Biomedical Instrumentation Devices information

Electrodes

Many medical instruments are electronic devices and thus it should a electrical signal for an input. In the cases where biopotential may acquire, some form of electrodes used between patient and instrument.

Medical Electrodes:

There are few electrodes,

ECG Electrodes:

1) Plate electrode
2) Suction Cup electrode
3) Needle electrode

Plate Electrode

It is use to connect patients extremities to ECG input amplifier.
It consists of a binding attached to 3cm*5cm metallic plate.
The plate is attached and held in placed against the patient’s skin by rubber strap surrounding the extremity.

Suction Cup Electrode

This is also use the same type of material as the electrode but us used in connecting the portion of body other than the extremities.
Most frequently the suction cup electrode is connected to the patient’s chest electrode.
A few advanced ECG machines also require a connection to the patient’s
face or head and three connections are also use the suction cup electrode.

Needle Electrode

This is used for ECG or EEG brain wave recording when patient is anesthetized.
The needle is inserted into the tissue just below patient’s skin surface.
The needle electrode is also used extensively in veterinary application.

Column Electrode

It gives significant amount of patient’s movement without creating a significant amount of artifacts.
ECG can be taken when the patient is in the fade null while being monitored, artifacts is minimal in most cases.
At least one manufacturer supplies a pad with three column electrodes mounted on a single foam rubber backing.
The electrodes are arranged in configuration that allows pickup of the ECG waveform when the patient lies on the pad.
This is mainly used in operating rooms for short term monitoring.

EEG Electrodes

The brain produces bioelectric signals that can be pickup through surface electrodes attached to the scalp.
These electrodes will be connected to an EEG amplifier that drives on either oscilloscope or strip chart recorder.

Problems in electrodes

One of the major problems is high impedance of human skin.
Thus use high input impedance when attempting to record biopotential.

Transducers

transducers used to convert some form of non-electrical physical parameters of stimulus such as force, pressure, and temp. Etc. to analogs electrical signal proportional to the value of original stimulus parameters. Thus, transducer is the devices, which converts some form of energy produced by a physical stimulus to an electrical analog of stimulus.

Strain Gauges

It is a resistive element that produces a change in its resistances proportional to an applied mechanical strain. A strain is face applied in either compression or tension.
These are two basic forms of strain gauges bonded and unbounded.

P-N Junction Diode

This type have one distinct advantages, its widely linear up to most temperature so o/p voltage may be processed in simple amplifier and requires no special circulatory to linearise the result.

Inductive Transducers

Physical movement of a permeable core within as inductor can vary inductance for exam, easily.
Inductors can be used to make transducers.
There are basic forms of inductive transducers.
1) Single coil
2) Reactive wheat stone bridge
3) LVDT (linear voltage differential transducer)

Electrocardiograph

The electrocardiograph system of heart consists of structure called node bundle of his atrioventricular node additional structure called as packing fibers.
The SA node serves as pacemakers for hearts, providing trigger signal under control CNS.

Standard positioning of electrodes
In standard ECG there are five electrode connected to the patient including right arm, left arm, left leg and chest .
In this m/c unipolar lead configuration is used.

R(Symbol) Red(Colour) Right arm(Electrode Positioning)
L(Symbol) Yellow(Colour) Left arm(Electrode Positioning)
F(Symbol) Green(Colour) Left leg(Electrode Positioning)
B(Symbol) Blue(Colour) Right leg(Electrode Positioning)

Technical Specifications

Power supply-mains 22-24+10% AC, 50HZ internal rechargeable battery 12v=0.8R. Power consumption-22-24 Wmax
Battery life-> 100 testing
Writing system-linear with thermomicodot
Heart sensitive chart paper-30mm*50mm
Safety normative –class1
Leads-12 std level simultaneously
A/D conversion-10 bits, 100 samples/channel
Memory-10sec , 12 leads
Frequency response-0.05-75(-3db)
Time constant ->3.25
Chart speed-5,25,50 min/sec
CMMR->100db
I/P impedance->100ohm
Filters-modified digital notch 50/35Hz
Functioning Temp -+10
Relative humidity-
Dimensions-24,5*17.3*6.2cm
Weight-1.7 kg

Phonocardiography

A Phonocardiogram or PCG is a plot of high fidelity recording of the sounds and murmurs made by the heart with the help of the machine called phonocardiograph, or "Recording of the sounds made by the heart during a cardiac cycle".

How to record a phonocardiogram
The phonocardiogram is recorded using microphones placed on the thorax in the main auscultatory areas;these microphones filter the sounds in between 25 and 200 Hz. usually done together with the ECG.

Electroencephalography

The electroencephalogram (EEG) is defined as electrical activity of an alternating type recorded from the scalp surface after being picked up by metal electrodes and conductive media.
The EEG measured directly from the cortical surface is called electrocardiogram while when using depth probes it is called electrogram.

The group of electrobiological measurements comprises items as electrocardiography (ECG, heart), electromyography (EMG, muscular contractions), electroencephalography (EEG, brain), magneto encephalography (MEG, brain), Electroencephalography is a medical imaging technique that reads scalp electrical activity generated by brain structures.

EEG Wave Patterns

Delta Wave

Delta is the frequency range up to 4 Hz. It tends to be the highest in amplitude and the lowest waves. It is seen normally in adults in slow wave sleep. It is also seen normally in babies.

Theta Wave

Theta is the frequency range from 4 Hz to 7 Hz. Theta is seen normally in young children. It may be seen in drowsiness or arousal in older children and adults; it can also be seen in meditation.

Alpha Waves

Alpha is the frequency range from 8 Hz to 12 Hz. Hans Berger named the first rhythmic EEG activity he saw, the "alpha wave. It emerges with closing of the eyes and with relaxation, and attenuates with eye opening or mental exertion. The posterior basic rhythm is actually slower than 8 Hz in young children.

Beta Waves

Beta is the frequency range from 12 Hz to about 30 Hz Beta activity is closely linked to motor behavior and is generally attenuated during active movements. It is the dominant rhythm in patients who are alert or anxious or who have their eyes open.

Applications

Monitor alertness, coma and brain death
Locate areas of damage following head injury, stroke, tumour, etc.
Test afferent pathways (by evoked potentials)
Monitor cognitive engagement (alpha rhythm)
Produce biofeedback situations, alpha, etc
Control anaesthesia depth (“servo anaesthesia”)
Investigate epilepsy and locate seizure origin
Test epilepsy drug effects
Assist in experimental cortical excision of epileptic focus
Monitor human and animal brain development
Test drugs for convulsive effects
Investigate sleep disorder and physiology

Electromyogram

The Bio Electric Potential associated with muscle activity constitutes the Electromyogram-EMG.
These potential may be measures at the surface of the body near a muscle of interest of directly from the muscle by penetrating the skin with a needle electrode.

Photoplethysmography

A plethsymograph is a device that measure the amount of blood in of the body.
A photoplethysymograph does this optically.
The output of the photoplethsymograph. will is four LED’s on your PIC board.

Blood Flow Meter

The principle of ultrasound blood flow measurement is the visualization and measurement ofblood flow velocity by the shift in frequency of a continuous ultrasonic wave.
The sensor usedhere is piezoelectric crystal. This sensor acts both as the transmitter and receiver.
The ultrasonicwaves transmitted by the transmitter are reflected by the motion of blood and is received by thereceiver but here they received frequency is Doppler shifted.
The Doppler frequency shift is ameasure of the size and direction of the flow velocity.

Indicator Intensity for Blood Flow Measurement

Two approaches are used as the Continuous infusion; Rapid injection bolus. Both only measure averaged flow over a number of heartbeats and do not pulsatile instantaneous flow Continuous Infusion to measure blood flow

DC flow meters

Use DC magnetic field Were not satisfactory due to Electrode-electrolyte potential at the same order as the induce voltage due to the flow emf can’t be detected
The ECG has similar frequency components and it has larger signal (near the heart) 1/f noise is large which yield poor SNR.
Use AC magnetic field

AC flow meters

AC magnet current (400Hz) is the problem of the transformer voltage higher than the flow voltage and proportional to dB/dt.
The solutions are Phantom electrode, Sampling Quadrature suppression, and Square-wave excitation.

Spirometer

Spirometry is the most common of the Pulmonary Function Tests (PFTs), measuring lung function, specifically the measurement of the amount (volume) and or speed (flow) of air that can be inhaled
and exhaled.
Spirometry is an important tool used for generating pneumotachographs which are helpful in assessing conditions such as asthma, pulmonary fibrosis and cystic fibrosis.
The spirometry test is performed using a device called a spirometer, which comes in several different varieties.

Volume Spirometers

Volume spirometers record the forced expiratory maneuver as it is produced. When the subject breathes into a mouthpiece, the air moves a cylinder, a plastic bell, or a rubber or plastic diaphragm, which in turn moves a pen that traces a curve on a moving paper graph. The water seal, dry rolling seal, and bellows spirometers are the three most widely used types of volume spirometers.

Flow Spirometers

Flow spirometers measure how quickly air flows past a detector and then derive the volume by electronic means.
They record the flow rate at very brief intervals, such as 30-300 times a second, and use the data obtained to reconstruct the flow rate at each point in time and volume.
This process is called digitization.
The most common types of flow spirometers are the pneumotachographs, hot wire anemometers, and rotating vanes.