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Lung F unction T esting in School-Age Children. Paul Aurora Great Ormond Street Hospital for Children, & Institute of Child Health, London. Structure of talk. Why bother? What do we need from a lung function test? What tests are available? Spirometry Other tests. Why bother?.

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Lung f unction t esting in school age children l.jpg

Lung Function Testingin School-Age Children

Paul Aurora

Great Ormond Street Hospital for Children,

& Institute of Child Health,


Structure of talk l.jpg
Structure of talk

  • Why bother?

  • What do we need from a lung function test?

  • What tests are available?

  • Spirometry

  • Other tests

Why bother l.jpg
Why bother?

  • LFTs aid diagnosis and prognosis, so are of benefit clinically and epidemiologically

  • Early identification of lung disease allows monitoring of progression

  • LFTs can be used as outcome measures to evaluate interventions

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What is the test for?

  • For the researcher, lung function tests need to show differences between groups, at cross-section, and over time or with interventions

  • For the clinician, lung function tests need to discriminate between individuals, or to monitor change in an individual over time or with intervention

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Airway function in infants with CF vs prospective healthy controls

Average reduction of

22 % in FEV0.5 in infants

with CF vs healthy infants

after adjustment for body size,

age, sex etc

Ranganathan et al

Lancet 2001


USA healthy

London CF

London healthy

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So, what do we need to know first? controls

  • Precision – usually expressed as coefficient of variation

  • Variability/repeatability

    • Between subjects

    • Within subject, between occasions

  • Reference data

  • Standardisation

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Between occasion repeatability controls






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Intervention controls






Between occasion repeatability

Chan E, Thorax. 2003 Apr;58(4):344-7.

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Accurate anthropometry essential for controls

meaningful interpretation of results

How often do you calibrate your stadiometer?

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Quality control controls

  • Study by Arets et al (ERJ 2001) reported spirometry in 446 school-age children who were experienced in the test

  • Only 60% met ATS and ERS adult criteria for start of test

  • Only 15% met the criterion for forced expired time

  • Only 80% met the criteria for reproducibility

  • Conclusion – adult QC criteria are not appropriate for children

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Commonly used techniques controls

  • Spirometry

    • tells you about airflow limitation and lung volumes

  • Plethysmography

    • tells you about airway resistance, total lung size, and trapped gas

  • Transfer factor

    • Tells you about alveolar function (also affected by pulmonary blood supply & VQ matching)

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Less c controlsommonly used techniques

  • Gas washout tests

    • Tell you about gas mixing (small airway function, heterogenous changes in compliance)

  • Interrupter resistance (Rint)

    • Tells you about airway resistance

  • Oscillometry

    • possibly tells you about small airways

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Diagnosing controlsasthma

  • Change in lung function

    • After bronchodilator

    • After bronchoconstriction (exercise, dry air, methacholine)

  • Commonly use spirometry as outcome measure, but can use any airway test (eg airway resistance, gas washout)

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Airway inflammation controls

  • Exhaled NO

  • Exhaled breath condensate

  • Induced sputum

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Exercise tests controls

  • Maximal tests (eg bicycle ergometer)

    • Monitor VO2, VCO2, lactate production etc

  • Submaximal tests (6-min walk, 3-min step, shuttle)

    • Monitor walk distance, SpO2, HR, breathlessness scores

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Other specialised tests controls

  • Fitness to fly (ask child to breath 15% O2, monitor SpO2)

  • Skin allergen testing (skin prick, skin patch)

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Spirometry controls

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What is a forced expiratory manoeuvre? controls

  • Breathe in to desired volume

  • exhale as fast as possible to RV

  • volume-time or flow-volume plots

  • easy for adults and children > 6, difficult for younger children, infants need assistance

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What is measured from forced expiration? controls

  • Volume-time

    • Timed expired volumes, FEVt

    • MEF75-25

  • Flow-Volume

    • PEF

    • Flow at fixed volumes, MEF%

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Forced Expiratory Flow-Volume Curve controls






Flow (L.s-1)










Expired Vital Capacity (%)

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What does the flow-volume curve tell you? controls

  • Flow-volume curves

    • maximal (MEFV) from TLC

    • partial (PEFV) from lower volume

  • slope of descending limb

    • inverse of time-constant of emptying

    • shape conveys information

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Why measure forced expiration? controls

  • Expiratory flow-limitation is achieved with reasonable effort during forced expiration

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Expiratory flow limitation controls

  • Once a certain minimum effort has been exceeded, maximum expiratory flow becomes independent of the effort applied

  • the maximum flow is thought to reflect the mechanical properties of the lungs and airways

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Demonstrating flow-limitation controls: Isovolume pressure-flow curves

  • Series of forced expirations at different lung volumes

  • Driving pressure must be measured

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Demonstrating flow-limitation controls

75% TLC

50% TLC

25% TLC

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Demonstrating flow-limitation controls

75% TLC

50% TLC

25% TLC

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Demonstrating flow-limitation controls

  • Isovolume pressure-flow curves

  • Increasing driving pressure

    • overlay curves

    • adding an oscillating pressure to jacket pressure during squeeze

    • applying negative pressure

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NEP Equipment for assessing flow controls

limitation during RVRTC

Jones et al 2000

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NEP to assess flow limitation controls

- Jones et al AJRCCM 2000

Flow limitation


No Flow limitation

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Theories to explain flow limitation controls

  • Equal pressure point (Mead et al. 1967)

  • Starling resistor (Pride et al. 1967)

  • Wave speed theory (Dawson et al. 1977)

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Body controlsPlethysmography

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Body plethysmography controls

  • Airway resistance calculated from the relationship between pressure difference and flow

  • Total lung volume can be calculated by breathing against an occlusion

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Multiple-breath washout controls

  • Tidal breathing test

  • The resident gas of the lung is ‘washed-out’ using air (eg SF6 or He washout), or oxygen (nitrogen washout)

  • The ventilation required to dilute the resident gas is a measure of (small) airway function

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A = Wash-in phase controls

B = Disconnection

C = Washout

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Interrupter technique: theory controls

  • Based on assumption that change in transpulmonary pressure observed immediately after sudden occlusion of airway is entirely explained by cessation of flow

  • Respiratory system resistance (Rrs) then calculated from change in pressure (Prs) and flow preceding occlusion

  • Assumes that pressure measured at mouth equilibrates along airways immediately after occlusion

  • Can now be measured by inexpensive portable device

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Impulse oscillation / Forced oscillation: theory controls

  • The mechanical characteristics of a system may be calculated by relating the applied stress to the resultant deformation

  • During breathing, pressure is generated by the respiratory muscles to produce deformation of the lung

  • If transpulmonary pressure is varied in a frequency domain different from that of respiratory muscle activity, we can study mechanics related to the applied transpulmonary pressure

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Impulse oscillation: technique controls

  • Signal of 6Hz or greater generated by computer, delivered through one or more loudspeakers placed at the mouth, at the chest or via a headbox (headbox aims to reduce upper airway artefact)

  • Measure angular velocity and frequency of applied pressure and resultant flow. From this can calculate the mechanical impedance of the respiratory system

    (Zrs, = Prs/ V’rs)

  • Pressure and flow are normally measured at same point (input impedance, Zrs,in)

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Forced Oscillation controls


standard generator

‘head’ generator to minimize upper airway artefact

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Nitric Oxide levels within the airway controls

  • NO formed in upper & lower respiratory tract

  • Diffusion into lumen conditions exhaled gas with NO

  • Alveolar NO is very low as NO taken up by haemoglobin in pulmonary capillaries

  • Nasal NO is high and may contaminate exhaled samples

  • Ambient NO may be very high. Measurement technique needs to prevent contamination of exhaled sample

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Inhale (NO free air) controls

  • Exhale to RV

  • Inhale to TLC over 2-3s

  • No nose clip (unless subject cannot avoid nasal inspiration)Inspired air passes through a scrubber to eliminate ambient NO [recommend: FINO < 5 ppb]

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Exhaled NO signal profiles controls

  • Flow 45-55 ml/sduration > 6s

  • NO profile:- washout phase- transition- plateau lasting >3s[NO] < 10% or if [NO] <5ppb, [NO] < 1ppb

  • Pressure 5-20 cmH2O.

  • Allow > 30s quiet breathing between tests

  • Repeatability:  2 tests with NO plateau within 10% of the mean

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Key points controls

  • Spirometry still the mainstay of the lung function lab, but

    • Other tests may be more sensitive

    • It may be possible to measure inflammation non-invasively

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Key points controls

  • Whatever test you use, remember

    • What is the test for?

    • What is precision, variability?

    • Quality control is essential

    • What are your reference data?