Lecture 6 MEASUREMENT OF FLOW AND VOLUME OF...

Bioinstrument

Lecture 6MEASUREMENT OF FLOW AND

VOLUME OF BLOOD Dr. ShamekhiSummer 2016

Sahand University of Technology

Dr. Shamekhi, Sahand University of Technology 2

Introduction

fourth-class

ECG

third-class Blood Pressure

second-class Blood Flow

Some of the primary measurements are the concentration of O2 and other nutrients in the cells.

Blood flow helps to understand basic physiological processes and e.g. the dissolution of a medicine into the body.

These are normally so difficult to measure therefore we force to use the second-class measurements of blood flow and changes in blood volume, (correlated with concentration of nutrients). Difficult to measure

Hard to measure If blood flow is difficult to measure the third-class measurement of blood pressure is used (correlates with blood flow)

If blood pressure cannot be measured, the physician may fall back on the fourth-class measurement of the ECG, (correlates adequately with blood pressure.

Usually the blood flow measurements are more invasive than blood pressure measurements / ECG


Normal blood flow velocity 0,5 m/s – 1 m/s (Systolic, large vessel)

It also helps to understand many pathological conditions, since many diseases alter the blood flow. Also the blood clots in the arterial system can be detected.


Blood flow measurements

Indicator Dilution Methods: • Continuous Infusion

•Fick Method• Rapid Injection Methods

• Dye Dilution• Thermo-dilution

The indicator-dilution methods do not measure instantaneous pulsatile flow but, rather, flow averaged over a number of heartbeats.


Indicator-dilution method based on continuous infusion

ConcentrationWhen a given quantity 𝑚𝑚0 of an indicator is added to a volume V, the resulting concentration C of the indicator is given by 𝐶𝐶 = 𝑚𝑚0/𝑉𝑉.

When an additional quantity 𝑚𝑚 of indicator is then added, the incremental increase in concentration is ∆𝐶𝐶 = 𝑚𝑚/𝑉𝑉.

When the fluid volume in the measured space is continuously removed and replaced, as in a flowing stream, then in order to maintain a fixed change in concentration, the clinician must continuously add a fixed quantity of indicator per unit time. That is, ∆𝐶𝐶 = ⁄⁄dm dt ⁄dV dt .

From this equation, we can calculate flow (Donovan and Taylor, 2006).


Fick technique

Fick technique

We can use flow equation to measure cardiac output (blood flow from the heart) as follows:


Fick technique

Figure 8.1 several methods of measuring cardiac output. In the Fick method, the indicator is O2; consumption is measured by a spirometer. The arterial-venous concentration difference is measured by drawing samples through catheters placed in an artery and in the pulmonary artery. In the dye-dilution method, dye is injected into the pulmonary artery and samples are taken from an artery. In the thermodilution method, cold saline is injected into the right atrium and temperature is measured in the pulmonary artery.

The exhaled CO2 is absorbed in a soda-lime canister, so the consumption of O2 is indicated directly by the net gas-flow rate. ( ⁄dm dt is calculated by a flow-meter in spirometer)

(𝑐𝑐a)(𝑐𝑐v)

( ⁄dm dt)


Fick technique

• The units for the concentrations of O2 represent the volume of O2 that can be extracted from a volume of blood. This concentration is very high for blood (vs. water), because large quantities of oxygen can be bound to hemoglobin.

• The Fick technique is nontoxic, because the indicator O2 is a normal metabolite that is partially removed as blood passes through the systemic capillaries.

• The cardiac output must be constant over several minutes so that the investigator can obtain the slope of the curve for O2 consumption.

• The presence of the catheter causes a negligible change in cardiac output.


Indicator-dilution method based on rapid injection

Figure 8.2 Rapid-injection indicator-dilution curve After the bolus is injected at time A, there is a transportation delay before the concentration begins rising at time B. After the peak is passed, the curve enters an exponential decay region between C and D, which would continue decaying alone the dotted curve to t1 if there were no recirculation. However, recirculation causes a second peak at E before the indicator becomes thoroughly mixed in the blood at F. The dashed curve indicates the rapid recirculation that occurs when there is a hole between the left and right sides of the heart.

The continuous-infusion method has been largely replaced by the rapid injection method, which is more convenient.A bolus (known amount of a substance such as a dye or radioactive isotope) of indicator is rapidly injected into the vessel (venous), and the variation in downstreamconcentration of the indicator (the arterial concentration) versus time is measured until the bolus has passed (has completely passed through given volume.)



EQUATIONAn increment of blood of volume d𝑉𝑉 passes the sampling site in time d𝑡𝑡. The quantity of indicator d𝑚𝑚 contained in d𝑉𝑉 is the concentration 𝐶𝐶(𝑡𝑡) times the incremental volume. Hence d𝑉𝑉 = 𝐶𝐶(𝑡𝑡)d𝑉𝑉. Dividing by d𝑡𝑡, and ⁄d𝑉𝑉 d𝑡𝑡 = 𝐹𝐹𝑖𝑖 the instantaneous flow; therefore d𝑚𝑚 = 𝐹𝐹𝑖𝑖𝐶𝐶(𝑡𝑡)d𝑡𝑡. Integrating over time, when the bolus has passed the downstream sampling point, we obtain

where 𝑡𝑡1 is the time at which all effects of the first pass of the bolus have died out (point E in Figure 8.2).

The mixing of the bolus and the blood within the heart chambers and the lungs smoothsout minor variations in the instantaneous flow 𝐹𝐹𝑖𝑖 produced by the heartbeat. Thus we can obtain the average flow 𝐹𝐹 from

If the initial concentration of indicator is not zero-as may be the case when there is residual indicator left over from previous injections-then (8.5) becomes


Rapid injection

DYE DILUTION

A common method of clinically measuring cardiac output is to use a colored dye, indocyanine green (cardiogreen).

It meets the necessary requirements for an indicator in that it is (1) inert, (2) harmless, (3) measurable, (4) economical, and (5) always intravascular. In addition, its optical absorption peak is 805 nm, the wavelength at which the opticalabsorption coefficient of blood is independent of oxygenation.

The dye is available as a liquid that is diluted in isotonic saline and injected directlythrough a catheter, usually into the pulmonary artery. About 50% of the dye is excreted by the kidneys in the first 10 min, so repeat determinations are possible.

The plot of the curve for concentration versus time is obtained from a constant-flow pump, which draws blood from a catheter placed in the femoral or brachial artery. Blood is drawn through a colorimeter cuvette which continuously measures the concentration of dye, using the principle ofabsorption photometry (as describe as in the next slide)


Rapid injection (Dye dulation)

The blood is drawn through a colorimetric cuvette and the concentration is measured using the principle of absorption photometry

amount of dye

1% peak C

Avg. flow

Photometry


The most common method of measuring cardiac output is that of injecting a bolus of cold saline as an indicator.

THERMODILUTION

- A special four-lumen catheter is floated through the brachial vein into place in thepulmonary artery:1- A syringe forces a gas through one lumen; the gas inflates a small, doughnut-shaped balloon at the tip. The force of the flowing blood carries the tip into the pulmonary artery. 2- The cooled saline indicator is injected through the second lumen into the right atrium.

* The indicator is mixed with blood in the right ventricle. * The resulting drop in temperature of the blood is detected by a thermistor

located near the catheter tip in the pulmonary artery.3- The third lumen carries the thermistor wires. 4- The fourth lumen, which is not used for the measurement of thermodilution, can be used for withdrawing blood samples.

*An artery puncture is not needed in this technique



A bolus of chilled saline solution is injected into the blood circulation system (right atrium). This causes decrease in the pulmonary artery temperature.

A standard technique for measuring cardiac output in critically ill patients

Several measurements can be done in relatively short time

Problems:(1) There may be inadequate mixing between the injection site and the sampling site.

(2) There may be an exchange of heat between the blood and the walls of the heart chamber.

(3) There is heat exchange through the catheter walls before, during, and after injection.

However, the instrument can be calibrated by simultaneously performing dye-dilution determinations and applying a correction factor that corrects for several of the errors.


ELECTROMAGNETIC FLOWMETERS

Based on Faraday’s law of induction that a conductor that moves through a uniform magnetic field, or a stationary conductor placed in a varying magnetic field generates emf on the conductor:

- For uniform 𝑩𝑩 and uniform velocity profile 𝒖𝒖, the induced emf is 𝒆𝒆 =BLu. where these three components are orthogonal.

𝑒𝑒 = �0

𝐿𝐿1𝒖𝒖 × 𝑩𝑩. d𝑳𝑳

ELECTROMAGNETIC FLOWMETERS:The electromagnetic flowmeter measures instantaneous pulsatile flow of blood.


Electromagnetic flowmeters

- If the vessel's cross section were square and the electrodes extended the full length of two opposite sides, the flowmeter would measure the correct average flow for any flow profile.

- The electrodes are small, however, so velocities near them contribute more to the signal than do velocities farther away.

-When an ac magnetic field is used, any flux lines cutting the shaded loop induce an undesired transformer voltage..

- Flow can be obtained by multiplying the blood velocity u with the vessel cross section A.



- Solid lines show the weighting function that represents relative velocity contributions (indicated by numbers) to the total induced voltage for electrodes at the top and bottom of the circular cross section.

- If the vessel wall extends from the outside circle to the dashed line, the range of the weighting function is reduced.

- It shows that the problem is less when the electrodes are located outside the vessel wall.

Figure 8.4 shows the weighting function that characterizes this effect for circular geometry.

The instrument measures correctly for a uniform flow profile.



- For axisymmetric nonuniform flow profiles, such as the parabolic flow profile resulting from laminar flow, the instrument measurement is correct if u is replaced by �𝑢𝑢, the average flow velocity.

- Because we usually know the cross-sectional area A of the lumen of the vessel, we can multiply A by �𝑢𝑢 to obtain F, the volumetric flow.

- However, in many locations of blood vessels in the body, such as around the curve of the aorta and near its branches, the velocity profile is asymmetric, so errors result.


Electromagnetic flowmetersOther factors can also cause error:

1. Regions of high velocity generate higher incremental emfs than regions of low velocity, so circulating currents flow in the transverse plane. These currents cause varying drops in resistance within the conductive blood and surrounding tissues.

2. The ratio of the conductivity of the wall of the blood vessel to that of the blood varies with the hematocrit (percentage of cell volume to blood volume), so the shunting effects of the wall cause a variable error.

3. Fluid outside the wall of the vessel has a greater conductivity than the wall, so it shunts the flow signal.

4. The magnetic-flux density is not uniform in the transverse plane; this accentuates the problem of circulating current.

5. The magnetic-flux density is not uniform along the axis, which causes circulating currents to flow in the axial direction.

To minimize these errors, most workers recommend calibration for animal work by using blood from the animal-and, where possible, the animal's own vessels also. Blood or saline is usually collected in a graduated cylinder and timed with a stopwatch.



DIRECT - CURRENT FLOWMETER:

The flowmeter can use a de magnetic field, so the output voltage continuously indicates the flow. Although a few early DC flowmeters were built, none were satisfactory, for the following three reasons. (1) The voltage across the electrode's metal-to-solution interface is in series with the flow signal. Even when the flowmeter has nonpolarizable electrodes, the random drift of this voltage is of the same order as the flow signal, and there is no way to separate the two.

(2) The ECG has a waveform and frequency content similar to that of the flowsignal; near the heart, the ECG's waveform is much larger than that of the flow signal and therefore causes interference.

(3) In the frequency range of interest, 0 to 30 Hz, 𝟏𝟏/𝒇𝒇 noise in the amplifier is large, which results in a poor SNR.



ALTERNATINO - CURRENT FLOWMETER:

- The clinician can eliminate the problems of the DC flowmeter by operating the system with an ACmagnet current of about 400 Hz. - Lower frequencies require bulky sensors, - whereas higher frequencies cause problems

due to stray capacitance. The operation of this carrier system results in the ac flow voltage shown in Figure 8.5.

- When the flow reverses direction, the voltage changes phase by 180°, so the phase-sensitive demodulator (described in Section 3.15) is required to yield directional output.

Figure 8.5



Although AC operation is superior to DC operation, the new problem of transformer voltage arises. If the shaded loop shown in Figure 8.3 is not exactly parallel to the B field, some AC magnetic flux intersects the loop and induces a transformer voltage proportional to 𝑑𝑑𝑑𝑑/𝑑𝑑𝑡𝑡 in the output voltage. Even when the electrodes and wires are carefully positioned, the transformer voltage is usually many times larger than the flow voltage, as indicated in Figure 8.5. The amplifier voltage is the sum of the transformer voltage and the flow voltage.



There are several solutions to this problem:

(1) It may be eliminated at the source by use of a phantom electrode. One of the electrodes is separated into two electrodes in the axial direction and a potentiometer is placed between them.

(2) Note in Figure 8.5 that we can sample the composite signal when the transformer voltage is zero. At this time the flow voltage is at its maximum, and the resulting gated signal measures only the flow voltage. However, if undesired phase shifts cause the gating to be done even a few degrees away from the proper time, large errors and drifts result.

(3) The best method for reducing the effects of transformer voltage is to use the quadrature suppression circuit shown in Figure 8.6.


• Comes in 1 mm increments for 1 ~ 24 mm diameter blood vessels

• Individual probes cost $500 each

• Made to fit snuggly to the vessel during diastole

• Only used with arteries, not veins, as collapsed veins during diastole lose contact with the electrodes

• Needless to say, this is an INVASIVE measurement!!!

• A major advantage is that it can measure instantaneous blood flow, not just average flow

Perivascular probe electromagnetic Flowmeter ProbesA toroidal laminated Permalloy core is wound with two oppositely wound coils.


ULTRASONIC FLOWMETERS

• Ultrasonic waves are sound waves above human hearing (>20 kHz) • Typical frequencies are 20 kHz - 20 MHz.

• A typical dynamic sensor is a thin, low mass diaphragm, stretched over passive electromagnet.

• Such diaphragms operates at frequencies up to 100 kHz

•Sound waves are longitudinal pressure waves caused by vibrations in a medium•Several types of ultrasonic sensors are available- the most common are dynamic or piezoelectric sensors

Basics



• Many ultrasonic flow sensors consist of pairs of transducers• Each transducer can operate as either a source or a detector of sound waves

Ultrasonic flow sensors



Dynamic Ultrasonic Transducer

• As a generator of ultrasonic waves: the drive current creates a magnetic field which pushes against the permanent magnet.

• As a detector: the motion of the element induces a current in the drive coil



Piezoelectric ultrasonic transducers

•The piezo transmits when an applied potential distorts crystal

• Receives when pressure wave distorts crystal



Why using ultrasonic types?

•Wide range of applications such as blood, pure water, wash water, sewage,process liquids, oils, and other light homogeneous liquids

•Clamp-on types measure flow through the pipe without any wetted parts,ensuring that corrosion and other effects from the fluid will not deteriorate thesensors.

•Clamp-on types simplify and speed up meter installation and minimizemaintenance.

•Ultrasonic flowmeters may be portable.

•Measurement accuracy can be in the range of 1% of flow rate, and speed ofresponse can be as fast as 1 s.



Types of Ultrasonic Flowmeters:

1- Transit time:This type of ultrasonic flowmeter makes use of the difference in the time for a sonic pulse to travel a fixed distance, first against the flow and then in the direction of flow.

2- Doppler:It makes use of the Doppler frequency shift caused by sound reflected or scattered from suspensions in the flow path


Transit Time Flowmeter


The Doppler effect

We can use the Doppler effect to measure the velocity of a fluid.

Doppler effect is a shift in frequency from a moving source of waves.


Doppler Measurements:

Ultrasound Doppler

The blood cells in the fluid scatter the Doppler signal diffusively.

In the recent years ultrasound contrast agents have been used in order to increase the echoes.

cvff cd 2=

f = 2 – 10 MHz

c = 1500 - 1600 m/s (1540 m/s)f = 1.3 – 13 kHzd

The ultrasound beam is focused by a suitable transducer geometry and a lens


Continuous wave and pulse wave

Continuous wave systems use continuous transmission and reception of ultrasound. Doppler signals are obtained from all vessels in the path of the ultrasound beam ( high velocity)The PW transducer both sends and receives the signal. It sends in short bursts and receives in the time when it is not sending( accurate region).


•the power decays exponentially because of the heating of the tissue. The absorption coefficient ~ proportional to frequency

•the far field operation should be avoided due to beam divergence.

λ4

2Ddnf = D = Transducer diameter (e.g. 1 – 5 mm)

•the backscattered power is proportional to f4

the resolution is related to the pulse duration. Improving either one of the parameters always affects inversely to the other

Ultrasound Doppler General Parameters


Liquids to be metered must have an excess of approximately 2% suspended solids by volume

Liquid linear velocities must exceed 0.15 m/sPiping material must be of a homogenous composition Pipe wall thickness cannot be greater than 1.91 cm

Limitations of Doppler flowmeters


Laser Doppler Flowmetry

The principle of measurement is the same as with ultrasound Doppler

The laser parameter may have e.g. the following properties:

5 mWHe-Ne-laser632.8 nm wavelength

The moving red blood cells cause Doppler frequency 30 – 12 000 Hz.

The method is used for capillary (microvascular) blood flow measurements


THERMAL-CONVECTION VELOCITY SENSORS

PRINCIPLE

In contrast with thermodilution methods (entire flow stream), thermal velocity sensors depend on convective cooling of a heated sensor and are therefore sensitive only to local velocity.

The thermistor 𝑅𝑅𝑢𝑢 is heated to a temperature difference ∆𝑇𝑇 above blood temperature by the power 𝑊𝑊 dissipated by current passing through 𝑅𝑅𝑢𝑢. Experimental observations show that these quantities are related to the blood velocity 𝑢𝑢 by

where a and b are constants. Thus the method is nonlinear, with a large sensitivity at low velocities and a small sensitivity at high velocities.

A linearizer is required(8.20)



Figure 8.13 Thermal velocity probes (a) Velocity sensitive thermistor 𝑅𝑅𝑢𝑢 is exposed to the velocity stream. Temperature compensating thermistor 𝑅𝑅𝑡𝑡 is placed within the probe. (b) Thermistors placed down- and upstream from 𝑅𝑅𝑢𝑢 are heated or not heated by 𝑅𝑅𝑢𝑢, thus indicating velocity direction. (c) Thermistors exposed to and shielded from flow can also indicate velocity direction.



1- First, the time constant of the sensor embedded in the probe is a few tenths of a second much too long to achieve the desired frequency response of 0 to 25 Hz. 2- Second, to achieve a reasonable sensitivity at high velocities, the sensor current must be so high that when the flow stops, lack of convection cooling increases the sensor temperature more than 5 °C above the blood temperature and fibrin coats the sensor.

A constant-current sensor circuit cannot be used for two reasons:

The constant-temperature sensor circuit shown in Figure 8.14 overcomes both of these problems (Figure 8.14 ).

The circuit is initially unbalanced by adjusting 𝑅𝑅1. The unbalance is amplified by the high-gain op amp, and its output is fed back to power the resistance bridge. Operation of the circuit is as follows: Assume that thermistor 𝑅𝑅𝑢𝑢 is 5 °C higher than blood temperature because of self-heating. If the velocity increases, 𝑅𝑅𝑢𝑢 cools and its resistance increases. A more positive voltage enters the noninverting op-amp terminal, so 𝑣𝑣𝑏𝑏 increases. This increases bridge power and 𝑅𝑅𝑢𝑢 heats up, thus counteracting the original cooling. The system uses high-gain negative feedback to keep the bridge always in balance. Thus 𝑅𝑅𝑢𝑢remains nearly constant, and therefore its temperature remains nearly constant.



Figure 8.14 Thermal velocity meter circuit A velocity increase cools 𝑅𝑅𝑢𝑢, the velocity-measuring thermistor. This increases voltage to the noninverting opamp input, which increases bridge voltage 𝑣𝑣𝑏𝑏 and heats 𝑅𝑅𝑢𝑢. 𝑅𝑅𝑡𝑡 provides temperature compensation (A temperature-compensating thermistor 𝑅𝑅𝑡𝑡 is added to keep the bridge in balance. 𝑅𝑅𝑡𝑡 must have a much lower resistance-temperature coefficient than 𝑅𝑅𝑢𝑢, to ensure that 𝑅𝑅𝑡𝑡 is a sensor of temperature and not of velocity.)

The high-gain negative feedback divides the sensor time constant by a factor equal to the loop gain, so frequency response is greatly improved.



- Calibration can be accomplished by using a sinusoidal-flow pump or a cylindrical pan of liquid rotating on a turntable.

- The main use of thermal-velocity sensors is to measure the velocity of blood and to compile velocity profiles in studies of animals, although such sensors have also been regularly used to measure velocity and accelerationof blood at the aortic root in human patients undergoing diagnosticcatheterization. - The same principle has also been applied to the measurement of the flowof air in lungs and ventilators by installing a heated platinum wire in a breathing tube.

A linearizer is required to solve (8.20). We may square 𝑣𝑣𝑏𝑏 to obtain Wand then use an antilog converter to obtain 𝑣𝑣O.

• Kinds of Plethysmography– Strain gage– Chamber plethysmography– Electrical-impedance plethysmography– Photoplethysmooraphy


PLETHYSMOGRAPHY

Plethysmography means the methods for recording volume changes of an organ or a body part (e.g. a leg or a hand)


Plethysmography

Strain gage is made of silicone rubber tubes, which are filled with conductive liquid (e.g. mercury) whose impedance changes with volume.

Venous occlusion cuff is inflated to 40 – 50 mmHg. In this way there will be the arterial inflow into the limb but no venous outflow.

Strain Gage Method


By timing these volume changes, we can measure flow by computing F=dV/dt.

A cuff is used to prevent venous blood from leaving the limb-hence the name venous occlusion plethysmography

As mentioned before, Plethysmographs measure changes in volume.

The only accurate way to measure changes in volume of blood in the extremities noninvasively is to use a chamber plethysmograph.

As the volume of the leg increases, the leg squeezes some kind of bladder and decreases its volume

The chamber has a rigid cylindrical outer container and is placed around the leg.

Volume transducer can be e.g. water filled tube (level) or air (pressure)

Plethysmography

Chamber Method


Chamber Plethysmography

- Water-filled plethysmographs are temperature controlled to prevent thermal drifts.

- Because of their hydrostatic pressure, they may constrict the vessels in the limb and cause undesirable physiological changes.

- Air may be used in the bladder and the resulting changes in pressure measured directly.

- Some systems do not use a bladder. They attempt to seal the ends of a rigid chamber to the limb, but then leaks may be a problem.

- One device uses a pneumotachometer to measure the flow of air into and out of the chamber. This flow is then integrated to yield changes in volume.

- This equipment is designed to accommodate a variety of limb sizes, so the chambers and bladders are made in a family of sizes.

Equipment


Chamber Plethysmography

- A calibration may be marked on the record by injecting into the chamber a known volume of fluid, using the volume calibration syringe.

- The venous-occlusion cuff is then applied to a limb and pressurized to 50 mm Hg (6.7 kPa), which prevents venous blood from leaving the limb. Arterial flow is not hindered by this cuff pressure, and the increase in volume of blood in the limb per unit time is equal to the arterial inflow.

- If the chamber completely encloses the limb distal to the cuff, the arterial flow into the limb is measured.

- If the chamber encloses only a segment of a limb, as shown in Figure 8.15, an arterial-occlusion cuff distal to the chamber must be inflated to 180 mm Hg (24 kPa) to ensure that the changes in chamber volume measure only arterial flow entering the segment of the limb.

-

Method

Early water- or air-filled chamber plethysmographs for measuring tumescence have been replaced by less bulky circular metal bands and elastic strain gages.


Figure 8.16 After venous-occlusion cuff pressure is turned on, the initial volume-versus-time slope is caused by arterial inflow. After the cuff is released, segment volume rapidly returns to normal (A). If a venous thrombosis blocks the vein, return to normal is slower (B).

- A few seconds after the cuffs are occluded, the venous pressure exceeds 50 mm Hg(6.7kPa), venous return commences, and the volume of blood in the limb segment plateaus. - When the clinician releases the pressure of the venouso-cclusion cuff, the volume of blood in the limb segment rapidly returns to normal (Figure 8.16, curve A). - If a venous thrombosis (vein clot) partially blocks the return of venous blood, the volume of blood in the veins returns to normal more slowly (Figure 8.16, curve B). This technique is a useful noninvasive test for venous thrombosis.- Therefore, peed of return is important parameter.


Plethysmography

Electric-Impedance Method- It is simple to attach electrodes to a segment of tissue and measure the resulting impedance of the tissue. - As the volume of the tissue changes in response to pulsations of blood (as happens in a limb) or the resistivity changes in response to increased air in the tissue (as happens in the lung), the impedance of the tissue changes (Hutten, 2006).

- Electrical-impedance plethysmography has been used to measure a wide variety of variables, but in many cases the accuracy of the method is poor or unknown.


Electric-Impedance Plethysmography

PrincipleThe derivation requires three assumptions: (1) The expansion of the arteries is uniform in healthy vessels(not in diseased ones).(2) The resistivity of blood, 𝜌𝜌𝑏𝑏, does not change. In fact, 𝜌𝜌𝑏𝑏, decreases with velocity because of alignment of the cells with flow streamlines and movement of cells toward the axis. Also,𝜌𝜌𝑏𝑏, is real for dc but has a small reactive component at higher frequencies. (3) Lines of current are parallel to the arteries. This assumption is probably valid for most limb segments, but not for the knee.

The accuracy is often poor or unknown



The shunting impedance of the blood, 𝑍𝑍𝑏𝑏, is due to the additional blood volume ∆𝐿𝐿 that causes the increase in cross sectional area ∆𝐴𝐴.

(a) A model for impedance plethysmography (Swanson's model). A cylindrical limb has length Land cross-sectional area ∆𝐴𝐴. With each pressure pulse, ∆𝐴𝐴 increases by the shaded area M.(b) This causes impedance of the blood, 𝑍𝑍𝑏𝑏, to be added in parallel to 𝑍𝑍. (c) Usually '1Z is measured instead of 𝑍𝑍𝑏𝑏.

But we must replace the 𝑍𝑍𝑏𝑏 of Figure 8.17(b) in terms of the normally measured of Figure 8.17(c). Now



1. It is desirable to use a current greater than 1 mA in order to achieve adequate SNR. At low frequencies this current causes an unpleasant shock. However, the current required for perception increases with frequency (Section 14.2). Therefore, frequencies above 20 kHz are used to avoid perception of the current.

2. The skin-electrode impedance decreases by a factor of about 100 as the frequency is increased from low values up to 100 kHz. High frequencies are therefore used to decrease both the skin-electrode impedance and the undesirable changes in this impedance that result from motion of the patient.

3. If a frequency much higher than 100 kHz is used, the low impedances of the stray capacitances make design of the instrument difficult.

I = 0,5 – 4 mA rms (SNR)f = 50 – 100 kHz (Zskin-electrode+shock)



Two or four electrodesFor reasons of economy and ease in application, some impedance plethysmographsuse two electrodes, as shown in Figure 8.18. The current i flows through the same electrodes used to measure the voltage v. This causes several problems.1. Weighted impedance of the tissue near electrodes2. Pulsations of blood in the tissue cause artifactual changes in the skin-electrode impedance3. The current density is not uniform in the region of interest, so (8.25) cannot be used.

* To solve these problems, clinicians use the four-electrode impedance plethysmograph shown in Figure 8.18.

- The current flows through the two outer electrodes, so the current density is more uniform in the region sensed by the two inner voltage electrodes.- Variations in skin-electrode impedance cause only a second-order error.

Constant current source

Voltage sensing amplifier and other circuits



ApplicationsElectrical-impedance plethysmography is used to measure a wide variety of changes in the volume of tissue:- Electrodes placed on both legs provide an indication of pulsations of volume are normal.

- A clinically useful noninvasive method for detecting venous thrombosis in the leg is venous occlusion plethysmography.

- Electrodes on each side of the thorax provide an excellent indication of rate of ventilation, but they give a less accurate indication of volume of ventilation. Such transthoracic electrical impedance monitoring is widely used for infant apnea monitoring to prevent sudden infant death syndrome (SIDS).

- Electrodes around the neck and around the waist cause current to flow through the major vessels connected to the heart (impedance cardiography).

- An eight-electrode catheter in the left ventricle injects current through band electrodes 1 and 8 and measures voltages from all the electrodes in between.

- Some systems claim to measure body water and body fat by measuring the electric impedance between the limbs.



Example: - If we place 16 electrodes around the thorax, we can obtain 120 independent measurements and can use these data to compute a two-dimensional image of resistivity distribution within the thorax. - The spatial resolution is only about 10%, but this electrical-impedance tomography may be useful for monitoring the development of pneumonia, measuring stomach emptying, or monitoring ventilation.

The number of independent measurements from N electrodes is equal to N(N -1)/2.

The advantages of electrical-impedance plethysmography are that it is noninvasive and that it is relatively simple to use.

Advantages:

The disadvantages are that it is not sufficiently accurate for many of the attempted applications and that even the cause of the changes in impedance is not clear in some cases.

Disadvantages:


Plethysmography

Photoplethysmography

Figure 8.20 (a) Light transmitted into the finger pad is reflected off bone and detected by a photosensor. (b) Light transmitted through the aural pinna is detected by a photosensor.

Light can be transmitted through a capillary bed. As arterial pulsations fill the capillary bed, the changes in volume of the vessels modify the absorption, reflection, and scattering of the light. Although photoplethysmography is simpleand indicates the timing of events such as heart rate, it provides a poor measure of changes in volume, and it is very sensitive to motion artifact.



Figure 8.21 In this photoplethysmograph, the output of a light-emittingdiode is altered by tissue absorption to modulate the phototransistor. Thede level is blocked by the capacitor, and switch S restores the trace. A

- A GaAs LED is used as the light source (with narrow band width 940 nm)- In the photosensor an infrared filter is used to prevent visible light interference- High-pass filter (0.05 Hz) is used to eliminate large baseline values

See Example 8.6.



Applications

- For a patient who remains quiet, the photoplethysmograph can measure heart rate.

- When the patient is in a state of shock, vasoconstriction causes peripheral flow to be greatly reduced, and the resulting small output may make the device unusable.

To prevent this problem, the device has been used to transmit light through the nasalseptum. This technique monitors terminal branches of the internal carotid artery and yields an output that correlates with cerebral blood flow.

Disadvantages

- It offers an advantage in that it responds to the pumping action of the heart and not to the ECG. - When properly shielded, it is unaffected by the use of electrosurgery, which usually disables the ECG.

Advantages

Date post:	13-Mar-2020
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