2The speed of the wave increases as arterial stiffness increases c is pulse wave velocityu is blood velocity (u<<c) is density of bloodIncrease stiffness by 2 increase wave speed of 1.4Increase stiffness by 4 increase wave speed of 2
3How to measure wave speed Detect pulse wave at two sites a known distance apartMeasure time it takes for the pulse wave to get from one site to the other (transit time,TT)Speed = distance/time
4Methods 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.)
5Optical detection of the diameter wave UpstreamprobeInfra red emitterDetectorDownstreamprobeSKINFLOWARTERYMUSCLE/BONELoukogeorgakis, et al. (2002). Physiological Measurement 23:
6Photo-transistor (detector) LED (emitter)Photo-transistor (detector)20 mm
7Highly scattering region (skin and wall) (Units mm-1)SourceABHighly scattering region (skin and wall)Weakly scattering region (blood)
10Validation experiments. Comparison of PPG withEcho 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?
11PPG/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
13PPG/Echo Tracking - Conclusions. PPG faithfully reproduces the diameter wave, when compared to high precision echo tracking system.
14Validation experiments. Comparison of PPG withEcho 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?
15PPG/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.
16PPG/Doppler hardware. PPG CPU Amplifier Multiplexer A/D converter ECG
17Comparison of PPG and Doppler transit times TT PPG [ms]350y = 0.90x r = 0.95300250Leg200Arm150100100150200250300350TT Doppler [ms]
18Comparison of PPG and Doppler. Difference v mean Doppler - PPG [ms]50+ 2SD250.0LegArm-25- 2SD-50100200300400Average [ms]
19PPG/Doppler - Conclusions. PPG transit times agree well with Doppler values recorded at the ‘same’ site.The difference plot showsthe difference between the PPG and the Doppler values is independent of the mean of each pair of estimatesthe 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).
20Validation experiments. Comparison of PPG withEcho 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?
21Subjects 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.
25Comparison of PPG and intra-arterial transit times PPG transit time [ms]110y = 0.68x + 22, r = 0.66, P < 0.005100908070605040405060708090100110Intra arterial transit time [ms]
26Comparison of PPG and intra-arterial transit times. Difference v mean I.A. - PPG [ms]30+ 2SD20100.0-10-20- 2SD-30405060708090100110Mean transit time [ms]
27Comparison of PPG and intra-arterial pulse wave velocities 6.08.0101214PPG PWV [ms-1]68Intra-arterial PWV [ms-1]y = 0.77x r = 0.62, P < 0.005
28Comparison of PPG and intra-arterial PWV. Difference v mean PWV I.A. - PWV PPG [ms-1]4.0+ 2SD2.00.0y = 0.24x + 2.2, r = 0.23, P: NS-2.0- 2SD-4.068101214Mean PWV [ms-1]
29Some limitations of the study. Non simultaneous measurement of proximal and distal signalsEthical constraint of one catheterProximal 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 investigatedCareful comparison between I.A. and external distance measurements required.
30PPG/Intra-arterial - Conclusions. Good correlation between intra-arterial and PPG transit times and pulse wave velocities.Mean difference between the two methods close to zeroSlight 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?
31Validation experiments. Comparison of PPG with:Echo Tracking.Similar waveforms in radial & dorsalis pedis arteries show that PPG method does measure large artery diameter.PPGU/S100150200250300350TT PPG [ms]TT Doppler [ms]y = 0.90x r = 0.95LegArm100150200250300350TT 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?68101214PPG PWV [ms-1]Intra-arterial PWV [ms-1]Reasonablywell
32Repeatability 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.
33Repeatability study methods Study (A) 3-hourly intervals5 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.
34Repeatability study methods Study (B) 10 minute intervals10 female volunteers.Protocol and measurement sites similar to the one used in the first study.3 recording sessions made approximately 10 minutes apart.