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

 Transducers  A transducer is a device that will convert one form of energy into another  Common Transducers  Generator - mechanical into electrical

 Microphone - sound into electrical

 Speaker - electrical into sound

 LED (light-emitting-diode) - electrical into light

 Piezoelectric (crystal) - pressure into electrical

 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

 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

Strain gauge - yields to stretching forces causes changes in resistance  Uses fine resistive wire  As wire is stretched, resistance increases in R2 and R3

 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

Thermistor  Changes resistive value in a predictable manner with changes in temperature  Has a positive or negative temperature coefficient » Positive coefficient - as temperature raises, resistance raises » Negative coefficient - as temperature raises, resistance falls  Solid state PN junction - resistance decreases as temperature increases (negative temperature coefficient)

 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

 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

 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

 Electrocardiographs  An electrocardiograph records small voltages about 1mv appear on the skin surface as a result of cardiac activity

 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

 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

 Electrode  An electrode is a device that converts ionic potentials into electronic potentials and establishes electrical contact with a nonmetallic part of a circuit

 Characteristics  Reusable  Usually offers better performance  Requires cleaning  Used on many patients  Disposable  One time use  More convenient  Reduces cross-contamination

 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

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

 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

 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

 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

 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)

 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

 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