The pulse wave
The speed of the wave increasesas arterial stiffness increases
c is pulse wave velocityu is blood velocity (u<<c) is density of blood
E p 2.(c u)2
c E p
2
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 detection.– Widely used.– Ability to detect deeper vessels.
• Photoplethysmography (PPG) :Diameter wave detection. – High sensitivity and time resolution.– Very easy to use. – Superficial arteries only.
(Eliakim et al. Am Heart J, 1971. 82: 448.)
Optical detection of the diameter wave
SKIN
ARTERYFLOW
Downstreamprobe
MUSCLE/BONE
Upstreamprobe
Infra red emitter Detector
Loukogeorgakis, et al. (2002). Physiological Measurement 23: 581-96.
20 mm
LED (emitter) Photo-transistor (detector)
Weakly scattering region (blood)
Highly scattering region (skin and wall)
Source A B
SKIN WALL BLOOD
Absorption 0.025 0.025 0.6
Scatter 4.3 3.5 0.15
(Units mm-1)
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
Rela
tive a
mp
litu
de
Ph
ase
0
100
200
300
400
0 2 4 6 8 10
Frequency (Hz)
0.01
0.1
1
Ultrasound
PPG
PPG
Ultrasound
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?
• Doppler.– How well do PPG derived pulse wave transit times compare
to measurements using an established method?
YES!
PPG/Doppler Methods.Comparison of PPG and Doppler ultrasound estimates of pulse wave transit time.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
ECG
Doppler
CPU
PPG/Doppler hardware.
Amplifier
Multiplexer
A/D converter
100
150
200
250
300
350
TT PPG [ms]
100 150 200 250 300 350
TT Doppler [ms]
y = 0.90x + 12.8 r = 0.95
Comparison of PPG and Doppler transit times
Leg
Arm
Comparison of PPG and Doppler.Difference v mean
-50
-25
0.0
25
50
Doppler - PPG [ms]
0 100 200 300 400
Average [ms]
+ 2SD
- 2SD
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?
• Doppler.– How well do PPG derived pulse wave transit times compare to
measurements using an established method?
YES!
• Intra-arterial pressure wave.– Do transcutaneous transit time and PWV measurements compare
with intra-arterial ones?
Not bad!
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
ECG
Pressure
Amplifier
Multiplexer
A/D converter
CPU
PPG/Intra-arterial hardware.
Catheter LaboratoryElectronics
Inguinal ligament
ECG
TP1 Pressuremeasurement pos. 1
Femoral arteriotomy
Inguinal ligament
Pressuremeasurement pos. 1
ECG
TP1
Pressuremeasurement pos. 2
TP2
PPG measurementpos.
TPPG
TP = TP2-TP1
TPPG = TPPG-TP1
PWVP = DP/ TP
PWVPPG = DPPG/ TPPG
TC
+TC
Femoral arteriotomy
40
50
60
70
80
90
100
110
PPG transit time [ms]
40 50 60 70 80 90 100 110
Intra arterial transit time [ms]
y = 0.68x + 22, r = 0.66, P < 0.005
Comparison of PPG and intra-arterial transit times
-30
-20
-10
0.0
10
20
30
I.A. - PPG [ms]
40 50 60 70 80 90 100 110
Mean transit time [ms]
+ 2SD
- 2SD
Comparison of PPG and intra-arterial transit times.Difference v mean
6.0
8.0
10
12
14
PPG PWV [ms-1]
6 8 10 12 14
Intra-arterial PWV [ms-1]
y = 0.77x + 1.9 r = 0.62, P < 0.005
Comparison of PPG and intra-arterial pulse wave velocities
+ 2SD
- 2SD
Comparison of PPG and intra-arterial PWV.Difference v mean
y = 0.24x + 2.2, r = 0.23, P: NS
-4.0
-2.0
0.0
2.0
4.0
PWV I.A. - PWV PPG [ms-1]
6 8 10 12 14
Mean PWV [ms-1]
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?
PPG
U/S
Validation experiments.
Echo Tracking. Similar waveforms in radial & dorsalis pedis arteries show that PPG method does measure large artery diameter.
100
150
200
250
300
350
TT PPG [ms]
100 150 200 250 300 350
TT Doppler [ms]
y = 0.90x + 12.8 r = 0.95
Leg
Arm
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?
Comparison of PPG with:
100
150
200
250
300
350
TT PPG [ms]
100 150 200 250 300 350
TT Doppler [ms]
6
8
10
12
14
PPG PWV [ms-1]
6 8 10 12 14Intra-arterial PWV [ms-1]
Reasonablywell
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.
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