Transcript Document
The pulse wave
The speed of the wave increases as arterial stiffness increases
E p
2 .(
c
u
) 2 c is pulse wave velocity u is blood velocity (u< c E 2 p Increase stiffness by 2 increase wave speed of 1.4 Increase stiffness by 4 increase wave speed of 2 • 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 • Tonometry : Pressure wave – – – detection. Good sensitivity and time resolution. Very sensitive to arterial movement. Superficial arteries only. • Doppler ultrasound : Flow wave – Widely used. – Ability to detect deeper vessels. detection. • Photoplethysmography (PPG) : Diameter wave – – – High sensitivity and time resolution. Very easy to use. Superficial arteries only. detection. ( Eliakim et al. Am Heart J, 1971. 82: 448.) Infra red emitter Upstream probe Detector Downstream probe SKIN FLOW ARTERY MUSCLE/BONE Loukogeorgakis, et al. (2002). Physiological Measurement 23: 581-96. LED (emitter) Photo-transistor (detector) 20 mm (Units mm -1 ) Absorption Scatter SKIN 0.025 4.3 Source A B WALL - 0.025 3.5 BLOOD 0.6 0.15 Highly scattering region (skin and wall) Weakly scattering region (blood) • 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? • • • • • • • • • 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 1 PPG Ultrasound 0.1 PPG Ultrasound 0.01 400 300 200 100 0 0 2 4 6 Frequency (Hz) 8 10 PPG faithfully reproduces the diameter wave, when compared to high precision echo tracking system. 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? • • • • • • • • 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 Amplifier Multiplexer A/D converter CPU Comparison of PPG and Doppler transit times TT PPG [ms] 350 300 250 200 150 100 100 y = 0.90x + 12.8 r = 0.95 150 200 250 TT Doppler [ms] 300 Leg Arm 350 Doppler - PPG [ms] 50 25 0.0 -25 -50 0 100 200 Average [ms] 300 + 2SD - 2SD 400 Leg Arm • 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). 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? • 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 Pressur e Catheter Laboratory Electronics Amplifier Multiplexer A/D converter CPU T P1 ECG Pressure measurement pos. 1 Inguinal ligament Femoral arteriotomy T P1 Pressure measurement pos. 1 ECG Inguinal ligament T PPG PPG measurement pos. T P T PPG = T P2 -T P1 = T PPG -T P1 +T C PWV P PWV PPG T P2 Pressure measurement pos. 2 = D P / T P = D PPG / T PPG T C Femoral arteriotomy Comparison of PPG and intra-arterial transit times PPG transit time [ms] 110 100 90 80 70 60 50 40 40 y = 0.68x + 22, r = 0.66, P < 0.005 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 20 10 0.0 -10 -20 -30 40 50 60 70 80 90 Mean transit time [ms] 100 110 + 2SD - 2SD and PPG PWV [ms -1 ] 14 y = 0.77x + 1.9 r = 0.62, P < 0.005 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 2.0 0.0 -2.0 -4.0 6 8 10 Mean PWV [ms -1 ] y = 0.24x + 2.2, r = 0.23, P: NS 12 + 2SD - 2SD 14 • • • 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. • 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? Comparison of PPG with: Echo Tracking. Similar waveforms in radial & dorsalis pedis arteries show that PPG method does measure large artery diameter. PPG Doppler. PPG derived pulse wave transit times compare well with measurements using an established method. TT PPG [ms] 350 300 250 200 150 y = 0.90x + 12.8 r = 0.95 TT PPG [ms] 350 300 250 200 150 100 100 150 200 250 TT Doppler [ms] 300 U/S Arm Intra-arterial pressure wave. Do transcutaneous transit time and PWV measurements compare with intra-arterial ones? 100 100 150 Reasonably well 200 250 -1 ] 14 300 10 8 6 6 8 10 12 Intra-arterial PWV [ms -1 ] 350 14 • 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.How to measure wave speed
Methods for the measurement of PWV
Optical detection of the diameter wave
Validation experiments.
PPG/Echo Tracking - Methods.
PPG/Echo Tracking Conclusions.
Validation experiments.
PPG/Doppler Methods.
PPG/Doppler hardware.
Comparison of PPG and Doppler.
Difference v mean
PPG/Doppler - Conclusions.
Validation experiments.
Subjects
PPG/Intra-arterial hardware.
Comparison of PPG
intra-arterial pulse wave velocities
Some limitations of the study.
PPG/Intra-arterial - Conclusions.
Validation experiments.
Repeatability studies.