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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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