Lung Function Testing in School-Age Children

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Lung Function Testingin School-Age Children

• Paul Aurora
• Great Ormond Street Hospital for Children,
• Institute of Child Health,
• London

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Structure of talk

• Why bother?
• What do we need from a lung function test?
• What tests are available?
• Spirometry
• Other tests

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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
Ranganathan et al Lancet 2001 AJRCCM 2002
Average reduction of 22 in FEV0.5 in infants
with CF vs healthy infants after adjustment
for body size, age, sex etc
USA healthy London CF London healthy
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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

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Between occasion repeatability
Intervention
Outcome
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1
Time
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Between occasion repeatability
Chan E, Thorax. 2003 Apr58(4)344-7.
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Accurate anthropometry essential for meaningful
interpretation of results
How often do you calibrate your stadiometer?
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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

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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)

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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

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Diagnosing asthma

• 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

• Exhaled NO
• Exhaled breath condensate
• Induced sputum

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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

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Other specialised tests

• Fitness to fly (ask child to breath 15 O2,
  monitor SpO2)
• Skin allergen testing (skin prick, skin patch)

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Spirometry
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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 gt 6, difficult for
  younger children, infants need assistance

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What is measured from forced expiration?

• Volume-time
• Timed expired volumes, FEVt
• MEF75-25
• Flow-Volume
• PEF
• Flow at fixed volumes, MEF

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Flow-volume and volume-time plots
FEV1
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Forced Expiratory Flow-Volume Curve
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PEF
9
MEF75
6
Flow (L.s-1)
MEF50
3
MEF25
0
100TLC
25
75
50
0RV
Expired Vital Capacity ()
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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

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Why measure forced expiration?

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

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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

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Demonstrating flow-limitation Isovolume
pressure-flow curves

• Series of forced expirations at different lung
  volumes
• Driving pressure must be measured

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Demonstrating flow-limitation
75 TLC
50 TLC
25 TLC
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Demonstrating flow-limitation
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Demonstrating flow-limitation
75 TLC
50 TLC
25 TLC
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Demonstrating flow-limitation

• 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 limitation
during RVRTC Jones et al 2000
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NEP to assess flow limitation - Jones et al
AJRCCM 2000
Flow limitation achieved
No Flow limitation
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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)

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Spirometry in infants
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• Choose appropriate game
• Set appropriate target
• Allow sufficient trials

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Body Plethysmography
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Body plethysmography

• 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

• 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
• B Disconnection
• C Washout

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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

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The interrupter technique
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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

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

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Forced Oscillation Technique
standard generator
head generator to minimize upper airway
artefact
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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

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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 lt 5 ppb

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Exhaled NO signal profiles

• Flow 45-55 ml/sduration gt 6s
• NO profile- washout phase- transition-
  plateau lasting gt3s ?NO lt 10 or if NO
  lt5ppb, ?NO lt 1ppb
• Pressure 5-20 cmH2O.
• Allow gt 30s quiet breathing between tests
• Repeatability ? 2 tests with NO plateau within
  10 of the mean

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Offline exhaled NO circuit
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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

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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?