Transcript Document

The pulse wave

The speed of the wave increases
as arterial stiffness increases
E p  2.(c  u)
c
2
c is pulse wave velocity
u is blood velocity (u<<c)
 is density of blood
Ep
Increase stiffness by 2 
increase wave speed of 1.4
2
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
Upstream
probe
Infra red emitter
Downstream
probe
Detector
SKIN
FLOW
ARTERY
MUSCLE/BONE
Loukogeorgakis, et al. (2002). Physiological Measurement 23: 581-96.
LED (emitter)
Photo-transistor (detector)
20 mm
(Units mm-1)
SKIN
Absorption
0.025
Scatter
4.3
Source
A
WALL
BLOOD
- 0.025
0.6
3.5
0.15
B
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
PPG
Relative amplitude
1
PPG
Ultrasound
0.1
0.01
400
Ultrasound
Phase
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
Amplifier
ECG
Multiplexer
A/D converter
Doppler
CPU
Comparison of PPG and Doppler transit times
TT PPG [ms]
y = 0.90x + 12.8 r = 0.95
350
300
250
Leg
200
Arm
150
100
100
150
200
250
TT Doppler [ms]
300
350
Comparison of PPG and Doppler.
Difference v mean
Doppler - PPG [ms]
50
+ 2SD
25
Leg
0.0
Arm
-25
- 2SD
-50
0
100
200
Average [ms]
300
400
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
Amplifier
ECG
Pressur
e
Multiplexer
Catheter
Laboratory
Electronics
A/D converter
CPU
TP1
Pressure
measurement pos. 1
ECG
Femoral arteriotomy
Inguinal ligament
TP1
Pressure
measurement pos. 1
ECG
TP
TPPG
= TP2-TP1 +TC
= TPPG-TP1
PWVP = DP/ TP
PWVPPG = DPPG/ TPPG
TP2
TPPG
Inguinal ligament
PPG measurement
pos.
Pressure
measurement pos. 2
TC
Femoral arteriotomy
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
Intra arterial transit time [ms]
110
Comparison of PPG and intra-arterial transit times.
Difference v mean
I.A. - PPG [ms]
30
+ 2SD
20
10
0.0
-10
-20
-30
40
- 2SD
50
60
70
80
90
Mean transit time [ms]
100
110
Comparison of PPG and intra-arterial pulse
wave velocities
PPG PWV [ms-1]
y = 0.77x + 1.9 r = 0.62, P < 0.005
14
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
+ 2SD
2.0
0.0
y = 0.24x + 2.2, r = 0.23, P: NS
-2.0
- 2SD
-4.0
6
8
10
Mean PWV [ms-1]
12
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
TT PPG [ms]
350
Doppler.
PPG derived pulse wave transit times
compare well with measurements using
an established method.
y = 0.90x + 12.8TT PPGr[ms]
= 0.95
350
300
300
250
200
250
150
100
100
200
150
200
Intra-arterial pressure wave.
300
U/S
150
100
100
250
TT Doppler [ms]
150
200
250
PPG PWV [ms-1]
300
Leg
350
Arm
350
14
TT Doppler12[ms]
Do transcutaneous transit time and PWV Reasonably
measurements compare with intra-arterial well
ones?
10
8
6
6
8
10
12
Intra-arterial PWV [ms-1]
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.