Assessment of an infant whole-body plethysmograph using an infant lung function model

Respiratory Follow Up of the Premature Neonates-Rationale and Practical Issues

The aim of the review was to present the state of knowledge about the respiratory pathology in former premature neonates (children that were born preterm-before 37 weeks of gestation-and are examined and evaluated after 40 weeks corrected age) other than chronic lung disease, in order to provide reasons for a respiratory follow-up program for this category of patients. After a search of the current evidence, we found that premature infants are prone to long-term respiratory consequences due to several reasons: development of the lung outside of the uterus, leading to dysmaturation of the structures, pulmonary pathology due to immaturity, infectious agents or mechanical ventilation and deficient control of breathing. The medium- to long-term respiratory consequences of being born before term are represented by an increased risk of respiratory infections (especially viral) during the first years of life, a risk of recurrent wheezing and asthma and a decrease in pulmonary volumes and airway flows. Late preterm infants have risks of pulmonary long-term consequences similar to other former premature infants. Due to all the above risks, premature neonates should be followed in an organized fashion, being examined at regular time intervals from discharge from the maternity hospital until adulthood-this could lead to an early detection of the risks and preventive therapies in order to improve their prognosis and assure a normal and productive life. The difficulties related to establishing such programs are represented by the insufficient standardization of the data gathering forms, clinical examinations and lung function tests, but it is our belief that if more premature infants are followed, the experience will allow standards to be established in these fields and the methods of data gathering and evaluation to be unified.

Forced expiratory flows and volumes in a Swedish cohort of healthy term infants

BACKGROUND

The use of pulmonary function tests (PFTs) in infants has increased during the last decades, making the need for equipment- and ethnic-specific reference data mandatory for appropriate interpretation of the results.

AIM

Our aim was to investigate how well the already published reference equations for infant spirometry fit a healthy population of Swedish infants.

METHOD

We performed forced tidal and raised volume expiratory maneuvers in healthy infants using Jaeger BabyBody equipment.

RESULTS

PFT data were collected from 91 healthy infants aged between 3 months to 2 years at 143 occasions. Mean (standard deviation) z-scores were 0.68(1.33) for maximal flow at functional residual capacity (V'_max FRC), -0.15(0.96) for forced vital capacity (FVC), 0.40(1.33) for the forced expired volume in the initial 0.5 seconds (FEV_0.5 ) and 0.52(0.93) for the ratio FEV_0.5 /FVC, respectively. Z-scores for all indices but FEV_0.5 /FVC were highly dependent on length.

CONCLUSIONS

We have shown that the use of previously published reference equations may result in an age-related misinterpretation of lung function measure in a Swedish infant population.

A Calibration Device to Compare Body Plethysmographs Among Pediatric Lung Function Laboratories

Multi-center studies in specific airway resistance have shown significant inter laboratory variability. Comparison of plethysmographic equipment using a lung model easily transportable from one site to another should be of help to international normative studies. A resistor made of parallel capillary tubes – insuring adequate linearity within 1 L/sec – was connected to a glass bottle. Thermal time constants were measured while the bottle was empty and while stuffed with steel wool. In the latter, isothermal condition was estimated to occur only at very low frequency (around 0.01 Hz) and gas compression was polytropic up to 0.6 Hz. With the empty analog, adiabatic gas compression was estimated to occur at frequencies ≥0.2 Hz, and more accurate volume estimation was obtained. The empty analog volume and specific resistance measured in a body plethysmograph on different days indicated within 5% accuracy as well as intersession repeatability. It is concluded that a physical analog built out of simple material provides accurate measurements of specific resistance. The apparatus should be of help to compare plethysmographic equipments from different laboratories.

Lung Function in Wheezing Infants after Acute Lower Respiratory Tract Infection and Its Association with Respiratory Outcome

Background:

Wheezing is common in early childhood and remains an important health concern. The aim of this study was to assess the lung function of wheezing infants and to investigate the relationship between lung function and respiratory outcome.

Methods:

Infants <2 years of age with acute lower respiratory tract infection (ALRTI) who had undergone lung function tests were included in the study. They were assigned to wheeze or no wheeze group based on physical examination. Infants without any respiratory diseases were enrolled as controls. Lung function was measured during the acute phase and 3 months after ALRTI. One-year follow-up for infants with ALRTI was achieved.

Results:

A total of 252 infants with ALRTI who had acceptable data regarding tidal breathing were included in the final analysis. Compared with the control and the no wheeze groups, infants in the wheeze group had significantly decreased time to peak tidal expiratory flow as a percentage of total expiratory time (TPTEF/TE) (20.1 ± 6.4% vs. 34.4 ± 6.2% and 26.4 ± 8.3%, respectively, P < 0.0001) and significantly increased peak tidal expiratory flow (PTEF) (90.7 ± 26.3 ml/s vs. 79.3 ± 18.4 ml/s and 86.1 ± 28.0 ml/s, respectively, P < 0.01), sReff and Reff. The infants in the wheeze group still had lower TPTEF/TE and volume to peak tidal expiratory flow as a percentage of total expiratory volume (VPTEF/VE) than the no wheeze infants 3 months after the ALRTI. Moreover, there was a significant inverse relationship between TPTEF/TE, VPTEF/VE, and the recurrence of wheezing and pneumonia.

Conclusions:

Impaired lung function was present in wheezing infants with ALRTI and the deficits persisted. In addition, the lower level of TPTEF/TE and VPTEF/VE was a risk factor for poor respiratory outcome.

New reference equations to improve interpretation of infant lung function

RATIONALE

With increasing use of infant pulmonary function tests (IPFTs) in both clinical and research studies, appropriate interpretation of results is essential.

OBJECTIVES

To investigate the potential bias associated with "normalising" IPF by expressing results as a ratio of body size and to develop reference ranges for tidal breathing parameters, passive respiratory mechanics (compliance [Crs] and resistance [Rrs]) and plethysmographic functional residual capacity (FRCp ) for white infants during the first 2 years of life.

METHODS

IPFTs were measured using the Jaeger BabyBody system and standardized protocols. Reference equations, adjusted for body size, age, and sex where appropriate, were created using multilevel modeling.

RESULTS

The ratio of lung function to body length changes markedly with growth, thereby precluding its use for any outcome. While the ratio of tidal volume and Crs to body weight remained relatively constant with growth, this was not the case for FRCp . Even in healthy infants, a strong inverse relationship was observed between lung function/body weight and weight z-score which could distort interpretation of results in growth-restricted infants with lung disease, such as cystic fibrosis. Reference equations were derived from 153 healthy white infants on 232 test occasions (median age 35.5 weeks [range: 2.6-104.7]). Crown-heel length was the strongest predictor of IPF.

CONCLUSIONS

When reporting IPF, use of size-corrected ratios should be discouraged, with interpretation instead based on appropriate reference equations. The current equations are applicable to white infants and young children up to 2 years of age, studied using the same commercially available equipment. The extent to which these equations are applicable to infants and young children of other ethnic backgrounds or who are tested with different equipment needs to be established.

Methacholine-induced lung function changes measured with infant body plethysmography

Several techniques have been applied to measure airway responsiveness (AR) in infants, but there are limited data on lung function changes measured by body plethysmography during induced bronchoconstriction. The aim of this study was to compare changes in maximum forced expiratory flow measured at functional residual capacity (V'(maxFRC)) by rapid thoracoabdominal compression (RTC) technique with plethysmographic measurements of specific airway conductance (sG(aw) ), and to investigate whether changes in functional residual capacity (FRC) occur during methacholine-induced bronchoconstriction in infants. We examined 94 infants with recurrent airway symptoms using methacholine airway challenge test including RTC and plethysmographic measurements. A significant association between changes in V'(maxFRC) and sG(aw) (r = 0.30; P = 0.004) was observed, but after adjustments with baseline variability the changes in V'(maxFRC) were greater and showed a closer association with changes in oxygen saturation. At the point of maximal airway obstruction, there was a poor agreement between V'(maxFRC) and sG(aw) to indicate a significant methacholine-induced bronchoconstriction. Airway challenge was also associated with a significant increase in FRC (P < 0.001), with decreasing V'(maxFRC). We conclude that in infants undergoing airway challenge with methacholine, plethysmographic measurements of sG(aw) correlate with the changes in V(maxFRC), but the agreement is poor and the methods cannot be used interchangeably. V(maxFRC) is also more sensitive to detect airway obstruction than sG(aw). However, methacholine-induced bronchoconstriction was associated with significant increases in FRC, which may affect the validity of V(maxFRC) measurements during the test.

Accuracy of whole-body plethysmography requires biological calibration

BACKGROUND

Specific airway resistance (sRaw) measured by whole-body plethysmography in young children is increasingly used in research and clinical practice. The method is precise and feasible. However, there is no available method for calibration of the resistance measure, which raises concern of accuracy. Our aim was to determine the agreement of sRaw measurements in six centers and expand normative sRaw values for nonasthmatic children including these centers.

METHOD

Identical hardware with different software versions was used at the six centers. Measurements followed a standard operating procedure: (1) seven healthy young children were brought to each of the six centers for sRaw measurements; and (2) 105 healthy preschool children (52 boys; mean age, 5.1 years; interquartile range, 4.3 to 6.0) were recruited locally for sRaw measurements.

RESULTS

(1) The sRaw of the seven-children study group was significantly lower at two centers compared with the other four centers, and one center had significantly higher sRaw than all the other centers (p < 0.05). Error in the factory settings of the software was subsequently discovered in one of the deviating centers. (2) Normative data (105 preschool children) were generated and were without significant difference between centers and independent of height, weight, age, and gender. We subsequently pooled these normative data (105 children) with our previous data from 121 healthy young children (overall mean sRaw, 1.27; SD, 0.25).

CONCLUSION

Control using biological standards revealed errors in the factory setting and highlights the need for developing methods for verification of resistance measures to assure accuracy. Normative data were subsequently generated. Importantly, other centers using such normative data should first consider proper calibration before applying reference values.

Lung function tests in neonates and infants with chronic lung disease: lung and chest-wall mechanics

This is the fifth paper in a review series that summarizes available data and critically discusses the potential role of lung function testing in infants and young children with acute neonatal respiratory disorders and chronic lung disease of infancy (CLDI). This review focuses on respiratory mechanics, including chest-wall and tissue mechanics, obtained in the intensive care setting and in infants during unassisted breathing. Following orientation of the reader to the subject area, we focused comments on areas of enquiry proposed in the introductory paper to this series. The quality of the published literature is reviewed critically with respect to relevant methods, equipment and study design, limitations and strengths of different techniques, and availability and appropriateness of reference data. Recommendations to guide future investigations in this field are provided. Numerous different methods have been used to assess respiratory mechanics with the aims of describing pulmonary status in preterm infants and assessing the effect of therapeutic interventions such as surfactant treatment, antenatal or postnatal steroids, or bronchodilator treatment. Interpretation of many of these studies is limited because lung volume was not measured simultaneously. In addition, populations are not comparable, and the number of infants studied has generally been small. Nevertheless, results appear to support the pathophysiological concept that immaturity of the lung leads to impaired lung function, which may improve with growth and development, irrespective of the diagnosis of chronic lung disease. To fully understand the impact of immaturity on the developing lung, it is unlikely that a single parameter such as respiratory compliance or resistance will accurately describe underlying changes. Assessment of respiratory mechanics will have to be supplemented by assessment of lung volume and airway function. New methods such as the low-frequency forced oscillation technique, which differentiate the tissue and airway components of respiratory mechanics, are likely to require further development before they can be of clinical significance.

Lung function tests in neonates and infants with chronic lung disease of infancy: functional residual capacity

This is the second paper in a review series that will summarize available data and discuss the potential role of lung function testing in infants and young children with acute neonatal respiratory disorders and chronic lung disease of infancy. The current paper addresses the expansive subject of measurements of lung volume using plethysmography and gas dilution/washout techniques. Following orientation of the reader to the subject area, we focus our comments on 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. Measurements of lung volume are important both for assessing growth and development of lungs in health and disease, and for interpreting volume-dependent lung function parameters such as respiratory compliance, resistance, forced expiratory flows, and indices of gas-mixing efficiency. Acute neonatal lung disease is characterized by severely reduced functional residual capacity (FRC), with treatments aimed at securing optimal lung recruitment. While FRC may remain reduced in established chronic lung disease of infancy, more commonly it becomes normalized or even elevated due to hyperinflation, with or without gas-trapping, secondary to airway obstruction. Ideally, accurate and reliable bedside measurements of FRC would be feasible from birth, throughout all phases of postnatal care (including assisted ventilation), and during subsequent long-term follow-up. Although lung volume measurements in extremely preterm infants were described in a research environment, resolution of several issues is required before such investigations can be translated into routine clinical monitoring.

Problems in the measurement of functional residual capacity

Accurate assessment of lung volume in infancy is important to determine the impact of disease and the efficacy of therapies. A new generation of infant plethysmographs with lower apparatus deadspace has been produced, but gives lower volume results than those from older traditional plethysmographs. We hypothesized that the new plethysmographs might have greater sensitivity to the adiabatic effect and hence they, rather than the traditional plethysmographs, produced erroneous results. Our aim was to assess the influence of the adiabatic effect on the results of a contemporary plethysmograph, an older traditional plethysmograph and a helium gas dilution system using a lung model. Altering the amount of copper wool within the lung model allowed the influence of the adiabatic effect on the plethysmographic results to be assessed. The measured compared to the actual volumes were significantly lower for the contemporary plethysmograph compared to the traditional plethysmograph (p < 0.001) and to the helium gas dilution system (p < 0.001). Under optimal testing conditions the contemporary plethysmograph under-recorded by 11-13%, whereas the other two systems gave similar results to the actual volumes. As the effect of the adiabatic effect was increased, the discrepancy between the results of the contemporary and the traditional plethysmographs increased. We conclude, the contemporary plethysmograph is more sensitive to adiabatic effects and hence under-records.

Limitations of electronic compensation for measuring plethysmographic airway resistance in infants

Electronic compensation to overcome thermal artifacts during plethysmographic estimations of airway resistance is now used routinely in adults and school-age children, and was shown to be a valuable means of discriminating airway function between preschool children with and without lung disease. A similar system is now commercially available for infants, which could increase the applicability of this technique. However, we noted marked discrepancies in electronically calculated values of airway resistance in this age group, both with respect to absolute values displayed and marked within-subject variability on a single test occasion. The aim of this technical report is to summarize our recent findings in order to alert other users to potential problems. Airway resistance (R(aw)) was measured in 62 infants (28 with cystic fibrosis (CF); 34 healthy). Three to five epochs of quiet regular tidal breathing were collected in each infant. Marked within-subject, within-test variability was observed, with the median coefficient of variation (CV) being 9.1% (range, 1.2-52.6%) within and 8.5% (0.1-112%) between epochs. Among healthy infants, R(aw) varied from 0.1-6.4 (kPa x liter(-1) x sec), with no relationship to either body or lung size and complete overlap of results with those from infants with CF, despite abnormal lung function in the latter when assessed by other means. The marked within- and between-subject variability in healthy infants, and lack of discriminative power of R(aw) when derived from electronically compensated values, currently preclude application in either clinical or research studies.

Difficult asthma in the pre-school child

The most important aspect of dealing with a pre-school child suspected of having difficult asthma, is to ensure that the diagnosis is correct, in order to avoid the inappropriate use of therapies such as inhaled corticosteroids. After exclusion of other diagnoses, if a pre-school child is thought to have asthma, difficult or otherwise, the corollary is, what sort of asthma? Is it a syndrome with airway inflammation susceptible to treatment, or one in which there is no inflammation and time alone will result in resolution of symptoms? Probably the most common mistake in this age group is to fail to recognise the latter and institute ever more aggressive and useless therapies. An approach to excluding other diagnoses, appropriate investigations to elicit the presence of airway inflammation and suggestions for subsequent management have been detailed in this review.

Progressive decline in plethysmographic lung volumes in infants: physiology or technology?

During the last 30 years, there has been an unexplained trend toward declining values for plethysmographic assessments of lung volume at functional residual capacity (FRC) in infants. The aim of this study was to compare data collected from healthy infants using contemporary equipment with published reference data and to explore reasons for discrepancies. Lung volumes were measured in 32 healthy infants (age, 4-93 weeks; weight, 3.9-12.4 kg) using a new, commercially available infant plethysmograph. Mean (SD) FRC was 19.6 (3.4) ml/kg (within subject coefficient of variation 3.4 [2.3%]), which was on average 7.0 [3.5] ml/kg and 2.3 [1.2] SD (Z) scores lower than the recently collated reference data from an American Thoracic Society task force. A total of 66% of these healthy infants had a FRC that was below the predicted normal range. Comparison of equipment, software, and protocols with those from previous reports revealed the importance of minimization of dead space and of adequate subtraction of all compressible occluded volume when calculating FRC in infants. These findings emphasize the need to establish reference data for lung function tests in infants that are appropriate for the equipment and protocols in current use.

The infant lung function model: a mechanical analogue to test infant lung function equipment

To facilitate international multicentre studies and quality control of infant pulmonary function measurements, the European Respiratory Society-American Thoracic Society (ERS-ATS) working group for infant lung function testing aims to develop specifications for standardized infant lung function equipment and software. However, a standardized test device is also needed to test whether existing infant lung function equipment is able to meet these requirements. The authors have built a "mechanical model baby" consisting of a linear pump which can reproduce prerecorded tidal flow waveforms with a precision of 0.5% (full stroke), enabling the simulation of tidal and forced flow patterns. This linear pump can be connected to a series of copper lung volumes (range 50-300 mL) with known time constants, so that lung volumes can be reproduced with a precision of +/-1% at frequencies 10-120bpm. Five airflow resistors were built using sinter material. When assessed using flows 0-300 mL.s(-1) all resistors showed a quasilinear pressure/ flow relationship, with slopes 1.0-5.6 kPa.L(-1).s. These resistances could be reproduced with a precision of +/-2.5%. The infant lung model can also be used to assess frequency responses of infant lung function equipment, since the pump is capable of delivering low amplitude volumes up to 20 Hz in a pseudorandom noise manner. In summary, based on error estimations, this infant lung model is able to test whether or not infant lung function equipment meets the requirements suggested by the European Respiratory Society-American Thoracic Society standardization group, that is: flow measurements within +/-2.5%, volume and resistance measurements within +/-5%, frequency response: magnitude attenuation <+/-10% and phase shift <+/-3 degrees at 10 Hz.