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The pulse wave. The speed of the wave increases as arterial stiffness increases c is pulse wave velocity u is blood velocity (u<

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Presentation on theme: "The pulse wave. The speed of the wave increases as arterial stiffness increases c is pulse wave velocity u is blood velocity (u<

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<

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. –Superficial arteries only. ( Eliakim et al. Am Heart J, : 448.)

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

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

7 Weakly scattering region (blood) Highly scattering region (skin and wall) SourceAB (Units mm -1 )

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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 Relative amplitude Phase Frequency (Hz) Ultrasound PPG Ultrasound

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

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

17 TT PPG [ms] TT Doppler [ms] y = 0.90x r = 0.95 Comparison of PPG and Doppler transit times Leg Arm

18 Comparison of PPG and Doppler. Difference v mean Doppler - PPG [ms] Average [ms] + 2SD - 2SD Leg Arm

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

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

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

24 Inguinal ligament Pressure measurement pos. 1 ECG T P1 Pressure measurement pos. 2 T P2 PPG measurement pos. T PPG  T P = T P2 -T P1  T PPG = T PPG -T P1 PWV P = D P /  T P PWV PPG = D PPG /  T PPG TCTC +T C Femoral arteriotomy

25 PPG transit time [ms] Intra arterial transit time [ms] y = 0.68x + 22, r = 0.66, P < Comparison of PPG and intra-arterial transit times

26 I.A. - PPG [ms] Mean transit time [ms] + 2SD - 2SD Comparison of PPG and intra-arterial transit times. Difference v mean

27 PPG PWV [ms -1 ] Intra-arterial PWV [ms -1 ] y = 0.77x r = 0.62, P < Comparison of PPG and intra-arterial pulse wave velocities

28 + 2SD - 2SD Comparison of PPG and intra-arterial PWV. Difference v mean y = 0.24x + 2.2, r = 0.23, P: NS PWV I.A. - PWV PPG [ms -1 ] 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 PPG U/S Validation experiments. Echo Tracking. Similar waveforms in radial & dorsalis pedis arteries show that PPG method does measure large artery diameter TT PPG [ms] TT Doppler [ms] y = 0.90x 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: TT PPG [ms] TT Doppler [ms] 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 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. Repeatability study methods

34 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. Repeatability study methods

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