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.

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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

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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 …

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FRC Measurement: Setup

Ultrasonic Flow / MM Sensor

Test gas supply

Valve in ‘open’ position

Valve in ‘closed’ position

Side-stream sampling tube

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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

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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

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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.