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In vitro Total control of confounding variables

In vitro Total control of confounding variables Vasomotion, temperature changes, autoregulation, mean BP Most accurate because vessel examined directly Best for detailed information about mechanical properties of vessel material In vivo (invasive) Realistic clinical information

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In vitro Total control of confounding variables

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  1. In vitro • Total control of confounding variables • Vasomotion, temperature changes, autoregulation, mean BP • Most accurate because vessel examined directly • Best for detailed information about mechanical properties of vessel material In vivo (invasive) • Realistic clinical information • Limited by technical problems • Measurement errors, transitory changes in diam. BP etc In vivo (non-invasive) • Further technical problems • Especially pressure

  2. Measurement of blood pressure • Invasive • Pressure catheter and transducer • Non invasive • Sphygmomanometry • Auscultation (by ear or automatically by microphone) • Oscillometry • Volume clamp • Tonometry

  3. Invasive Accurate reproduction of central pressure waveforms Risk of thrombosis and arrhythmias Non-invasive Quick, cheap, widely used Lack of central pressure measurement Requires skilled and experienced operators Advantages/ drawbacks

  4. Sphygmomanometry Manometer (mercury or capsule type) Pulse detector (stethoscope or microphone) www.fmshk.com.hk/sahk/lecture_noninvasive.pp

  5. Sphygmomanometry • 1896 Blood pressure cuff (Riva Rocci) • 1905 First report of audible detection of heart sounds used with cuff (Korotkov) • 1968 Microphone used for automatic pressure measurement (Stegall)

  6. Sphygmomanometry Capsule manometer Replacing mercury spymomanometer Mercury sphygmomanometer

  7. Korotkov Soundscaused by vibration collapse of the arterial wall?? • Korotkoff IV is a better indication of diastolic pressure according to theory • However Korotkoff V is the commonly recommended measuring point except in pregnant patients because • It is associated with less inter-observer variations • It is easier to detect by most observers Systolic Cuff pressure Diastolic www.fmshk.com.hk/sahk/lecture_noninvasive.pp

  8. Errors • Korotkoff sounds compared to invasive blood pressure measurement • Korotkoff IV is on average 8mm Hg above the invasively measured diastolic blood pressure • Korotkoff V is on average 2mm Hg above the invasively measured diastolic blood pressure

  9. Oscillometry • Cuff round the arm • Pressurise cuff (> systolic) • Allow pressure to drop slowly to zero • Measure pressure in the cuff during deflation

  10. Oscillometry: set up Micro- processor Display Air pump Pressure transducer Bleed valve

  11. Principle of oscillometry Filtered signal Of cuff pressure Variation of cuff pressure as cuff is deflated

  12. Limitations • Inaccurate / unreliable in shock patients • Inaccurate / unreliable in patients with arrhythmias • The algorithm of measurement assumes a regular pulse, so the reading is unreliable in patients with irregular pulse Advantages • No skill required • No subjective errors

  13. Volume clamp Infra red emitter To pump Detected signal Pressure Diameter Finger Air Change cuff pressure Air Artery Measure cuff pressure Detector

  14. Applanation tonometry Detects pressure of arterial pulsations through the skin

  15. Aortic and peripheral pressures are different. The heart doesn’t care what the pressure is in the radial artery. It only “sees” aortic pressure. Aortic pressure is difficult (impossible?) to measure non-invasively Can we reconstruct the aortic waveform from the radial? Problem: Radial Aortic 120 Systolic 100 Mean Diastolic 80

  16. Yes we can. At least in principle • Record radial waveform with tonometry • Apply inverse transfer function • “Reconstruct” aortic waveform • What is an inverse transfer function? • How do we reconstruct the waveform?

  17. H1 H1 + H2 H2 H3 H1 + H2 + H3 H1+H2+H3+H4 2 Mean H4 Measured 1 0 90 180 270 360 -1 -2 Fourier analysis

  18. aortic pressure radial artery pressure Pa(t) = pa0 + pa1Cos(t - a1) + pa2Cos(t - a2) + pa3Cos(t - a3) + ... Pb(t) = pr0 + pr1Cos(t - r1) + pr2Cos(t - r2) + pr3Cos(t - r3) + ... For each harmonic (n) Transfer function phase = an - rn Transfer function amplitude = pan / prn

  19. AA - RA CA - RA AA - CA Amplification of the pulse

  20. How to derive the central pressure from peripheral measurements • Compare Fourier series of “typical” aortic pressure waves with Fourier series of the radial pressure computed from tonometric measurements. • Calculate the amplitude ratio and phase difference for each harmonic • Apply this ratio and phase difference to each harmonic of the measured radial wave and reconstruct aortic wave that would when transmitted down the arm, produce the measured radial wave

  21. Question • How well does the typical transfer function apply to people of different ages and disease states Answer • Surprisingly well considering the changes that occur in the arterial system with age and vascular disease • However, most believe that more work is needed to validate the method

  22. Pressure transducers(for invasive measurement) Diaphragm manometer To pressure to be measured, (via an intra arterial cannula) Fluid filled chamber Stiff diaphragm Measure its movement electronically • Advantages • Cheap, disposable • easy to use • Accurate mean pressure • Disadvantages • Clotting in cannula, air bubbles • Therefore errors in pulse pressure

  23. Cannula tip manometer Semi conducting strain gauge Diameter may be as small as 0.67 mm Pressure transducers(for invasive measurement - 2) • Advantages • High accuracy • Especially in very small vessels • Disadvantages • No calibration possible when in position • Expensive • Fragile

  24. Pressure: comparison of methods

  25. Flow Measurement • Invasive • Electromagnetic flow velocimetry • Ultrasonic transit time • Non invasive • Doppler ultrasound • Ultrasonic transit time • Optical (small superficial vessels only) • MRI

  26. Flow measurement • 1870 Fick principle described Flow in a given period of time = Amount of substance injected in that time/concentration difference before and after point of entry • 1886 Fick method first used by Grehart & Quinquardt • Modern instruments • Optical • Electromagnetic 1936-1937 Kolin • Ultrasonic transit time 1959 • Ultrasonic Doppler 1961 • MRI 1990’s (not commercial)

  27. Vessel diameter E = H.d.V Mean blood velocity Induced voltage Magnetic field strength Electrode i.d. 0.5 - 26 mm

  28. c l Principle of Doppler flow velocimetry c c + v v l = = ' c f = l ' f f ' cf ( c v ) f = + ' cf cf vf = + ' f f v - = f c vf f D = c

  29. Flow: comparison of methods

  30. Diameter Measurement • Mechanical • Optical • Ultrasonic • Implanted crystals • Pulse echo • Cine-angiography • MRI

  31. Invasive Diameter Measurement • Ultrasound (external transducers) • IVAS • TV • Mechanical • Cine angiography

  32. Non-invasive diameter measurement • Pulse echo ultrasound (direct) • PWV (indirect) • Diameter wave • Flow wave • Pressure wave

  33. Springy stainless steel Differential transformer Artery Other diameter methods TV camera Ultrasonic crystals (glued or sutured) Measure time delay Transmitter Receiver

  34. Principle of pulse echo ultrasound Measure time delay between transmitted and received pulse

  35. Diameter: comparison of methods

  36. Elasticity measurement • Direct • Stress • pressure, tension, area, wall thickness • Strain • length, diameter • Indirect • Pulse wave velocity • detect pressure, diameter or flow pulse

  37. PWV Methods • Pressure pulse • Tonometry • Flow pulse • Doppler • Diameter Pulse • PPG

  38. Nature of the PPG Signal • Commonly regarded as a measure of changes in tissue volume due to arteriolar and capillary blood flow  time varying absorption of light or i.r. • When detected in the vicinity of a large superficial artery, the signal is dominated by changes in the diameter (volume) of the artery.

  39. Upstream probe Downstream probe Infra red emitter Detector Optical detection of the diameter wave SKIN ARTERY FLOW MUSCLE/BONE Loukogeorgakis, et al. (2002). Physiological Measurement 23: 581-96.

  40. d t PhotoPlethysmoGraphy (PPG) for pulse wave velocity measurement. How does it work? • Infra red probes detect transitory change in conduit artery volume due to the passage of the pulse wave • Measure time delay and distance between the probes • Pulse wave velocity = d/t • Pulse wave velocity  (compliance)-1/2

  41. LED (emitter) Photo-transistor (detector) 20 mm 20 mm

  42. Validation experiments. Comparison of PPG with • Echo Tracking. • Does PPG method really measure large artery 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 measurements compare with intra-arterial ones?

  43. PPG/Echo tracking methods Probes on the posterior tibial artery NIUS ultrasound probe PPG PPG NIUS ultrasound probe Probes on the radial artery

  44. 1 PPG Ultrasound PPG Relative amplitude 0.1 Ultrasound 0.01 400 300 Phase 200 100 0 0 2 4 6 8 10 Frequency (Hz)

  45. PPG/Echo Tracking - Conclusions. • PPG reproduces the diameter wave with reasonable fidelity, when compared to high precision echo tracking system. • Timing of the foot is close

  46. 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?

  47. Probes on the posterior tibial artery Doppler PPG PPG Doppler ECG PPG Doppler Probes on the radial artery ECG used as time reference PPG/Doppler methods

  48. Comparison of PPG and Doppler transit times TT PPG [ms] y = 0.90x + 12.8 r = 0.95 350 300 250 Leg 200 Arm 150 100 100 150 200 250 300 350 TT Doppler [ms]

  49. Doppler - PPG [ms] 50 25 0.0 -25 -50 0 100 200 300 400 Average [ms] Comparison of PPG and Doppler.Difference v mean + 2SD Mean difference = 8.6 ms Leg Arm - 2SD

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