ERS 2007 - 1/19
What is ultrasonic plethysmographyand how can we use it
in airway disease?
Christian Buess
ndd Medizintechnik AG, Zürich, Switzerland
ERS 2007 - 2/19
Background
Until approx. the year 2000 mainly the following flow sensor principles were used in lung function testing:
Differential pressure (variable or fixed resistance)Turbines (rotation speed)Heated wires (cooling effect)
All these principles have potential limitations:
Hygiene: Difficult to cleanUnable to measure very low flowsNeed of regular calibrationDependency on gas composition and/or temperature
ERS 2007 - 3/19
Operating Principle
Exchangeable Flow Tube
Sound Transparent
Windows
Integrated Mouthpiece
Ultrasonic Transducer
Ultrasonic flow measurement is commonly used for liquid flow.It is now also used for air flow measurement in lung function diagnostics.
ERS 2007 - 4/19
Operating Principle
Transmission of Ultrasonic Pulse
Sound Trans-mission Path
Sound Receiver
Ultrasonic transit-time measurement. Please note that the principle differs to the Doppler principle used for blood flow measurement.
ERS 2007 - 5/19
Operating Principle
Transmission of Ultrasonic Pulse
Sound ReceiverSound transmission and receptionis repeated every 1 to 5 ms.Sampling rates 200 to 1000 Hz.
ERS 2007 - 6/19
Measurement principle (simple)
New York Zürich
7 Hours
9 Hours
Time difference is proportional to speed of Jet stream
Ultrasonic flow sensor: Time difference up to 1 μs
ERS 2007 - 7/19
ϕcos⋅−=
ucLtd
ϕcos⋅+=
ucLtu
upstreamtransit-time
(Zürich – NY)
downstreamtransit-time(NY – Zürich)
Flow measurement depends only on flow and geometryNo dependency on temperature, gas composition, humidity or pressure
ud
ud
ttttLu⋅−
⋅⋅
=ϕcos2
Flow
Measurement principle: Flow
Speed of sound depends on temperature and gas composition
ERS 2007 - 8/19
ϕcos⋅−=
ucLtd
ϕcos⋅+=
ucLtu
Molar mass measurement depends on transit-times andtemperature along the sound transmission pathMolar mass measurement ‘for free’ – no additional sensor requiredMolar mass measurement is not specific
Molar Mass
MR
LT
t tt t
u d
u d= ⋅ ⋅
⋅+
⎛⎝⎜
⎞⎠⎟
42
2κ
Measurement principle: Molar Mass
MTRc ⋅⋅
=κspeed of
sound
upstreamtransit-time
downstreamtransit-time
ERS 2007 - 9/19
Strengths and Weaknesses
No calibration since flow measurement depends only on time measurement and geometry of sensor
Completely open, unobstructed tube
Good hygienic solution
High resolution and dynamic range
Molar mass measurement “for free”
Relatively high cost of sensor
ERS 2007 - 10/19
Spirometry: DevicesExamples of devices using ultrasound technology
Standalonespirometer
for office use
Flow sensors for spirometry and exercise testing
Flow sensor for Intensive Care
application
ERS 2007 - 11/19
ApplicationThe technology can be used for a large range
HorsesResolution: < 80 ml/sRange: 100 l/s
NeonatologyResolution: < 1 ml/sRange: 500 ml/s
AdultResolution: < 10 ml/sRange: 18 l/s
ERS 2007 - 12/19
Spirometry: Studies
Performance of 70 devices over 3 to 6 month periodR. Pérez-Padilla et al.: Long-term stability of portable spirometers. (Respir Care 2006)
Performance of 6 devices over 200 daysJ. Walters et al.: Stability of the EasyOne ultra-sonic spirometer for use in general practice. (Respirology 2006)
Ultrasonic spirometer used by world-wide BOLD study (Burden of Obstructive Lung Disease).Ultrasonic spirometer used by Platino study in Latin America.
ERS 2007 - 13/19
FRC Measurement: BackgroundMeasurement of FRC using multiple breath wash-out / wash-in of a tracer gas by measuring molar mass of in- and expired gas flow.
Main stream molar mass signal is synchronous with flow signalSimple instrument setup compared to mass spectrometryBest results are only achieved with side-stream MM sensor
start of tracer wash-in
expiratory side-stream MM trace
wash-in of tracer gas
Computation of FRC, LCI …
ERS 2007 - 14/19
FRC Measurement: Setup
Ultrasonic Flow / MM Sensor
Test gas supply
Valve in ‘open’ position
Valve in ‘closed’ position
Side-stream sampling tube
ERS 2007 - 15/19
FRC Measurement: StudiesDirect comparison of ultrasonic system with mass spectrometerFuchs et al., Pediatr Pulmol, 2006, 41.
r2 = 1.00, mean difference -0.2%
Setup Problems: Gas leaks etc.Most devices and most software are in prototype stage!The above results were obtained using a side-stream MM sensor If applied carefully results compare well with mass spectrometersAdditional studies: J. Pillow et al., Schibler et al. etc.
ERS 2007 - 16/19
Tidal Breathing Analysis: BackgroundComparable to capnography the molar mass analysis of tidal breathing
shows the expiratory molar mass curve vs. expired volume.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0.0 0.2 0.4 0.6 0.8 1.0
Volumen [l]
Trial 1Trial 2Trial 3
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0.0 0.2 0.4 0.6 0.8 1.0
Volumen [l]
Trial 1Trial 2Trial 3
Several parameters can be computed from the curve data:Slope of phase 2 S2Slope of phase 3 S3Dead Space volume Vd
S2
S3
Vd
healthy control patient with COPD
ERS 2007 - 17/19
Tidal Breathing Analysis: Method
Statistical Analysis
90 seconds of tidal breathingrecording flowand molar mass
{S2,S3,Vd, Vi,Ve,f,…}1
{S2,S3,Vd, Vi,Ve,f,…}2
{S2,S3,Vd, Vi,Ve,f,…}3
{S2,S3,Vd, Vi,Ve,f,…}4
{S2,S3,Vd, Vi,Ve,f,…}5
{S2,S3,Vd, Vi,Ve,f,…}6
{S2,S3,Vd, Vi,Ve,f,…}7
{S2,S3,Vd, Vi,Ve,f,…}8
{S2,S3,Vd, Vi,Ve,f,…}9
Determination of mean expiratory MM curve
S2, S3, Vd Diagnosis
ERS 2007 - 18/19
Tidal Breathing Analysis: Studies
MMI
0
5
10
15
20
25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Studies analyzing the Molar Mass Index in side-streamMMI = S3 / S2 ·100
Normal controlsVery severe to moderate COPDRestricted
Beckert, Buess, Magnussen: Not yet published
Fuchs, Gaultier, Buess, Gappa: ERS 2007.
Correlation of MMI and LCI, both determined by ultrasonic flow sensor
ERS 2007 - 19/19
Conclusion
Ultrasonic flow measurement is a new technology with many advantages: No calibration, good hygienic solution and high accuracy
Ultrasonic molar mass measurement is the mass spectrometer for everybody – its use may have a large potential for measuring multi-breath FRC.
Analysis of expiratory molar mass curve during quiet breathing: Potentially interesting for analysis of gas distribution in the lungs. Further investigations are required.
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