Download Electromagnetic Interference

Electromagnetic
Interference
Hospital Device Immunity
RF Transmitters
RF Transmitters – 2 Major Categories:
1.
Intentional

Portable (handheld or mobile):
Cellular Telephones, 2-Way Radio, Telemetry

Installations:
Paging Tower, Broadcast, Radar
2.
Unintentional


Electrically powered equipment including Medical
Devices
Nature
RF Transmitters
Effective Radiated Power
Function of Output Power and Antenna
Efficiency
 Usually fixed except Cellular
 More power = greater range at same
wavelength

RF Transmitters
Field Strength (V/m)
Distance dependant
In free space far field strength is inversely
proportional to distance
 Reflections can cause higher than expected
strength
 Referenced to wavelength

RF Transmitters
Wavelength
Short wavelength (high frequency) more
problematic than long wavelength (low
frequency)
 Conductors measuring ¼ or ¾ of the
wavelength are most susceptible to
interference

RF Propagation
1.
Radiation
Transmitter and Receiver
E.g. Radio or TV Broadcast Transmission
2.
Conduction
Physical connection
Does not need to be directly coupled
E.g. ESU coupled to Pt. monitor via Pt.
3.
Induction
Magnetic or Capacitive coupling
E.g. signal cables routed parallel to each other
EMI Induced Problems
1.
Bit Corruption
Associated with digital devices
Artifact changes byte value
Usually fixed with error detection schemes
2.
Junction Rectification
Associated with analog devices
High frequency AC produces DC at semiconductor
junction
Produces DC off set voltage or modulated signal
Electric Field Strength
in Hospital locations
(500 kHz to 1.5 MHz)
Nursing Unit
0.5 – 2.0 V/m
Radiology Room
0.5 – 3.0 V/m
Emergency Room
Operating Room
5.0 – 10.0 V/m
30.0 V/m
(1m from active ESU)
Ad Hoc Testing
Simultaneous learning, test design
and test execution.
 Informal, one time testing with no
designed procedures.
 Flexible, customized procedures for
task at hand.
 Generally exploratory in nature –
sometimes called Exploratory
Testing.

Ad Hoc Testing
Must incorporate reporting procedure
 Need a method to quantify and
compare data collected
 Test data should be reproducible!
 Useful for defining formal test
procedures.

Electromagnetic
Vulnerability
Medical Device Selection
Criteria
Electromagnetic Vulnerability
1.
Criticality of the Device
Is it life supporting
Critical patient monitoring
Diagnostic
Medication delivery
2.
Impact of Device Failure
Potential of injury or death to patient
Can it cause harm to staff
Electromagnetic Vulnerability
3.
Compliance with applicable EMC
Standards
Has the device been tested for EMC by an outside
party?
4.
Known EMI Problems with Device
Type of device, model or manufacturer history.
5.
Suspected EMI Problems with Device
Erratic performance of device
Electromagnetic Vulnerability
6.
Sensitive Components or Circuitry
High gain amplifiers, microprocessors, patient
leads, any antenna resembling traces
7.
Frequent No Fault Found Repairs
Repeated reported issues with device yet
testing reveals no anomalies OR problems
disappear when removed from user area
Device Immunity to RF
Hospital devices have conductors of
various lengths.
 Many of these devices are designed
to amplify very weak signals.
 Many of these devices have
connecting wires that can act as
antennas.

EMI Recognition
Device works in shop but is erratic
on floors.
 Patient’s condition does not match
data provided by device.
 Intermittent malfunction during
certain weather conditions or when
transmitter is nearby.
 Inexplicable alarms
