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