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

2. 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

3. Brief history
1628
1727
1929
1956 

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

5. PHYSIOLOGICAL PRESSURE RANGES
NORMAL
ANOMALOUS

7. 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

8. 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

9. 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

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

11. 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

12. 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

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

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

15. 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

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

17. 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

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

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

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

21. POSSIBLE RESPONSES

22. Pressure wave distorsion

23. MikroTip® vs CTS

24. Frequency Response

25. 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

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

27. 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?

28. INDIRECT MEASUREMENT
Auscultatory Method with sphygmomanometer

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

30. MEDIUM PRESSURE BY OSCILLOMETRIC METHOD

31. 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.

32. 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

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

34. 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

35. 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

36. 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

37. 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

38. 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: InTech.

39. Pressure-Velocity Relationship
Bernoulli Equation

40. 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)

41. 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

42. Goldmann Tonometer

43. Non-contact applanation Tonometer

44. Bibliography
Webster JG 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.