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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,

London


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


What is the test for l.jpg
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


Slide5 l.jpg

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

AJRCCM 2002

USA healthy

London CF

London healthy


So what do we need to know first l.jpg
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


Slide7 l.jpg

Between occasion repeatability controls

Intervention

Outcome

2

1

Time


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Intervention controls

Outcome

2

3

1

Time

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


Commonly used techniques l.jpg
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)


Less c ommonly used techniques l.jpg
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


Diagnosing asthma l.jpg
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)


Airway inflammation l.jpg
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)


Spirometry l.jpg
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


What is measured from forced expiration l.jpg
What is measured from forced expiration? controls

  • Volume-time

    • Timed expired volumes, FEVt

    • MEF75-25

  • Flow-Volume

    • PEF

    • Flow at fixed volumes, MEF%



Forced expiratory flow volume curve l.jpg
Forced Expiratory Flow-Volume Curve controls

12

PEF

9

MEF75%

6

Flow (L.s-1)

MEF50%

3

MEF25%

0

100TLC

75

50

25

0RV

Expired Vital Capacity (%)


What does the flow volume curve tell you l.jpg
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


Demonstrating flow limitation l.jpg
Demonstrating flow-limitation controls

75% TLC

50% TLC

25% TLC



Demonstrating flow limitation28 l.jpg
Demonstrating flow-limitation controls

75% TLC

50% TLC

25% TLC


Demonstrating flow limitation29 l.jpg
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


Slide30 l.jpg

NEP Equipment for assessing flow controls

limitation during RVRTC

Jones et al 2000


Slide31 l.jpg

NEP to assess flow limitation controls

- Jones et al AJRCCM 2000

Flow limitation

achieved

No Flow limitation


Theories to explain flow limitation l.jpg
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)



Slide34 l.jpg


Body plethysmography l.jpg
Body controlsPlethysmography


Body plethysmography36 l.jpg
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


Slide40 l.jpg

A = Wash-in phase controls

B = Disconnection

C = Washout


Interrupter technique theory l.jpg
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



Impulse oscillation forced oscillation theory l.jpg
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


Impulse oscillation technique l.jpg
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)


Slide46 l.jpg

Forced Oscillation controls

Technique

standard generator

‘head’ generator to minimize upper airway artefact


Nitric oxide levels within the airway l.jpg
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


Inhale no free air l.jpg
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]


Exhaled no signal profiles l.jpg
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



Key points l.jpg
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


Key points52 l.jpg
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?