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Plethysmographic assessment of
airway resistance
Jane Kirkby PhD
University College London,
Sheffield Children’s Hospital
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 I have no real or perceived conflicts of interest that relate to this presentation.
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Aims:
• Physiology of Airflow Resistance
• How we measure airflow resistance:
• Whole Body Plethysmography
• Plethysmographic Specific Airway Resistance
• Principles
• Clinical applications (different methodologies)
• Future directions
How does airflow occur?
• Flow through a tube only occurs if a pressure
difference exists between each end.
A
B
0
-5
• During inspiration the diaphragm contracts:
• Depresses the floor of the thorax
• Raises the ribcage
• Thus elongates & widens the thorax:
• ↑Volume and ↓Pressure
• Creates a pressure gradient → airflow
What is Airways Resistance (Raw)?
Raw = pressure difference that must be applied
between the alveoli and the external atmosphere to
produce a gas flow of 1 L/s at the airway opening.
Raw= Palv / Flow
Flow: measured by PNT
Palv: Pressure transducers are used to measure
pressure changes in box signal calibrated in terms of
alveolar pressure from Pm/ VB during airway
occlusion
Resistance to flow
•
Resistance = Pressure / Flow
•
Poiseuille's Law : R =
•
•
8nl
πr4
R is proportional to length of tube
R Inversely prop to 4th power of radius
½ length = ½ resistance
½ radius = 16x resistance
What happens to resistance in the lungs?
Airways get
progressively
narrower
BUT….
Number
of airways
increases
What happens to resistance in the lungs?
As cross sectional
area in the lungs ↑
Resistance ↓
In
Health!!
What influences airway calibre?
Narrowing of the airways:
•Low flows
•High resistance
Therefore, to generate
“normal flows” the subject
needs to ↑Pressure to
overcome the ↑resistance
(i.e. ↑work of breathing)
Implications of increased airways resistance
• Increased Airways Resistance during tidal breathing
means a bigger peak and mean Pressure change is
required to achieve the same expiratory and
inspiratory flows.
• Thus there is an increase in the work of breathing
• sense of uncomfortable breathing (dyspnoea)
• Particularly emphasised during exercise or when the
respiratory system is “stressed”
• Can lead to “Dynamic Hyperinflation”
Dynamic hyperinflation:
• ↑Raw slows down expiratory flow (expiratory flow limitation)
• The next inspiration starts before the system has come to rest:
“intrinsic PEEP” (positive end-expiratory pressure)
• Elevated FRC
• Palv remains positive at end-expiration.
• Positive effect:
• expiratory flow rates increase at higher lung volumes.
• Raw decreases at higher lung volumes
• Negative effect:
• Work of breathing increases at higher lung volumes
How do we measure Airway Resistance?
Whole Body Plethysmograph
A sealed airtight chamber
Measures:
• Partitioned Lung Volumes
• Airway Resistance (Raw)
• Specific Airways Resistance
(sRaw)
Body Plethysmography
• First described by Dubois, 1956
• Methodology scarcely changed
• Still most accurate measurement
for assessing Raw
• Principle is the same as that for
absolute lung volumes
• Alveolar pressure (Palv) changes
are calibrated in terms of ∆Pbox
by slow panting against a closed
shutter followed by panting with
the shutter open
Partitioned lung volume: Principles
• Boyle’s Law: For a fixed mass of gas at constant
temperature: P1V1 = P2V2
• Subject sits within ‘airtight’ plethysmograph
• Brief occlusion at airway-opening to seal a fixed mass of
gas in the lungs (V1) where P1 ~ ambient P
Respiratory efforts against the occlusion:
Occlusion
End Expiration:
Occlusion at airway
opening
Insp effort:
Lung Vol ↑
Palv ↓
Exp effort
Lung: Vol ↓
Palv ↑
How do we measure change in alveolar
pressure during occlusion?
When no airflow (occlusion), pressures equilibrate throughout
respiratory system and can be measured at airway opening
Principles (2)
• P2 and V2 represent the pressure and volume in the lungs
after a respiratory effort against the occlusion.
• FRC = (VBox /  Pmouth ) x Pambient
 lung volume from  box volume ( VBox)
 alveolar pressure from  Pmouth
Provided no airflow, pressures equilibrate throughout
respiratory system and alveolar pressure can be measured
at airway opening
Raw: Traditional 2-step technique

 Pmo
Flow
Pbox
Step 1:
Tidal breathing records the
relationship between flow and
change in box pressure
(which is proportional to
change in alveolar pressure).
Pbox
Step 2:
Brief occlusion determines the
relationship between change in
mouth pressure (which is
assumed to be EQUAL to
alveolar pressure) against box
pressure to be derived.
Rawpleth
Shutter open:
Shutter closed:
∆Pbox * ∆Palv
∆V’
∆Pbox
= Raw
Rawpleth
Shutter open:
Shutter closed:
∆Pbox * ∆Palv
∆V’
∆Pbox
∆Palv/ ∆V’ = Raw
= Raw
Raw =
Vbox spont / flow
Vboxocc/ Pmo
Performing spirometry post
occlusion allows the calculation
of partitioned lung volumes:
RV = FRC-ERV
TLC = RV + FVC
BUT young children
rarely tolerate breathing
against the shutter
Rawpleth
• During the closed shutter manoeuvre, lung volume (FRC) is
measured
• Enables us to measure Specific Airways Resistance (sRaw)
Resistance adjusted for lung volume
sRaw = Raw x FRC
• sRaw measurements can be calculated from normal tidal
breathing without the closed shutter
• Particularly useful since young children rarely tolerate the
shutter
Growth
sRaw
X
FRC
Raw
Airways Resistance and Growth:
Growth
Growth
Why sRaw?
• With growth, both lung size and airway calibre increase:
• ↑ Lung volume, ↓ Airways Resistance
• Thus sRaw remains relatively consistent throughout life IN HEALTH
• In airway disease, both Raw and FRC (hyperinflation) will tend to , so
sRaw is elevated
• sRaw found to be elevated in pre-school children
• with wheeze (Lowe Arch Dis Child 2004)
• and CF (Nielsen AJRCCM 2004, Aurora AJRCCM 2005)
Single step procedure
1)
sRaw = Raw x FRCp
Single step procedure
1)
2)
sRaw = Raw x FRCp
Raw =
Vbox spont / flow
Vboxocc/ Pmo
o
Nb flow often written as V’ or V in text books
Single step procedure
1)
sRaw = Raw x FRCp
Vbox spont / flow
Vboxocc/ Pmo
2)
Raw =
3)
FRCp = Vboxocc/ Pmo x (Pamb – PH20)
Single step procedure
1)
sRaw = Raw x FRCp
Vbox spont / flow
Vboxocc/ Pmo
2)
Raw =
3)
FRCp = Vboxocc/ Pmo x (Pamb – PH20)
Therefore:
sRaw=
 Vboxspont/flow x  Vboxocc /  Pmo x(Pamb-PH2O)
 Vboxocc/ Pmo
Single step procedure
1)
sRaw = Raw x FRCp
Vbox spont / flow
Vboxocc/ Pmo
2)
Raw =
3)
FRCp = Vboxocc/ Pmo x (Pamb – PH20)
Therefore:
sRaw=
 Vboxspont/flow x  Vboxocc /  Pmo x(Pamb-PH2O)
 Vboxocc/ Pmo
Single step procedure
1)
sRaw = Raw x FRCp
Vbox spont / flow
Vboxocc/ Pmo
2)
Raw =
3)
FRCp = Vboxocc/ Pmo x (Pamb – PH20)
Therefore:
sRaw=
 Vboxspont/flow x  Vboxocc /  Pmo x(Pamb-PH2O)
 Vboxocc/ Pmo
Single step procedure
1)
sRaw = Raw x FRCp
Vbox spont / flow
Vboxocc/ Pmo
2)
Raw =
3)
FRCp = Vboxocc/ Pmo x (Pamb – PH20)
Therefore:
sRaw=
 Vboxspont/flow x  Vboxocc /  Pmo x(Pamb-PH2O)
 Vboxocc/ Pmo
sRaw = V box spont / flow x (Pamb – PH20)
sRaw summary
sRaw can also be measured by a single step procedure
from the relationship between simultaneous
measurements of airflow and change of box volume
without closing the shutter to measure FRC
sRaw = Pbox / flow x (Pamb – PH20)
Resistance (pressure/flow) loops
The higher the resistance the larger the
pressure (box volume) change for any given flow
Causes of elevated sRaw
• Increased resistance
• airway obstruction, bronchoconstriction, congenitally
small airways
• Increased FRC
• Hyperinflation secondary to  airway resistance, CF,
wheezing illnesses
sRaw in healthy preschool children and those
with CF (Aurora AJRCCM 2005)
sRaw z-score (ICH controls)
6
srtotzs
sR

healthy

CF
4
2
aw
zsc
or
e
0
-2
-4
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
age (years)
43% of those with CF had sRaw above 97% limits of normality
Bronchodilator responsiveness in
preschool children (2-7 y)
• 55 asthmatics
• 37 healthy
•
Some response to
terbutaline even in
healthy, but more
marked in
asthmatics
Nielsen & Bisgaard - AJRCCM 2001
Change in sRaw in response to cold air challenge
in healthy and asthmatic preschool children
Nielsen & Bisgaard – AJRCCM 2000
•
•
•
•
Differences in quality control
BTPS / electronic thermal compensation
sRaw parameter reported
Mask or mouthpiece
• Method of reporting (mean vs median)
• Breathing frequency and flow rate
• Manufacturer differences
Phase lag due to changes in temperature &
humidity of respired gas thru breath
200
Insp
0
Exp
-200
Change in Vbox between
points of zero flow due to
heating and humidification
VBox
Body temperature, pressure
saturated(BTPS) corrections
• To minimise box pressure changes due to changes in
humidity and temperature thru breath, adults
encouraged to pant
• ?feasibility
• Heated rebreathing bags maintained at BTPS used in
sleeping infants
• Infection control issues
1980’s: Electronic ‘Compensation’
• Introduced to compensate for phase shift between
flow and VB caused by breathing air at ambient
conditions
• Use inspiratory part of the loop to flatten
• Now widely used
• In preschool children, ‘electronic’ sRaw correlated
with, but higher than, that measured at BTPS
• ? Different algorithm for each manufacturer
Effect of electronic compensation:
Klug and Bisgaard 1997
sRaw parameters:
• ‘Total’ – between points of peak flow or pressure
• ‘Effective’ – throughout the breath
• ‘Inspiratory’/’expiratory’ – e.g.P/F relationship
between 0 and 0.5L/s
• All will give different values – and hence need
separate prediction equations!
sReff: Average Pressure/Flow relationship
throughout breath
1
Inspiration
Flow
L.s -1
0.5
VBox
0
mL
- 0.5
-1
Expiration
sRtot: P/F between points of maximum
(total) Vbox
1
Inspiration
Flow
L.s -1
0.5
Vbox
0
mL
0.5
1
Expiration
sR0.5: P/F over fixed flow range: +/- 0.5 L/s
Flow L.s -1
1
Inspiration
0.5
VBox
0
mL
0.5
1
Expiration
Unacceptable - changes in breathing
frequency from 25 to 60 bpm
Note 50% increase in sRaw with increasing rates
Effect of breathing frequency and flows
a) 30/min sRtot = 1.0 kPa.s
b) 50/min sRtot = 1.5 kPa.s
Effect of Respiratory Frequency
Klug & Bisgaard 1997
Summary
• Preschool children rarely tolerate airway
occlusions: routine measures of FRCpleth not
feasible<6 years
• Specific resistance is easily measured during tidal
breathing in 3-5 year olds and may be highly
discriminative between health and disease
• Future work involves standardisation with regards
to equipment specification and data collection
techniques.