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.