Measurement of CV 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 Limited by technical problems –Measurement errors, transitory changes in diam. BP etc In vivo (non-invasive) Further technical problems –Especially pressure

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

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

Sphygmomanometry Pulse detector (stethoscope or microphone) Manometer (mercury or capsule type) Manometer (mercury or capsule type) d d + 20%

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)

Sphygmomanometry Capsule manometer Replacing mercury spymomanometer Mercury sphygmomanometer

Korotkov Sounds caused by vibrational collapse of the arterial wall?? Cuff pressure Systolic Diastolic –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

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

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

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

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

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

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

Applanation tonometry Detects pressure of arterial pulsations through the skin

Problem: AorticRadial 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? Systolic Diastolic Mean

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?

Fourier analysis H1 + H2 H H1 + H2 + H3 H4 Mean H1 H2 Measured H1+H2+H3+H4

Pa(t) = pa 0 + pa 1 Cos(  t -  a 1 ) + pa 2 Cos(  t -  a 2 ) + pa 3 Cos(  t -  a 3 ) +... Pb(t) = pr 0 + pr 1 Cos(  t -  r 1 ) + pr 2 Cos(  t -  r 2 ) + pr 3 Cos(  t -  r 3 ) +... For each harmonic (n) Transfer function phase =  a n -  r n Transfer function amplitude = pa n / pr n aortic pressureradial artery pressure

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

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, producing the measured radial wave

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

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

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

Pressure: comparison of methods