Physiological Pressure Measurements TBMT09
today s lecture Literature: Lecture notes/slides! See also Togawa, Tamura, Öberg: Biomedical Transducers and Instruments Ask: Tryckmätning Blom ch 3
Lecture outline Indirect/direct pressure measurement methods Pressure transducers Pressure measurement system
The first invasive blood pressure measurement The first invasive attempt to measure blood pressure was made by Stephen Hales in 1733. From LA Geddes: The direct and indirect measurement of blood pressure Year Book Medical Publ 1970
Blood pressure levels Normal blood pressure Hypertension and Hypotension Narrow pressure windows
Pressure measurements Indirect methods Arm-occluding cuff Auscultation Palpation Oscillometric method Automatic methods Aplanation tonometry Direct methods Hydraulic systems Catheter tip systems
Indirect methods
Auscultatory blood pressure measurement Auscultatory blood pressure measurement using a pressure cuff and a stethoscope
Effects of height difference on the measured blood pressure From: T Togawa, T Tamura, PÅ Öberg: Biomedical transducers and instruments, CRC Press, 1997
The reference level in humans From: T Togawa, T Tamura, PÅ Öberg: Biomedical transducers and instruments, CRC Press, 1997
Cuff pressure measurements No sound No sound Fluttering sound Weak noise
Korotkoff sounds From: RC Cobbold. Transducers for biomedical measurements, John Wiley, 1974
Oscillometric principle of pressure cuff Pressure detection Blood vessel Transducer arm cuff
Automatic Oscillometric blood pressure devices Might be unreliable due to geometry of the cuff, pressure detection algorithms and location of the cuff. Can be used for monitoring but diagnostic value of measurements is questionable
PPG and BP measurements Zero-balance technique with read light transmission (PPG) and a shaker driven pressure cuff keeping the light signal constant. The transducer signal will follow the time course of the blood pressure From: T Togawa, T Tamura, PÅ Öberg: Biomedical transducers and instruments, CRC Press, 1997
Zero-balance technique with light transmission and shaker driven pressure cuff The transmitted red light reflects the blood volume in the finger. The shaker applies a pressure to the finger cuff. By keeping the optical/volume signal constant by varying the shaker signal, the pressure in the cuff will follow instantaneous blood pressure changes
Maximal pulsations reflect maximally relieved blood vessels. The DC component of the PPG signal at this point is used as a set value of the feedback system From: T Togawa, T Tamura, PÅ Öberg: Biomedical transducers and instruments, CRC Press, 1997
Finapres
Micropipettes and BP measurements Zero-balance technique for measuring pressure in blood capillaries using glass micropipettes fill with KCl immersed in the blood. The impedance of the pipette is measured and kept constant by applying a counter pressure from a bellow to the pipette. From: RC Cobbold. Transducers for biomedical measurements, John Wiley, 1974 The impedance differ very much depending on if the tip of the pipette is filled with high molar KCl or blood/physiological NaCl (se next page). By using a feedback system keeping the impedance constant the pressure applied to the pipette will follow that of the capillaries
Impedance of a micropipette Impedance versus counter pressure of a micropipette filled with 3M KCl and immersed in solutions of various concentrations. With a moderate concentration of this solution (as in the case of blood/physiological saline) there is dramatic impedance change around 5 mmhg making the impedance signal suitable as the control signal of the feedback system. From: RC Cobbold. Transducers for biomedical measurements, John Wiley, 1974
Tonimetry for intraocular pressuremeasurements Tonimeter with eye microscope
When approaching a flat surface to the eye the cornea will be flattened (applanated). In principle the force on the flat surface is the intraocular pressure times the applanated area of the cornea. Knowing the area, the pressure can be measured from the force. Errors in this measurement are the surface adhesion of the tear fluid on one hand and the bending force of the cornea on the other. With the shown Mackay-Marg tonometer only the central (in this case 1.5 mm) of the flat surface is pressure sensitive. From: RC Cobbold. Transducers for biomedical measurements, John Wiley, 1974 1. Pressure increases as surface flattens the cornea. 2. Force corresponding to sum of component from intraocular pressure and bending force. 3. Force reflecting intra ocular pressure only since bending force is outside the pressure sensitive surface
Goldman applanation tonimeter With this technique the cornea is applanated to a predetermined area (measured by a split image technique) where in average the forces due to corneal bending and tear fluid adhesion should cancel From: T Togawa, T Tamura, PÅ Öberg: Biomedical transducers and instruments, CRC Press, 1997
Non-contact applanation tonimeter The pneumatic system creates an air jet flow velocity which is controlled by a piston pump. The jet creates a stagnation pressure that displaces/ flattens the corneal surface. The flattened surface is detected by an optical system. Knowing jet velocity and the flattened area the intraocular pressure can be estimated. From: T Togawa, T Tamura, PÅ Öberg: Biomedical transducers and instruments, CRC Press, 1997
Direct methods
The pressure signal One have to measure not only the values of systolic and diastolic pressure but the whole accurate time course of the signal. Requires: 1. Accuracy 2. Bandwidth
Historic background Direct pressure measurement by Fick 1864 From LA Geddes: The direct and indirect measurement of blood pressure Year Book Medical Publ 1970
Intra-cardiac measurement First intracardiac measurements by Marey & Chauveau 1860 s From LA Geddes: The direct and indirect measurement of blood pressure Year Book Medical Publ 1970
Example of ancient recording Recordings by Marey &Chauveau 1861 From LA Geddes: The direct and indirect measurement of blood pressure Year Book Medical Publ 1970
Landois hematogram 1872 From LA Geddes: The direct and indirect measurement of blood pressure Year Book Medical Publ 1970
Early pressure transducer Pressure transducer with a dome for connecting catheters and a metal membrane linked to strain-gauge sensing elements (Statham 1940`s) From LA Geddes: The direct and indirect measurement of blood pressure Year Book Medical Publ 1970
Disposable pressure transducer From: T Togawa, T Tamura, PÅ Öberg: Biomedical transducers and instruments, CRC Press, 1997
Clinical pressure transducers Modern pressure transducers with a dome (above) and one where the sensing element is included in the Luer tubing (blow)
Technical development of pressure transducers From: RC Cobbold. Transducers for biomedical measurements, John Wiley, 1974
Illustration of the piezo-resistivity of silicone. Plotting resistance change as a function of material strain p- and n-type have the opposite characteristics. p-type shows a more linear behavior and is therefore preferred From: RC Cobbold. Transducers for biomedical measurements, John Wiley, 1974
p-type piezo-resistive elements diffused into silicone material for various geometries Membrane Rod/cantilever From: RC Cobbold. Transducers for biomedical measurements, John Wiley, 1974
Catheter and Transducer Changing pressure, p Catheter compliance C radius, r Transducer compliance, Co Displacement of the surface and Volume displacment Mass of the fluid Fluid resistance Damping
Hydraulic coupled system From: RC Cobbold. Transducers for biomedical measurements, John Wiley, 1974
Resistance, inertance, compliance The flow resistance is given by: 8ηl R = π 4 r The Inertance is given by: L ρl = 2 π r The compliance is given by: C C = + 2 0 C
Pressure transfer function The pressure transfer function: which can be written as a second order transfer function: 0 0 0 1 1 ) ( ) ( sc R sl sc s p s p i + + = 2 0 0 2 2 0 0 2 ) ( ) ( ω ω ω + + = s D s s p s p i
Resonance frequency Identification gives us the resonance frequency: f 0 = 2π 1 LC 0 = 2 1 πρ r lc 0 The resonance frequency is proportional to the radius and inversely proportional to the square root of the catheter length and compliance.
Damping The damping is obtained as: D = R C0 4η lc = 3 2 L πρ r 0 The damping depends on the square root of the catheter length and compliance and inversely to the radius to the power of three.
Step response of a second order system The response of a second order system as a function of the damping
Example under-damped system Correctly measured aortic pressure (upper trace) The same signal measured with un under-damped low band-width hydraulic system (lower trace)
Areas of damping and resonance frequency resulting in an accurate recording of the blood pressure signal
Hydraulic parallel damping of a pressure measurement system
Importance of pressure sensor placements 1 P = P + ρv 2 1 P = P ρv 2 P = P 2 2 Depending on the direction of the catheter tip, the measured pressure can consist of both static and dynamic terms. (according to the Bernoulli equation)
A catheter-tip pressure transducer The transducer element is etched in silicon to a pressure sensing membrane
Transducer element A detailed view of the transducer element etched in silicon forming a membrane
Fiber optic pressure transducer From: RC Cobbold. Transducers for biomedical measurements, John Wiley, 1974
Example of optic sensor
Principles of a fiber optic pressure transducer From: RC Cobbold. Transducers for biomedical measurements, John Wiley, 1974
Fiber optic pressure transducer From: RC Cobbold. Transducers for biomedical measurements, John Wiley, 1974
Fiber optic pressure transducer The principle behind how a fiber optic pressure transducer works. A fiber (1) emits light within a cone in a certain angle. The other fiber (2) receives light within a similar cone. The overlap between these cones is larger the larger the distance is between the fibers and the reflecting metal membrane. Pressure The surface is close pressure on membrane is high Pressure Distance is larger pressure on membrane is smaller
Fiber optic miniature pressure transducer If the glass-beam is displaced downwards it approaches a glossy silicone corner surface and more of the light is reflected.