BIOMEDICAL ENGINEERING�BIOMEDICAL TRANSDUCERS (BMF)

PRESSURE MEASUREMENT

Rossana E. Madrid

LAMEIN – DBI – FACET/UNT – INSIBIO/CONICET

Latest update April 2025

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

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Brief history

1628

1727

1929

1956 🡪

PRESSURE UNITS

• Physiological Pressure Units [mmHg] or [cmH₂O]

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1 mmHg = 133,32 Pa = 0.1333 kPa

1 cmH₂O = 98,0665 Pa

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• Expressed relative to ATMOSPHERIC PRESSURE [atm]

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1 atm = 101,325 kPa 1 mbar = 0.1 kPa

PHYSIOLOGICAL PRESSURE RANGES

NORMAL

ANOMALOUS

PRESSURE IN THE CARDIOVASCULAR SYSTEM Characteristics

PRESSURE MEASUREMENT IN THE CARDIOVASCULAR SYSTEM

DIRECT MEASUREMENT

INDIRECT MEASUREMENT

INTRAVASCULAR

SENSOR

EXTRAVASCULAR

SENSOR

AUSCULTATORY METHOD

OSCILOMETRIC

METHOD

DOPPLER ULTRASOUND METHOD

Detection of arterial wall motion

Detection of Doppler blood flow velocity in artery

DIRECT PRESSURE MEASUREMENT

• 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

• Catether Pressure Transducer (Intravascular)
• Diaphragm Pressure Transducer (Extravascular)

PRESSURE CATETHERS

With a catheter you can...

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• Blood pressure waves

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• Cardiac Minute Volume

Indicator Dilution Principle

Método de Fick

• CTS

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• Angiography

• Transduction Principles
• Semiconductor Strain gauges
• Capacitive Sensors
• Optical Methods

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Ej: Mikro-Tip® Catheter Pressure Transducer

P = -50 a 300 mmHg f_resonance= 35 a 50 kHz

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

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DISADVANTAGES

• Fragile
• Expensive

INTRAVASCULAR SENSOR

CATHETER TIP PRESSURE TRANSDUCER

EXTRAVASCULAR SENSOR � DIAPHRAGM DISPLACEMENT TRANSDUCER

ELASTIC DIAPHRAGM

Strain gauge

Variable Capacitor

Optical Sensor

Inductive Sensor

EXTRAVASCULAR SENSORES

External Transducers

Statham Transducer

Blood Pressure Elastic deflextion Electrical Signal

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

• ELASTICITY

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• MASS

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• FRICTION

CTS Distributed parameters System

But... Clinical sets Second order system

Rt << Rc

and

Lt << Lc

Cc << Cd

Bubble

ΔP: 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 🡪

η: Viscosity

dF/dt: Flow Derivative

a: Acceleration [m/s²]

A: Area [m²]

It reduces to:

m: Liquid Mass [Kg]

ρ: Liquid Density [kg/m³]

Ed: Diaphragm Elasticity Module

By Kirchoff Law v_i vs v_o:

Elasticity Mass Friction

Determine two important parameters

ξ

ω

and

V_d: Displaced Volume in the transducer

CTS Time Response

¿How to measure?

Pressure Step Response:

ξ 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 20^th armonic

CLINICAL AREA

INTENSIVE CARE

Medium pressure important

instead waveform

CTS Response with and without bubble

CTS Dynamic response (Two techniques):

ω_n and ξ

BW Requirements

System fully

characterized

A variable frequency pressure generator is used to analize frequency response

ω_n : How fast the system can oscillate

ξ: How quickly the system returns to rest

Why?

1

2

INDIRECT MEASUREMENT

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Auscultatory Method with sphygmomanometer

• Occlusion Method

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• AHA (American Heart Association)

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Dimensions of the inflatable cuff

Difficulties:

• Ambient noie
• Motion artifacts

INDIRECT MEASUREMENT

MEDIUM PRESSURE BY OSCILLOMETRIC METHOD

ADVANTAGES

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The maximun oscillation amplitude is easily detectable

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Easy to automate

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Suitable for continuous monitoring of blood pressure.

DISADVANTAGES

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• Sistolic and Medium Pressure

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but …

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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

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Doppler Ultrasound Ps and Pd

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• 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:

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• Doppler shift at the OPENING

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Ordinarily observed at a range of: 200 – 500 Hz

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• Doppler shift at the CLOSING

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Observed at a range of: 30 – 100 Hz

ADVANTAGES

• It can be used in infants, hipotensive persosns and in noisy enviroments

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DISADVANTAGES

• Movement of the sensor 🡪 Errors

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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

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Difracción patterns

Use: high f and big transducers

Detecting arterial blood velocity

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.

Emitter and

receiver

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1.5 E5 cm/s

Pressure-Velocity Relationship

Bernoulli Equation

INTRA OCULAR PRESSURE (IOP) MEASUREMENT

• It measures IOP by providing force which flattens the cornea 🡪 Applanation Tonometry

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• Types of applanation tonometers
• Goldmann Tonometer
• Non-contact Tonometer
• Halbergn Tonometer
• Guard ring Tonometer (Mackay and Marg)

Goldmann Tonometer

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

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Webster JG. 1998. Medical Instrumentation: Application and

Design. New York: John Wiley & Sons Inc.

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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.

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