BIOMEDICAL ENGINEERING BIOMEDICAL TRANSDUCERS (BMF) PRESSURE MEASUREMENT Rossana E. Madrid LAMEIN – DBI – FACET/UNT – INSIBIO/CONICET Latest update 03/04/14

Table of contents Pressure units and ranges Pressure measurement in the cardiovascular system Direct Measurement Indirect Measurement Catheter-Transducer System Oscilometric Method Dopplet Ultrasound Methods Intraocular pressure measurement

Brief history 1628 1727 1929 1956 

PRESSURE UNITS Physiological Pressure Units [mmHg] or [cmH2O] 1 mmHg = 133,32 Pa = 0.1333 kPa 1 cmH2O = 98,0665 Pa Expressed relative to ATMOSPHERIC PRESSURE [atm] 1 atm = 101,325 kPa 1 mbar = 0.1 kPa

PHYSIOLOGICAL PRESSURE RANGES NORMAL ANOMALOUS

PRESSURE IN THE CARDIOVASCULAR SYSTEM Characteristics PARAMETER PRIMARY SIGNAL CHARACTERISTICS PRESSURE RANGES Blood Pressure (arterial, direct) Range of f: DC to 200 Hz 20 a 300 mmHg Blood Pressure (arterial, indirect) Range of f: DC to 5 Hz Blood Pressure (venous, direct) Range of f: DC to 40 Hz -5 a 20 mmHg

PRESSURE MEASUREMENT IN THE CARDIOVASCULAR SYSTEM INTRAVASCULAR SENSOR DIRECT MEASUREMENT EXTRAVASCULAR SENSOR AUSCULTATORY METHOD OSCILOMETRIC METHOD INDIRECT MEASUREMENT Detection of arterial wall motion DOPPLER ULTRASOUND METHOD Detection of Doppler blood flow velocity in artery

DIRECT PRESSURE MEASUREMENT Catether Pressure Transducer (Intravascular) Diaphragm Pressure Transducer (Extravascular) PRESSURE CATETHERS Needle (different diameter and shapes) Flexible plastic catethers X Rays to place the catether must be radiopaque Blood coagulation must be avoid Special material Heparine

With a catheter you can... Blood pressure waves Cardiac Minute Volume Indicator Dilution Principle Método de Fick CTS Angiography Angiografía. Inyección de líquidos opacos

CATHETER TIP PRESSURE TRANSDUCER INTRAVASCULAR SENSOR CATHETER TIP PRESSURE TRANSDUCER Transduction Principles Semiconductor Strain gauges Capacitive Sensors Optical Methods Ej: Mikro-Tip® Catheter Pressure Transducer P = -50 a 300 mmHg fresonance= 35 a 50 kHz

CATHETER TIP PRESSURE TRANSDUCER INTRAVASCULAR SENSOR CATHETER TIP PRESSURE TRANSDUCER ADVANTAGES No delays Flat response up to several kHz No need of saline solution to avoid coagulation Less affected by mechanical movement of the catheter DISADVANTAGES Fragile Expensive

EXTRAVASCULAR SENSOR DIAPHRAGM DISPLACEMENT TRANSDUCER ELASTIC DIAPHRAGM Strain gauge Variable Capacitor Optical Sensor Inductive Sensor

EXTRAVASCULAR SENSORES External Transducers Blood Pressure Elastic deflextion Electrical Signal Statham Transducer Standard for Blood Pressure Measurement Hg manometer

DYNAMIC PROPERTIES OF DIRECT PRESSURE MEASURMENT CATHETER TRANSDUCER SYSTEM Hydraulic Model of a pressure transducer P(t) = Applied Pressure M = Fluid Mass K = Stiffness Electrical Model Dynamic system of a 2nd order system Stiffness=Rigidez ELASTICITY MASS FRICTION

CTS Distributed parameters System But... Clinical sets Second order system Rt << Rc Lt << Lc and Bubble Cc << Cd

Liquid Resistance Inertance Compliance P: P Diff through the segment [Pascal] F: Flow (m3/seg) A: Cross section of the catheter [m2] v: Average flow speed [m/seg] By applying the Poisseuille Law  : Viscosity Inertance dF/dt: Flow Derivative a: Acceleration [m/s2] A: Area [m2] It reduces to: m: Liquid Mass [Kg] : Liquid Density [kg/m3] Compliance Ed: Diaphragm Elasticity Module

ω ξ By Kirchoff Law vi vs vo: Elasticity Mass Friction Determine two important parameters ω ξ

Natural Frequency Damping Coefficient and Vd: Displaced Volume in the transducer

CTS Time Response ¿How to measure? Pressure Step Response:  Influences in the Overshoot and the Rise Time

POSSIBLE RESPONSES

Pressure wave distorsion

MikroTip® vs CTS

Frequency Response

BAND WIDE REQUEREMENTS SCIENTIFIC Wide BW in the audiofrequency range CATHETERSIM LABORATORY Accurate reproduction of dP/dt Higher reuqeriments of BW Flat response up to 20th armonic Medium pressure important instead waveform CLINICAL AREA INTENSIVE CARE

CTS Response with and without bubble More compliant system With Bubbles More Underdamped Without Bubbles

CTS Dynamic response (Two techniques): 1 System fully characterized ωn and  ωn : How fast the system can oscillate : How quickly the system returns to rest A variable frequency pressure generator is used to analize frequency response 2 BW Requirements Why?

INDIRECT MEASUREMENT Auscultatory Method with sphygmomanometer

INDIRECT MEASUREMENT Occlusion Method Difficulties: Ambient noie AHA (American Heart Association) Dimensions of the inflatable cuff Difficulties: Ambient noie Motion artifacts

MEDIUM PRESSURE BY OSCILLOMETRIC METHOD

The maximun oscillation amplitude is easily detectable ADVANTAGES The maximun oscillation amplitude is easily detectable Easy to automate Suitable for continuous monitoring of blood pressure.

DISADVANTAGES Sistolic and Medium Pressure but … Pd may be obtained from the calculated Ps, Pm and the volume plethysmographic waveform is similar to the waveform of blood pressure

It can be considered that the PV relationship is linear in the range of amplitude of the pressure pulse ε  5-7 mmHg

BLOOD PRESSURE MEASUREMENT BY DOPPLER ULTRASOUND Auscultatory Method Doppler Ultrasound Ps and Pd There are 2 Methods: Detectiong Arterial Wall Motion Detecting Arterial Blood Velocity under the occluding cuff

Detecting Arterial Wall Motion With an 8MHz US signal: Doppler shift at the OPENING Ordinarily observed at a range of: 200 – 500 Hz Doppler shift at the CLOSING Observed at a range of: 30 – 100 Hz

To use several different crystals for emitters and receivers ADVANTAGES It can be used in infants, hipotensive persosns and in noisy enviroments DISADVANTAGES Movement of the sensor  Errors To use several different crystals for emitters and receivers Solution

Detecting arterial blood velocity The same principle of the Doppler Ultrasound Flowmeters Ps and Pd similar to the previous case THE TRANSDUCER Piezoelectric Crystals piezoeléctricos Finite diameter Difracción patterns dnf= Profundidad del campo cercano Use: high f and big transducers

Detecting arterial blood velocity Emitter and receiver ? Fe : Frecuencia del cristal. Ej: 8 MHz; c= velocidad del sonido en la sangre: 1.5 E5 cm/s 1.5 E5 cm/s Azhim, A. and Kinouchi, Y. Arterial Blood velocity measurement by portable wireless system for healthcare evaluation: The related effects and significant reference data. Recent Adv. In Biomed. Eng. Ganeish R. Neik, Ed. (2009). ISBN: 978-953-307-004-9. InTech.

Pressure-Velocity Relationship Bernoulli Equation

INTRA OCULAR PRESSURE (IOP) MEASUREMENT It measures IOP by providing force which flattens the cornea  Applanation Tonometry Types of applanation tonometers Goldmann Tonometer Non-contact Tonometer Halbergn Tonometer Guard ring Tonometer (Mackay and Marg)

GoldmannTonometer P = F / A Based ib the Imbert-Fick law: pressure within a sphere (P) is roughly equal to the external force (F) needed to flatten a portion of the sphere divided by the area (A) of the sphere which is flattened: P = F / A It applies to surfaces which are perfectly spherical, dry, flexible, elastic and infinitely thin

Goldmann Tonometer

Non-contact applanation Tonometer

Bibliography Webster JG. 1998. Medical Instrumentation: Application and Design. New York: John Wiley & Sons Inc. Biomedical transducers and instruments. Tatsuo Togawa, Toshiyo Tamura and P. Åke Öberg. CRC Press, Boca Raton, New York, 1997.   Sensors and signal conditioning. Ramón Pallá-Areny and John G. Webster. John Wiley & Sons, INC., 1991.