bio pot en cia les

8/14/2019 Bio Pot en CIA Les

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BiopotencialesBIOPOTENCIALES


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Agenda

Primera parte

Introduccin a los biopotenciales Mtodos de medida

Tradicional: ECG, EEG, EMG, EOG

Novedoso: VCG

Segunda parte

Mediciones Electrnica

Electrodos


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Qu son lo biopotenciales?

Biopotencial: Es un potencial elctrico que puede medirse entre dos

puntos en clulas vivientes, tejidos y organismos y que esconsecuencia de algunos de sus procesos bioqumicos.


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Mecanismos detrs de los biopotenciales

La concentracin de iones potasio(K+) es 30-50 veces mayor en el ladointracelular, comparado con elextracelular.

La concentracin de iones sodio(Na+) es 10 veces mayor afuera de lamembrana que en el interior.

En el estado de reposo, la membranaes permeable solo a iones potasio.

mVVm

100...70

mVVm

100...70


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Mecanismos de los biopotenciales

Cuando la estimulacin de una membranaexcitable excede un umbral de alrededor de

20 mV, ocurre un potencial de accin:1. Las permeabilidades de Na y K cambian.

2. La permeabilidad al Na se incrementa muyrapidamente al principio, permitiendo a losiones Na fluir desde el exterior al interior,haciendo el medio intracelular ms positivo.

3. La permeabilidad del K se incrementa mslentamente, permitiendo al potasio fluir desdedel interior al exterior, as retornando elpotencial de membrana a su valor de reposo.

4. Durante el reposos, la bomba Na-K ATPasarestaura las concentraciones inicas a susvalores originales.

El nmero de iones que fluyen a travs de uncanal abierto es >106/segundo

El cuerpo es un conductor de volumen,permitiendo a estos flujos ionicos generarpotenciales medibles en la superficie del


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Electrocardiografa (ECG)

Mide galvnicamente la actividad elctrica del corazn.

Las primeras ECG fueron realizadas por Augustus Wallermediante el electrmetro capilar, en el ao 1887.

Muy utilizado en el ambiente clnico.

De muy alto valor diagnstico.

1. Atrialdepolarization

2. Ventriculardepolarization

3. Ventricular repolarization


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ECG fundamentos bsicos

Amplitud: 1-5 mV

Ancho de banda: 0.05-100 Hz

Fuentes de error:

Artificios de movimiento

Interferencia con la lnea de voltaje 50 Hz

Aplicaciones tpicas:

Diagnstico de isquemia

Arritmias

Defectos de la conduccin


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Medicin del ECG con 12 electrodos.

El ms utilizado en la clnica.

Las seales elctricas son medidas no invasivamente con 9 electrodos Existen bases de datos y referencias internacionales con este sistemaa.

Este mtodo fue adoptado por razones histricas, aun cuando en la actualidadest algo obsoleto.

Einthoven leads: I, II & III Goldberger augmented leads: VR, VL & VF Precordial leads: V1-V6


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Electroencefalografa (EEG)

Mide la actividad elctrica del cerebromedido a nivel del cuero cabelludo.

La seal medida resulta de la actividadde billones de neuronas.

Amplitud: 0.001-0.01 mV

Ancho de banda: 0.5-40 Hz

Errores:

Ruido de RF termal Lneas elctricas de 50 Hz.

Aplicaciones tpicas:

Estudios del sueo.

Estudios de convulsiones.

Mapas corticales.


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Mediciones del EEG

El sistema de10-20electrodos es el ms usado

en la clnica. Permite la localizacin de

rasgos diagnsticos en lavecindad de un electrodo.

A menudo se utilizan

electrodos de hilo o unamalla de goma con loselectrodos posicionados.

Los investigadores delcerebro utilizan gorras con256 o 512 canales.


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Electromiografa (EMG)

Mide la actividad elctrica de las fibras musculares.

Los electrodos estn posicionados muy cerca delos grupos de msculos bajo estudio.

Seales rectificadas o integradas proveen una indi-

cacin cruda de la actividad muscular.

Electrodos de aguja pueden ser usados para medir

fibras musculares individuales.

Amplitud: 1-10 mV

Ancho de banda: 20-2000 Hz

Fuentes de error principales son la interferencia de RF y las lneas de 50 Hz.

Applicacones: funcin muscular, enfermedades de la placa neuromuscular,prtesis.


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Vectorcardiograma (VCG o EVCG)

En vez de desplegar la amplitud escalar(curva ECG), la activacin elctrica delcorazn es desplegada como un vector.

Tiene amplitud y direccin

El diagnstico est basado en la curva que lapunta de este vector se dibuja en dos o tresdimensiones.

El contenido de informacin del VCG esaproximadamente el mismo que el sistemade ECG con 12 electrodos. La ventaja vienede la manera en que la informacin esdesplegada.


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Un pequeo

descanso


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EL ORGANO ELECTRICO DE LA

ANGUILA


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En el reposo, las bombas Na-KATPasa generan un potencialde membrana de -100 mV. La

llegada de un potencial deaccin a la placa terminalcausa la entrada de Na a laclula.

En reposo, el voltaje total de loselectrocitos apilados es compensado por elpotencial positivo de la membrana de

superficie suave. La despolarizacinreversa el potencial de membranaresultando en una sumacin de lospotenciales individuales, causando unacorriente neta positiva


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The biopotential amplifier

Small amplitudes, low frequencies, environmental and biologicalsources of interference etc.

Essential requirements for measurement equipment:

High amplification

High differential gain, low common mode gain high CMRR

High input impedance

Low Noise Stability against temperature and voltage fluctuations

Electrical safety, isolation and defibrillation protection


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The Instrumentation Amplifier

Potentially combines the best features desirable for biopotentialmeasurements

High differential gain, low common mode gain, high CMRR, high input resistance

A key design component to almost all biopotential measurements!

Simple and cheap, although high-quality OpAmps with high CMRR should beused

1

2

121R

RG +=

3

4

2

R

RG =

CMRR fine tuning


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Application-specific requirements

ECG amplifier

Lower corner frequency 0.05 Hz, upper 100Hz

Safety and protection: leakage current below safety standard limit of 10 uA

Electrical isolation from the power line and the earth ground

Protection against high defibrillation voltages

EEG amplifier

Gain must deal with microvolt or lower levels of signals

Components must have low thermal and electronic noise @ the front end

Otherwise similar to ECG

EMG amplifier

Slightly enhanced amplifier BW suffices

Post-processing circuits are almost always needed (e.g. rectifier + integrator)

EOG amplifier High gain with very good low frequency (or even DC) response

DC-drifting electrodes should be selected with great care

Often active DC or drift cancellation or correction circuit may be necessary


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Electrical Interference Reduction

Power line interference (50 or 60 Hz) is always around us

Connects capacitively and causes common mode interference

The common mode interference would be completely rejected by theinstrumentation amplifier if the matching would be ideal

Often a clever driven right leg circuit is used to further enhance CMRR

Average of the VCM is inverted and driven back to the body via reference electrode

0RiV

DCM=

1

2

0

21R

R

RiV

D

CM

+

=


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Filtering

Filtering should be included in the front end of the InstrAmp

Transmitters, motors etc. cause also RF interference

Small inductorsor ferrite beadsin the lead wires

block HF frequencyEM interference

RF filtering withsmall capacitors

High-pass filterto reject DC drifting

Low-pass filteringat several stages

is recommended toattenuate residual

RF interference


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50 or 60 Hz notch filter

Sometimes it may be desirable to remove the power line interference

Overlaps with the measurement bandwidthMay distort the measurement result and have an affect on the diagnosis!

Option often available with EEG & EOG measuring instruments

Twin T

notch filter

Determinesnotch

frequency

Notch

tuning


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Artifact reduction

Electrode-skin interface is a major source of artifact

Changes in the junction potential causes slow changes in the baseline

Movement artifacts cause more sudden changes and artifacts

Drifting in the baseline can be detected by discharging the high-passcapacitor in the amplifier to restore the baseline


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Electrical isolation

Electrical isolation limits the possibility of passage of any leakagecurrent from the instrument in use to the patient

Such passage would be harmful if not fatal!

1. Transformer

Transformers are inherently highfrequency AC devices

Modulation and demodulation needed

2. Optical isolation

Optical signal is modulated inproportion to the electric signal andtransmitted to the detector

Typically pulse code modulated tocircumvent the inherent nonlinearityof the LED-phototransistorcombination


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Defibrillation Protection

Measuring instruments can encounter very high voltages

E.g. 15005000V shocks from defibrillator Front-end must be designed to withstand these high voltages

1. Resistors in the inputleads limit the current

3. Protection against

much higher voltagesis achieved withlow-pressure gasdischarge tubes

(e.g. neon lamps)

(note: even isolationcomponents such astransformers and

optical isolators needthese spark gaps)

Discharge @ ~100V

2. Diodes or Zener diodesprotect against high

voltages

Discharge @ 0.7-15V


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Electrodes Basics

High-quality biopotential measurements require

Good amplifier design

Use of good electrodes and their proper placement on the patient

Good laboratory and clinical practices

Electrodes should be chosen according to the application

Basic electrode structure includes:

The body and casing

Electrode made of high-conductivity material

Wire connector

Cavity or similar for electrolytic gel

Adhesive rim

The complexity of electrode design often neglected

El d B i


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Electrodes - Basics

Skin preparation by abrasion or cleansing

Placement close to the source being measured

Placement above bony structures where there is less muscle mass

Distinguishing features of different electrodes:

How secure? The structure and the use of strong but less irritant adhesives

How conductive? Use of noble metals vs. cheaper materials

How prone to artifact? Use of low-junction-potential materials such as Ag-AgCl

If electrolytic gel is used, how is it applied? High conductivity gels can help reducethe junction potentials and resistance but tend to be more allergenic or irritating

Baseline drift due to thechanges in junction

potential or motion artifacts

Choice of electrodes Muscle signalinterference

PlacementElectromagnetic

interference Shielding

A A Cl Sil Sil Chl id El t d


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Ag-AgCl, Silver-Silver Chloride Electrodes

The most commonly used electrode type

Silver is interfaced with its salt silver-chloride Choice of materials helps to reduce junction potentials

Junction potentials are the result of the dissimilarelectrolytic interfaces

Electrolytic gel enhances conductivity and also reduces

junction potentialsTypically based on sodium or potassium chloride,

concentration in the order of 0.1 M weak enough to notirritate the skin

The gel is typically soaked into a foam pad or applieddirectly in a pocket produced by electrode housing

Relatively low-cost and general purpose electrode

Particularly suited for ambulatory or long term use

G ld El t d


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Gold Electrodes

Very high conductivity suitable for low-noise meas.

Inertness suitable for reusable electrodes

Body forms cavity which is filled with electrolytic gel

Compared to Ag-AgCL: greater expense, higherjunction potentials and motion artifacts

Often used in EEG, sometimes in EMG

Conductive polymer electrodes Made out of material that is simultaneously conductive and adhesive

Polymer is made conductive by adding monovalent metallic ions

Aluminum foil allows contact to external instrumentation

No need for gel or other adhesive substance

High resistivity makes unsuitable for low-noise meas.

Not as good connection as with traditional electrodes

M t l b l t d


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Metal or carbon electrodes

Other metals are seldom used as high-quality noblemetal electrodes or low-cost carbon or polymeric

electrodes are so readily available Historical value. Bulky and awkward to use

Carbon electrodes have high resistivity and are noisierbut they are also flexibleand reusable

Applications in electrical stimulation and impedance plethysmography

Needle electrodes

Obviously invasive electrodes

Used when measurements have to be taken from the organ itself

Small signals such as motor unit potentials can be measured

Needle is often a steel wire with hooked tip

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