern of response in either index of forced expiration. Validated clinical scores or alternative physiological techniques may be preferable to respiratory mechanics in assessing bronchodilator response.

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.

Relative ability of full and partial forced expiratory maneuvers to identify diminished airway function in infants with cystic fibrosis

The tidal and raised volume rapid thoracoabdominal compression techniques are increasingly used to detect diminished airway function in infancy. The aim of this study was to assess the relative ability of parameters measured by these techniques to identify diminished airway function in infants newly diagnosed with cystic fibrosis (CF) with and without clinical evidence of prior lower respiratory illness. A cross-sectional, prospective study design was used in which maximal flow at functional residual capacity (VmaxFRC) from the tidal technique and FVC, FEV0.5, FEF75, and FEF25-75 from the raised volume technique were measured in 47 infants with CF and 187 healthy infants of similar body size, sex distribution, ethnic group, and exposure to maternal smoking. Multiple linear regression was used to assess group differences and to calculate SD scores for each parameter for the infants with CF. Airway function was also compared with clinical assessments of respiratory status made by pediatric pulmonologists. FEV0.5 was significantly diminished in 13 infants with CF, of whom 4 had been identified by clinicians as having normal respiratory status. Only one infant with CF had a VmaxFRC below the estimated normal range. Airway function is diminished in infants with CF irrespective of prior lower respiratory illness and in those whose respiratory status is considered normal by pediatric pulmonologists. In infants with CF, the raised volume technique identified diminished airway function more frequently than the tidal technique.

Effect of airway inflation pressure on forced expiratory maneuvers from raised lung volume in infants

The raised lung volume technique is increasingly used to measure forced expiratory maneuvers in infants. However, there is no consensus regarding the optimal airway inflation pressure (P(inf)) required for such maneuvers, or the influence of small changes in P(inf) within and between infants. The aim of this study was to assess the effect of small differences (0.2-0.3 kPa) in P(inf) on forced vital capacity (FVC), forced expired volume in 0.5 sec (FEV(0.5)), and forced expired flow at 75% of vital capacity (FEF(75)), all derived from the raised volume rapid thoraco-abdominal compression (RVRTC) technique. Randomized paired forced expiratory maneuvers were obtained in 32 healthy infants ( 3.9-39.3 weeks old, 3.8-9.9 kg) with the safety pressure relief valve for P(inf) set to 2.7 kPa or 3.0 kPa (27 or 30 cm H(2)0). When mean (SD) P(inf) was increased by 8.4 (2.8)%, there was a significant (P < 0.01) increase in mean (SD) FVC, FEV(0.5), and FEF(75) by 5.8 (5.7)%, 6.1 (6)%, and 8.3 (16.2)%, respectively. In conclusion, relatively small differences in P(inf) will result in significant differences in FVC, FEV(0.5), and FEF(75) by RVRTC technique. Precision in setting and reporting the applied P(inf) is therefore essential, particularly if data are to be compared between centers.

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.

Assessment of airway function using partial expiratory flow-volume curves: How reliable are measurements of maximal expiratory flow at frc during early infancy?

We investigated the extent to which measurements of maximal expiratory flow at FRC (V EmaxFRC) are influenced by the dynamic increase of FRC in young infants by superimposing partial forced expiratory flow-volume curves on those obtained after lung inflation to 2 kPa (20 cm H2O) in 12 infants during the first month of life. The elastic equilibrium volume (EEV) of the respiratory system was estimated by extrapolating the passive expiratory time constant (obtained after lung inflation but prior to forced deflation) to zero flow. There was a very strong relationship between V EmaxFRC (which ranged from 11 to 190 ml/s) and the extent to which FRC was dynamically increased above EEV (range: 0 to 5 ml/kg), r2 = 0.88. The results of this study suggest that, although V EmaxFRC remains a useful means of measuring peripheral airway function in infants, its values should be interpreted with caution during the neonatal period. In particular, the relatively high V EmaxFRC values reported in healthy newborn infants may reflect differences in breathing strategy rather than airway structure. More meaningful within- and between-infant comparisons of peripheral airway function may be obtained by calculating forced expiratory flows at a fixed interval (e.g., 3 ml/kg) above EEV, rather than at the FRC that is operational at the time of measurement.