1. The pulse wave

2. The speed of the wave increases as arterial stiffness increases
c is pulse wave velocity
u is blood velocity (u<<c)
 is density of blood
Increase stiffness by 2 
increase wave speed of 1.4
Increase stiffness by 4 
increase wave speed of 2

3. 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

4. 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 detection.
Widely used.
Ability to detect deeper vessels.
Photoplethysmography (PPG) :Diameter wave detection.
High sensitivity and time resolution.
Very easy to use.
(Eliakim et al. Am Heart J, : 448.)

5. Optical detection of the diameter wave
Upstream
probe
Infra red emitter
Detector
Downstream
probe
SKIN
FLOW
ARTERY
MUSCLE/BONE
Loukogeorgakis, et al. (2002). Physiological Measurement 23:

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

7. Highly scattering region (skin and wall)
(Units mm-1)
Source A B
Highly scattering region (skin and wall)
Weakly scattering region (blood)

10. 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?

11. 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

12. PPG Ultrasound Frequency (Hz) 1 Relative amplitude 0.1 0.01 400 300
Phase
200
100 2 4 6 8 10
Frequency (Hz)

13. PPG/Echo Tracking - Conclusions.
PPG faithfully reproduces the diameter wave, when compared to high precision echo tracking system.

14. 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?

15. PPG/Doppler Methods. 10 subjects (age range 20 - 53).
Comparison of PPG and Doppler ultrasound estimates of pulse wave transit time.
10 subjects (age range ).
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.

16. PPG/Doppler hardware. PPG CPU Amplifier Multiplexer A/D converter ECG

17. Comparison of PPG and Doppler transit times
TT PPG [ms]
350
y = 0.90x r = 0.95
300
250
Leg
200
Arm
150
100
100
150
200
250
300
350
TT Doppler [ms]

18. Comparison of PPG and Doppler. Difference v mean
Doppler - PPG [ms] 50
+ 2SD 25
0.0
Leg
Arm
-25
- 2SD
-50
100
200
300
400
Average [ms]

19. 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).

20. 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?
YES!
Not bad!
Intra-arterial pressure wave.
Do transcutaneous transit time and PWV measurements compare with intra-arterial ones?

21. 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.

22. PPG/Intra-arterial hardware.
Amplifier
Multiplexer
A/D converter
CPU
ECG
Catheter Laboratory
Electronics
Pressure

23. TP1 ECG Pressure measurement pos. 1 Femoral arteriotomy
Inguinal ligament

24. TP = TP2-TP1 +TC TPPG = TPPG-TP1 PWVP = DP/ TP PWVPPG = DPPG/ TPPG
Pressure
measurement pos. 1
TP2
ECG TC
+TC
TP = TP2-TP1
TPPG = TPPG-TP1
PWVP = DP/ TP
PWVPPG = DPPG/ TPPG
PPG measurement
pos.
TPPG
Pressure
measurement pos. 2
Femoral arteriotomy
Inguinal ligament

25. Comparison of PPG and intra-arterial transit times
PPG transit time [ms]
110
y = 0.68x + 22, r = 0.66, P < 0.005
100 90 80 70 60 50 40 40 50 60 70 80 90
100
110
Intra arterial transit time [ms]

26. Comparison of PPG and intra-arterial transit times. Difference v mean
I.A. - PPG [ms] 30
+ 2SD 20 10
0.0
-10
-20
- 2SD
-30 40 50 60 70 80 90
100
110
Mean transit time [ms]

27. Comparison of PPG and intra-arterial pulse wave velocities
6.0
8.0 10 12 14
PPG PWV [ms-1] 6 8
Intra-arterial PWV [ms-1]
y = 0.77x r = 0.62, P < 0.005

28. Comparison of PPG and intra-arterial PWV. Difference v mean
PWV I.A. - PWV PPG [ms-1]
4.0
+ 2SD
2.0
0.0
y = 0.24x + 2.2, r = 0.23, P: NS
-2.0
- 2SD
-4.0 6 8 10 12 14
Mean PWV [ms-1]

29. 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.

30. 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?

31. 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
U/S
100
150
200
250
300
350
TT PPG [ms]
TT Doppler [ms]
y = 0.90x r = 0.95
Leg
Arm
100
150
200
250
300
350
TT PPG [ms]
TT Doppler [ms]
Doppler.
PPG derived pulse wave transit times compare well with measurements using an established method.
Intra-arterial pressure wave.
Do transcutaneous transit time and PWV measurements compare with intra-arterial ones? 6 8 10 12 14
PPG PWV [ms-1]
Intra-arterial PWV [ms-1]
Reasonably
well

32. 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.

33. Repeatability study methods
Study (A) 3-hourly intervals
5 volunteers - age range 20 to 47.
Volunteers allowed to relax supine for 15 minutes.
Initial blood pressure reading taken.
Proximal probe placed over right carotid artery and distal probe over:
the radial artery at the wrist (‘arm’ segment).
femoral artery, below the inguinal ligament (‘aortic’ segment).
below the medial malleolus of tibia (‘leg’ segment).
Recordings made for 30 seconds.
Measurements made 4 times at 3 hourly intervals starting at 9 am.

34. Repeatability study methods
Study (B) 10 minute intervals
10 female volunteers.
Protocol and measurement sites similar to the one used in the first study.
3 recording sessions made approximately 10 minutes apart.