4. Indirect Pressure • Blood pressure measurement – Intermittent mode • Korotkoff (auscultatory) method • Oscillometric method • Ultrasound Doppler method – Continuous mode • Tonometry method • Unloading (volume compensation) method • Pulse wave transit time (pulse arrival time) method • Intraocular pressure measurement • Intracranial pressure measurement 1 Riva-Rocci Method Riva-Rocci, 1896 Brachial artery Ulnar artery Radial artery When the cuff pressure is higher than SBP, artery is closed, no pulse can be sensed When the cuff pressure drops below SBP, blood can pass through the artery and the pulse will be felt The pulse will continue to be felt as the pressure in the cuff falls down to zero 2 Characteristics of Korotkoff Sounds http://en.wikipedia.org/wiki/Nikolai_Korotkov Phase 1 2 3 4 5 time Fundamental sound frequencies (Hz) Relative sound intensities Cuff pressure (mmHg) Faint clear tapping sounds Silence Murmur-like sounds Thumping sounds Thumping Sudden muffling Thumping Silence Korotkoff, 1905 3 Korotkoff Method Stethoscope 4
When a flat surface is pressedagainst an object that has aflexible boundary on theoutside and is filled with fluid,the internal pressure P can bemeasured by the force Fexerted on the plane and thecontact area A
P=F/A
P=F/A
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Guard-ring Tonometry
P=F/A
F is measured by the centralpart of the contact surface, andthe surface on the guard ring ismaintained in the same planeas the measurement surface.The major advantage is thatalthough a deformation at theedge of the contact surfacecauses elastic force, it does notaffect the force on themeasurement surface located atthe center.
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Multiple-element TonometryBecause the range of the planar region is less than the arterial diameter, thusthe positioning of the force sensor is critical.An array of force sensors provide the best signal by selecting one of the propersensor cells even if the position of the probe is shifted to some extent.
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Continuous BP Measurement by Tonometric Monitor
Brachial arteryDirect BP
TonometerIndirect BP
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Vascular Unloading Method
when the blood in the finger increases, light absorption will increase and the transmitted light will decrease, driving signal will become larger to inflate finger cuff in order to follow the BP increase.
-
+
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Open-Close Loop Operation
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Start Gain adjustment
Close loop operationOpen loop operation
transmural pressure
continuous waveform
maximum oscillation point
mean pressure
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Pulse Wave PropagationChanges in blood pressure & flow at the ascending aorta propagate throughout the cardiovascular network in the form of waves pulse waves
Systole
Diastole
Peripheralresistance
Peripheralresistance
propagation of vibration rather than the blood
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Hydromechanics Theory• Pulse wave velocity depends on blood velocity and
wave velocity
• Blood velocity depends on pressure gradient, vessel diameter and blood viscosity
• Wave velocity depends on elasticity of vessel wall and blood density
vuv
d
aPEpA
Av
1
22
14 R
rxpRru
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PWV and BP Pulse Pressure, Mean Pressure
ds ppds e
RhE
LppRv
20
2
128
pm pp
p eRhEp
LRv 6
10
2
128
Systolic Pressure, Diastolic Pressure
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Pulse Wave Transit Time (PWTT)
PWTT
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PWTT and BP
PEP = pre-ejection period = The interval in the ECG between the onset of the QRS peak and cardiac ejection
Higher BP shorter PWTT Lower BP longer PWTT
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• PWTT and PWV have inversely proportional relationship
• During short term period, vessel elasticity （E0、γ） keeps constant. Systolic BP and PWV have same change tendency
• Change in PWTT can be used to estimate fast component in systolic BP
• Intermittent BP calibration can be used to estimate slow component in systolic BP
PWTT BasisAge dependent relationship between pressure and volume.
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PWTT MethodFL = 0.00053HzFH = 0.004HzBPF
PWTT
Calibration
Fast components
Slow components
FL FH
Interpolation
BP estimation
T = 5min25
4y F
36y M
71y M
Comparison with IBP
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Intraocular PressureFlat plate
Cornea
Split image viewerMicroscope
Operator
Applied force
Too small Too large match
The force is adjusted to press the flat plate against the cornea
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Quartz Crystal Tonometer
Force is detected as a charge induced by the piezoelectric effect of the quartz crystal.
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Pressure in the jet tube depends on the flow resistance of the gap, which varies with the displacement of the membrane.
Pneumatic Tonometer
The membrane remains at an equilibrium positionwhen the pneumatic force on the jet tube balances intraocular force on the contact point of the membrane and the guard ring
In equilibrium state, pressure in the jet tube is proportional to intraocular pressure. Intraocular pressure can be measured by measuring gas pressure in the jet tube
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Noncontact Tonometry
An air pulse is produced by acylinder and a piston driven by asolenoid.
The curvature of the cornealsurface is monitored by acollimated light beam.
When applanation is achieved, the cornea acts as a plane mirrorand causes a maximum signal at the detector.
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Applanation of Corneaan air pulse is linearly increased to impinge on the cornea.
light is reflected from the corneal surface and records amaximal signal by the detector at the instant of applanation.
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IntracranialPressure
An applanation sensor consists of a plungerand guard ring.
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