New reference equations to improve interpretation of infant lung function

Authors

  • The Thanh Diem Nguyen MD,

    Corresponding author
    1. Portex Respiratory Unit, UCL Institute of Child Health and Great Ormond Street Hospital for Children NHS Foundation Trust, London, UK
    2. Department of Respiratory Medicine, Centre Hospitalier Universitaire Sainte-Justine, Montreal, Quebec, Canada
    • Portex Respiratory Unit, UCL Institute of Child Health, 30 Guilford Street, London WC1N 1EH, UK.
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  • Ah-Fong Hoo PhD,

    1. Portex Respiratory Unit, UCL Institute of Child Health and Great Ormond Street Hospital for Children NHS Foundation Trust, London, UK
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  • Sooky Lum PhD,

    1. Portex Respiratory Unit, UCL Institute of Child Health and Great Ormond Street Hospital for Children NHS Foundation Trust, London, UK
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  • Angie Wade PhD,

    1. Centre for Paediatric Epidemiology and Biostatistics, UCL Institute of Child Health, London, UK
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  • Lena P. Thia MBChB Msc,

    1. Portex Respiratory Unit, UCL Institute of Child Health and Great Ormond Street Hospital for Children NHS Foundation Trust, London, UK
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  • Janet Stocks PhD

    1. Portex Respiratory Unit, UCL Institute of Child Health and Great Ormond Street Hospital for Children NHS Foundation Trust, London, UK
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  • None of the authors has any potential or actual conflict of interest relevant to the topic discussed in the present manuscript.

Abstract

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. Pediatr Pulmonol. 2013; 48:370–380. © 2012 Wiley Periodicals, Inc.

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