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Chapter: Medical Electronics : Bio-Chemical and Non Electrical Parameter Measurement

Blood Pressure

One of the oldest physiological measurements.Observation of blood pressure allows dynamic tracking of pathology and physiology affecting to the cardiovascular system, which has profound effects to all other organs of the body



One of the oldest physiological measurements.Observation of blood pressure allows dynamic tracking of pathology and physiology affecting to the cardiovascular system, which has profound effects to all other organs of the body


·        Originates from the heart


·        Commonly refers to arter ial blood pressure


Value depends on 3 factors:


·        cardiac output diameter o f arteries the quantity of blood


·        Values should be lower than 120 / 80 mmHg(systolic pressure (SP) / diastolic pressure (DP))


·        High value increases the risk of heart attack and strokes

·        Low value increases the r isk of lower oxygen perfusion e.g. in brains.


However, the ’normal values’ differ from person to another


Pulse Pressure(PP ) = SP - DP


Mean pressure (MP)


Average pressure during one cardiac cycle driving force of the periph eral perfusion. an estimate can be done by using an empirical formula:


M P = DP+PP/3


SP and DP may vary significan tly throughout the arterial system but MP is quite uniform (in normal situations)


1. Non-Invasive


Palpatory Method(Riva- Rocci Method)

Auscultatory Method

Ultrasonic Method

Oscillometric Method





2. Invasive


Extravascular Sensor

Intravascular Sensor

General on System Para meters




Indirect measurement = non-invasive measurement

Brachial artery is the most comm on measurement site 

Close to heart

Convenient measurement 

Other sites are e.g.:

forearm / radial artery wrist (tends to give much higher SP)


The most common indirect methods are auscultation and oscillometry an occlusive cuff is placed on arm and inflated to P > SP. Then the cuff is deflated gradually and the measurement of blood flow is done .


The occlusive cuff should be of a correct size in order to transmit the pressure to the artery evenly and thus to obtain accurate results .A short cuff requires special attention in placement. Longercuff reduces this problem. The cuff should be placed at the heart level in order to minimize the hydrostatic effects .




When the cuff is deflated, there is a palpable pulse in the wrist. P = BP .Several measurements should be done as the respiration and vasomotor waves modulate the blood pressure levels



The blood pressure can be measured in noisy environment too

Technique does not requi re much equipment



Only the systolic pressur e can be measured (not DP)

The technique does not give accurate results for infants and hypotensive patients




Pulse waves that propag ate through the brachial artery, generate Kor otkoff sounds. There are 5 distinct phases in the Korotkoff sounds, which define SP and DP The Korotkoff sounds are ausculted with a stethoscope or microphone (automatic measurement

The frequency range is 20-300 Hz and the accuracy is +/- 2mmHg (SP) and +/- 4mmHg (DP). Also with this method, several m easurements should be done.



Auscultatory technique iss simple and does not require much equipmen t




Auscultatory tecnique ca nnot be used in noisy environment The observations differ f rom observer to another


A mechanical error might be introduced into the system e.g. mercury leakage, air leakage, obstruction in the cuff etc.


The observations do not always correspond with intra-arterial pressure

The technique does not give accurate results for infants and hypotensive patients




A transcutaneous (through the skin) Doppler sensor is applied here. T he motion of blood-vessel walls in various states of occlusion is measured. The vessel opens and closes with each heartbeat when DP < P < SP .


The frequency difference between transmitted (8 MHz) and received signal is 40-500 Hz and it is proportional to velocities of the wall motion and the blood. As the cuff pressure is increased, the time between opening and closing decreases until they coincide.


Advantages & Disadvantages

Can be also used in noisy environment

Can be used with infants and hypotensive individuals


Subject’s movements change the path from sensor to vessel




The intra-arterial pulsation is transmitted via cuff to transducer (e.g. piezo-electric) .The cuff pressure is deflated either linearly or stepwise. The arterial pressure oscillations (which can be detected throughout the measurement i.e. when P > SP and P < DP) are superimposed on the cuff pressure SP and DP are estimated from the amplitudes of the oscillation by using a (proprietary) empirical algorithm .



In the recent years, oscillometric methods have become popular for their simplicity of use and reliability.


MP can be measured reliably even in the case of hypotension



Many devices use fixed algorithms leading to large variance in blood pressures




Linear array of pressure sensors is pressed against a superficial artery, which is supported from below by a bone (radial artery). A sensor array is used here, because at least one of the pressure sensors must lay directly above the artery .When the blood vessel is partly collapsed, the surrounding pressure equals the artery pressure

The pressure is increased continuously and the measurements are ma de when the artery is half collapsed. The hold-down pressure varies between individuals and therefore a ’calibration’

must be done




Can be used for non-inv asive, non-painful, continuous measurement




Relatively high cost


The wrist movement and tendons result in measurement inaccuracies




Direct measurement = Invasive measurement


A vessel is punctured an d a catheter (a flexible tube) is guided in The most common sites are brachial and radial arteries but also other sites can be used e.g. femoral artery A division is made into extravascular and intr avascular sensor systems .This method is precise but it is also a complex procedure involving many risks Used only when essential to determine the blood pressure continuously and accurately in dynamic circumstances




The sensor is located be hind the catheter and the vascular pressure is transmitted via this liquid-filled catheter.


The actual pressure sensor can be e.g. strain gage, variable inductance,variable capacitance Optoelectronic, piezoelectric, etc…

The hydraylic link is the major source of errors. The system’s natural frequency may be damped and degraded due (e.g.):


too narrow catheter too long tubing


various narrow connections air bubbles in the catheter


The catheter-sensor system mus t be flushed with saline-heparine solution every few minutes in order to prevent blood from clotting at the tip.


Normally the interesting frequency range is 0 – 100 Hz. If only MP is measured the bandwidth is 20 Hz (harmonics > 10 are ignored)




sensor is located in the tip of the catheter. This way the hydraulic connection is replaced with an electrical or optical connection .The dispacement of the diaphragm is measured .The frequency response is not limited by the hydraulic properties of the system.

No time delay.


Electrical safety and isolation when using fiber optics Breaks easily


More expensive


Disposable Sensors


Disposable sensors decrease the risk of patient cross-contamination and reduce the amount of handling by hospital personnel


Cheaper and more reliable than reusable pressure sensors




Even minute air bubbles in catheter have a dramatic effect on frequency response The natural frequency and the length of the catheter have a following relationship:

The catheter diameter has a linear relationship to natural frequency Stiffer catheters have a higher frequency response

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