Electrodes and Transducers

1. Electrodes and Transducers OBJECTIVE Without reference, identify at least four out of six principles pertaining to the application of transducers related to patient care.

2. Electrodes and Transducers (Continued) • Transducers • A transducer is a device that will convert one form of energy into another • Common Transducers • Generator - mechanical into electrical

3. Electrodes and Transducers (Continued) • Microphone - sound into electrical

4. Electrodes and Transducers (Continued) • Speaker - electrical into sound

5. Electrodes and Transducers (Continued) • LED (light-emitting-diode) - electrical into light

6. Electrodes and Transducers (Continued) • Piezoelectric (crystal) - pressure into electrical

7. Electrodes and Transducers (Continued) • Types of Transducers • Resistive transducers - any element that changes its resistance as a function of a physical variable • Pressure • Pressure causes displacement which causes a change in resistance by moving the arm of a potentiometer

8. Electrodes and Transducers (Continued) • Ways of moving the potentiometer • Linear displacement - shaft on a diaphragm • Rotational displacement - turning a potentiometer • Strain gauge - yields to stretching forces causes changes in resistance • Uses fine resistive wire • As wire is stretched, resistance increases in R2 and R3

9. Electrodes and Transducers (Continued) • Strain gauge - yields to stretching forces causes changes in resistance • Uses fine resistive wire • As wire is stretched, resistance increases in R2 and R3

10. Electrodes and Transducers (Continued) • Resistance in R1 and R4 decreases • All resistors are connected into an unbalanced wheat-stone bridge • All changes influence output voltage in the same direction • The strain gauge transducer changes the force of pressure into an electrical output

11. Electrodes and Transducers (Continued) • Thermistor • Changes resistive value in a predictable manner with changes in temperature • Has a positive or negative temperature coefficient • Positive coefficient - as temperatureraises, resistanceraises • Negative coefficient - as temperature raises, resistance falls • Solid state PN junction - resistance decreases as temperature increases (negative temperature coefficient)

12. Electrodes and Transducers (Continued) • Doppler effect • Send sound waves from transmitter • As sound waves hit a moving object, the waves will change in frequency • The measured frequency shift is proportional to the change in velocity • An ultrasound transducer receives the reflected sound waves and converts them into an electrical output • Used for ultrasound monitoring

13. Electrodes and Transducers (Continued) • Inductive transducer • Physical movement of a permeable core within an inductor • Affects the iron / ferrite core inside of the coil or the magnetic field of the core • Capacitive transducer • Causes capacitance of the transducer to vary with a stimulus • Uses a stationary plate or plates and a moveable plate that changes position under the influence of a stimulus

14. Electrodes and Transducers (Continued) • Thermocouple • Two dissimilar conductors or semiconductors joined together at one end (junction) • A potential is generated when the junction is heated and the electrons begin to flow

15. Electrodes and Transducers (Continued) • Electrocardiographs • An electrocardiograph records small voltages about 1mv appear on the skin surface as a result of cardiac activity

16. Electrodes and Transducers (Continued) • Signal Acquisition • Most medical instruments are electronic devices requiring an electrical signal for an input • Bioelectric potentials generated in the body are ionic potentials, produced by ionic current flow

17. Electrodes and Transducers (Continued) • Efficient measurement requires these ionic potentials to be converted into electronic potentials • Electrodes are used between the patient and the equipment where biopotentials must be acquired • An electrode converts ionic potentials into electric potentials

18. Electrodes and Transducers (Continued) • Electrode • An electrode is a device that converts ionic potentials into electronic potentials and establishes electrical contact with a nonmetallic part of a circuit

19. Electrodes and Transducers (Continued) • Characteristics • Reusable • Usually offers better performance • Requires cleaning • Used on many patients • Disposable • One time use • More convenient • Reduces cross-contamination

20. Electrodes and Transducers (Continued) • Types • Suction cup • Used for connecting portions of the body other than the extremities (head, face, chest) • Electrode is made from silver/silver chloride due to its superior conductive characteristics • Disadvantage - during long recordings, the electrode is prone to movement or slippage

21. Electrodes and Transducers (Continued) • Plate • Connected to patient's extremities held in place by a rubber strap • 3 cm x 5 cm metallic plate constructed with silver/silver chloride • One time use • Column • Reduces motion artifact generated by patient movement by eliminating electrode slippage

22. Electrodes and Transducers (Continued) • Used for long term applications • Held in place by adhesive • Needle electrode • Disposable • Uses • EEG monitoring - to reduce interface impedance and movement artifact • ECG monitoring - during surgery or when extremely fast electrode application is desired

23. Electrodes and Transducers (Continued) • Electromyography monitoring - tracing of muscle action potentials • Fetal ECG monitoring • Construction • Stainless steel hypodermic needles • Fine copper or platinum wire • Length is two to six inches

24. Electrodes and Transducers (Continued) • Using Electrodes • At least two electrodes are required to detect an ECG • Third is used as a reference to reduce electrical interference • Single electrode pair cannot completely represent the electrical activity of heart • Several electrodes arranged in standard configurations (leads) are used

25. Electrodes and Transducers (Continued) • Groups of Lead Configurations • Bipolar • Measures ECG signal between two specific electrodes • Lead 1 measures between left arm (LA) and right arm (RA) • Lead 2 measure between right arm (RA) and left leg (LL) • Lead 3 measures between left arm (LA) and left leg (LL)

26. Electrodes and Transducers (Continued) • Augmented • Measures voltage between one limb electrode (RA, LA, LL) and an average of remaining two electrodes • AVR measures potential at RA using LA and LL to form indifferent electrode • AVL measures potential at LA using RA and LL to form indifferent electrode • AVF measures potential at LL using RA and LA to form indifferent electrode

27. Electrodes and Transducers (Continued) • Precordial • Chest electrodes labeled V1, V2, V3, V4, V5, and V6 • Measures voltage between one chest electrode and the average of all limb electrodes • Cardiologist commonly work with 12 lead ECG • 10 electrodes • Signals from various groupings of these electrodes provide a complete view of heart's electrical activity