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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

Lung Function Testingin School-Age Children

Paul Aurora

Great Ormond Street Hospital for Children,

& Institute of Child Health,

London

structure of talk
Structure of talk
  • Why bother?
  • What do we need from a lung function test?
  • What tests are available?
  • Spirometry
  • Other tests
why bother
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
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

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
So, what do we need to know first?
  • Precision – usually expressed as coefficient of variation
  • Variability/repeatability
    • Between subjects
    • Within subject, between occasions
  • Reference data
  • Standardisation
slide7

Between occasion repeatability

Intervention

Outcome

2

1

Time

slide8

Intervention

Outcome

2

3

1

Time

Between occasion repeatability

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

slide9

Accurate anthropometry essential for

meaningful interpretation of results

How often do you calibrate your stadiometer?

quality control
Quality control
  • 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
Commonly used techniques
  • 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
Less commonly 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
Diagnosingasthma
  • 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
Airway inflammation
  • Exhaled NO
  • Exhaled breath condensate
  • Induced sputum
exercise tests
Exercise tests
  • 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
other specialised tests
Other specialised tests
  • Fitness to fly (ask child to breath 15% O2, monitor SpO2)
  • Skin allergen testing (skin prick, skin patch)
what is a forced expiratory manoeuvre
What is a forced expiratory manoeuvre?
  • 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
What is measured from forced expiration?
  • Volume-time
    • Timed expired volumes, FEVt
    • MEF75-25
  • Flow-Volume
    • PEF
    • Flow at fixed volumes, MEF%
forced expiratory flow volume curve
Forced Expiratory Flow-Volume Curve

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
What does the flow-volume curve tell you?
  • 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
why measure forced expiration
Why measure forced expiration?
  • Expiratory flow-limitation is achieved with reasonable effort during forced expiration
expiratory flow limitation
Expiratory flow limitation
  • 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
demonstrating flow limitation isovolume pressure flow curves
Demonstrating flow-limitation: Isovolume pressure-flow curves
  • Series of forced expirations at different lung volumes
  • Driving pressure must be measured
demonstrating flow limitation
Demonstrating flow-limitation

75% TLC

50% TLC

25% TLC

demonstrating flow limitation28
Demonstrating flow-limitation

75% TLC

50% TLC

25% TLC

demonstrating flow limitation29
Demonstrating flow-limitation
  • Isovolume pressure-flow curves
  • Increasing driving pressure
    • overlay curves
    • adding an oscillating pressure to jacket pressure during squeeze
    • applying negative pressure
slide30

NEP Equipment for assessing flow

limitation during RVRTC

Jones et al 2000

slide31

NEP to assess flow limitation

- Jones et al AJRCCM 2000

Flow limitation

achieved

No Flow limitation

theories to explain flow limitation
Theories to explain flow limitation
  • Equal pressure point (Mead et al. 1967)
  • Starling resistor (Pride et al. 1967)
  • Wave speed theory (Dawson et al. 1977)
slide34

Choose appropriate game

  • Set appropriate target
  • Allow sufficient trials
body plethysmography36
Body plethysmography
  • Airway resistance calculated from the relationship between pressure difference and flow
  • Total lung volume can be calculated by breathing against an occlusion
multiple breath washout
Multiple-breath washout
  • 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
A = Wash-in phase

B = Disconnection

C = Washout

interrupter technique theory
Interrupter technique: theory
  • 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
Impulse oscillation / Forced oscillation: theory
  • 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
Impulse oscillation: technique
  • 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

Forced Oscillation

Technique

standard generator

‘head’ generator to minimize upper airway artefact

nitric oxide levels within the airway
Nitric Oxide levels within the airway
  • 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
Inhale (NO free air)
  • 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
Exhaled NO signal profiles
  • 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
Key points
  • 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
Key points
  • Whatever test you use, remember
    • What is the test for?
    • What is precision, variability?
    • Quality control is essential
    • What are your reference data?
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