Measurement of CV variables

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Measurement of CV variables
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• 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

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Measurement of blood pressure

• Invasive
• Pressure catheter and transducer
• Non invasive
• Sphygmomanometry
• Auscultation (by ear or automatically by
  microphone)
• Oscillometry
• Volume clamp
• Tonometry

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Advantages/ drawbacks

• 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

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Sphygmomanometry
Manometer (mercury or capsule type)
d
Pulse detector (stethoscope or microphone)
d 20
www.fmshk.com.hk/sahk/lecture_noninvasive.pp
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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)

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Sphygmomanometry
Capsule manometer Replacing mercury spymomanometer
Mercury sphygmomanometer
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Korotkov Soundscaused by vibrational collapse of
the arterial wall??

• 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
9
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

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Oscillometry

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

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Oscillometry set up
Micro- processor
Display
Air pump
Pressure transducer
Bleed valve
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Principle of oscillometry
Filtered signal Of cuff pressure
Variation of cuff pressure as cuff is deflated
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• 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

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Volume clamp
Infra red emitter
To pump
Detected signal
Pressure
Diameter
Finger
Air
Change cuff pressure
Air
Artery
Measure cuff pressure
Detector
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Applanation tonometry
Detects pressure of arterial pulsations through
the skin
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Problem

• Aortic and peripheral pressures are different.
• The heart doesnt 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?

Aortic
Radial
120
Systolic
100
Mean
Diastolic
80
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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?

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Fourier analysis
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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 - ?r
2) pr3Cos(?t - ?r 3) ...

For each harmonic (n) Transfer function phase
?an - ?rn Transfer function amplitude pan / prn
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Amplification of the pulse
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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

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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

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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 electronicall
y

• Advantages
• Cheap, disposable
• easy to use
• Accurate mean pressure

• Disadvantages
• Clotting in cannula, air bubbles
• Therefore errors in pulse pressure

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Pressure transducers(for invasive measurement -
2)

• Advantages
• High accuracy
• Especially in very small vessels

• Disadvantages
• No calibration possible when in position
• Expensive
• Fragile

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Pressure comparison of methods