The pulse wave



The speed of the wave increases as arterial stiffness increases

E p

 2  .(

c



u

) 2 c is pulse wave velocity u is blood velocity (u<

c



E

2 

p

Increase stiffness by 2  increase wave speed of 1.4

Increase stiffness by 4  increase wave speed of 2 

How to measure wave speed

• Detect pulse wave at two sites a known distance apart • Measure time it takes for the pulse wave to get from one site to the other (transit time,TT) • Speed = distance/time

Methods for the measurement of PWV

• Tonometry : Pressure wave – – – detection. Good sensitivity and time resolution. Very sensitive to arterial movement.

Superficial arteries only. • Doppler ultrasound : Flow wave – Widely used.

– Ability to detect deeper vessels.

detection.

• Photoplethysmography (PPG) : Diameter wave – – – High sensitivity and time resolution.

Very easy to use. Superficial arteries only.

detection.

(

Eliakim et al. Am Heart J, 1971. 82: 448.)

Optical detection of the diameter wave

Infra red emitter Upstream probe Detector Downstream probe SKIN FLOW ARTERY MUSCLE/BONE Loukogeorgakis, et al. (2002).

Physiological Measurement

23: 581-96.

LED (emitter) Photo-transistor (detector) 20 mm

(Units mm -1 ) Absorption Scatter SKIN 0.025

4.3

Source A B

WALL - 0.025

3.5

BLOOD 0.6

0.15

Highly scattering region (skin and wall) Weakly scattering region (blood)

Validation experiments.

•

Comparison of PPG with

Echo Tracking.

– Does PPG method really measure diameter?

• Doppler.

– How well do PPG derived pulse wave transit times compare to measurements using an established method?

• Intra-arterial pressure wave.

– Do transcutaneous transit time and PWV measurements compare with intra-arterial ones?

PPG/Echo Tracking - Methods.

• • • • • • • • • 6 subjects (age range 20-47).

Subjects lay supine.

ECG reference signal obtained by a two electrode chest strap.

Diameter was measured at the radial artery using an high precision echo tracking ultrasound system (NIUS, Omega Electronics, Switzerland).

PPG probe positioned between 5 and 10 mm distal to the ultrasonic probe.

Simultaneous recordings taken for 30 seconds.

Procedure repeated with both probes placed over the right dorsalis pedis artery.

Time measured between ECG R wave and ‘foot’ of the diameter wave.

Timing and shape of curve from each probe compared by Fourier analysis

1

PPG Ultrasound

0.1

PPG Ultrasound

0.01

400 300 200 100 0 0 2 4 6

Frequency (Hz)

8 10

PPG/Echo Tracking Conclusions.

PPG faithfully reproduces the diameter wave, when compared to high precision echo tracking system.

Validation experiments.

Comparison of PPG with

• Echo Tracking.

– Does PPG method really measure diameter?

YES!

• Doppler.

– How well do PPG derived pulse wave transit times compare to measurements using an established method?

PPG/Doppler Methods.

• • • • • • • •

Comparison of PPG and Doppler ultrasound estimates of pulse wave transit time.

10 subjects (age range 20 - 53).

Subjects lay supine.

ECG reference signal obtained by a two electrode chest strap.

8 MHz pencil type Doppler probe placed over the left radial artery at the wrist.

PPG probe placed < 5mm proximal to Doppler probe Simultaneous recordings made for 20 seconds.

.

Procedure repeated with probes placed over the left dorsalis pedis artery.

Time delay between ECG R wave and the ‘foot’ of the Doppler and PPG waves was measured.

PPG/Doppler hardware.

PPG ECG Doppler Amplifier Multiplexer A/D converter CPU

Comparison of PPG and Doppler transit times

TT PPG [ms]

350 300 250 200 150 100 100 y = 0.90x + 12.8 r = 0.95

150 200 250

TT Doppler [ms]

300 Leg Arm 350

Comparison of PPG and Doppler.

Difference v mean

Doppler - PPG [ms]

50 25 0.0

-25 -50 0 100 200

Average [ms]

300 + 2SD - 2SD 400 Leg Arm

PPG/Doppler - Conclusions.

• PPG transit times agree well with Doppler values recorded at the ‘same’ site.

• The difference plot shows – the difference between the PPG and the Doppler values is independent of the mean of each pair of estimates – the transit time estimated by the Doppler instrument is consistently greater than that derived from the PPG signals (mean difference 8.6 ms) • The discrepancy may be due to the Doppler signal processing – (further experiments will test this).

Validation experiments.

Comparison of PPG with

• Echo Tracking.

– Does PPG method really measure diameter?

YES!

• Doppler.

– How well do PPG derived pulse wave transit times compare to measurements using an established method?

Not bad!

• Intra-arterial pressure wave.

– Do transcutaneous transit time and PWV measurements compare with intra-arterial ones?

Subjects

• 21 subjects (8 female).

• Age range 33 to 78 years, (mean 57 years).

• Measurements in all subjects were performed after routine coronary angiography, under the approval of the regional research ethics committee.

PPG/Intra-arterial hardware.

PPG ECG Pressur e Catheter Laboratory Electronics Amplifier Multiplexer A/D converter CPU

T P1 ECG Pressure measurement pos. 1 Inguinal ligament Femoral arteriotomy

T P1

Pressure measurement pos. 1

ECG

Inguinal ligament

T PPG PPG measurement pos.



T P



T PPG = T P2 -T P1 = T PPG -T P1 +T C PWV P PWV PPG T P2 Pressure measurement pos. 2 = D P /



T P = D PPG /



T PPG T C

Femoral arteriotomy

Comparison of PPG and intra-arterial transit times

PPG transit time [ms]

110 100 90 80 70 60 50 40 40 y = 0.68x + 22, r = 0.66, P < 0.005 50 60 70 80 90 100

Intra arterial transit time [ms]

110

Comparison of PPG and intra-arterial transit times.

Difference v mean

I.A. - PPG [ms]

30 20 10 0.0

-10 -20 -30 40 50 60 70 80 90

Mean transit time [ms]

100 110 + 2SD - 2SD

Comparison of PPG

and

intra-arterial pulse wave velocities

PPG PWV [ms -1 ]

14 y = 0.77x + 1.9 r = 0.62, P < 0.005

12 10 8.0

6.0

6 8 10 12

Intra-arterial PWV [ms -1 ]

14

Comparison of PPG and intra-arterial PWV.

Difference v mean

PWV I.A. - PWV PPG [ms -1 ]

4.0

2.0

0.0

-2.0

-4.0

6 8 10

Mean PWV [ms -1 ]

y = 0.24x + 2.2, r = 0.23, P: NS 12 + 2SD - 2SD 14

Some limitations of the study.

• • • Non simultaneous measurement of proximal and distal signals – Ethical constraint of one catheter Proximal signal not transcutaneous – ‘Hybrid’ measurements will avoid this. i.e. aortic signal from Doppler, distal signal from PPG.

– Current hardware and software will allow this.

Effect of errors in distance between measurement sites not investigated – Careful comparison between I.A. and external distance measurements required.

PPG/Intra-arterial - Conclusions.

• Good correlation between intra-arterial and PPG transit times and pulse wave velocities.

• Mean difference between the two methods close to zero • Slight but non-significant tendency for difference between I.A. and PPG to increase with increasing PWV.

• Transcutaneous estimation of pulse wave transit time provides an acceptable estimate of its intra-arterial value.

– Differences due to errors in external length measurement?

Validation experiments.

Comparison of PPG with:

Echo Tracking. Similar waveforms in radial & dorsalis pedis arteries show that PPG method does measure large artery diameter.

PPG Doppler.

PPG derived pulse wave transit times compare well with measurements using an established method.

TT PPG [ms]

350 300 250 200 150

y = 0.90x + 12.8 r = 0.95

TT PPG [ms]

350 300 250 200 150 100 100 150 200 250

TT Doppler [ms]

300 U/S Arm Intra-arterial pressure wave.

Do transcutaneous transit time and PWV measurements compare with intra-arterial ones?

100 100 150

Reasonably well

200 250

-1 ]

14 300 10 8 6 6 8 10 12

Intra-arterial PWV [ms -1 ]

350 14

Repeatability studies.

• Variation of aortic PWV over different time scales – – (A) 3 separate recording sessions 10 minutes apart.

(B) 4 separate recording sessions made at three hourly intervals.