03 Electromagnetic Interference

8/2/2019 03 Electromagnetic Interference

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Electromagnetic

InterferenceHospital Device Immunity


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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 MedicalDevices

Nature


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RF Transmitters

Effective Radiated Power

Function of Output Power and AntennaEfficiency

Usually fixed except Cellular

More power = greater range at samewavelength


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RF Transmitters

Field Strength (V/m)Distance dependant

In free space far field strength is inverselyproportional to distance

Reflections can cause higher than expectedstrength

Referenced to wavelength


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RF Transmitters

WavelengthShort wavelength (high frequency) more

problematic than long wavelength (low

frequency)Conductors measuring or of the

wavelength are most susceptible tointerference


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RF Propagation

1. Radiation

Transmitter and Receiver

E.g. Radio or TV Broadcast Transmission

2. ConductionPhysical connection

Does not need to be directly coupled

E.g. ESU coupled to Pt. monitor via Pt.

3. InductionMagnetic or Capacitive coupling

E.g. signal cables routed parallel to each other


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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 semiconductorjunction

Produces DC off set voltage or modulated signal


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Electric Field Strengthin 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 5.0 10.0 V/m

Operating Room 30.0 V/m

(1m from active ESU)


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Ad Hoc Testing

Simultaneous learning, test designand test execution.

Informal, one time testing with no

designed procedures. Flexible, customized procedures for

task at hand.

Generally exploratory in nature sometimes called ExploratoryTesting.


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Ad Hoc Testing

Must incorporate reporting procedure

Need a method to quantify andcompare data collected

Test data should be reproducible!

Useful for defining formal test

procedures.


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Electromagnetic

Vulnerability

Medical Device SelectionCriteria


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Electromagnetic Vulnerability

1. Criticality of the DeviceIs it life supporting

Critical patient monitoring

DiagnosticMedication delivery

2. Impact of Device FailurePotential of injury or death to patientCan it cause harm to staff


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3. Compliance with applicable EMCStandards

Has the device been tested for EMC by an outsideparty?

4. Known EMI Problems with DeviceType of device, model or manufacturer history.

5. Suspected EMI Problems with DeviceErratic performance of device


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6. Sensitive Components or CircuitryHigh gain amplifiers, microprocessors, patient

leads, any antenna resembling traces

7. Frequent No Fault Found RepairsRepeated reported issues with device yet

testing reveals no anomalies OR problemsdisappear when removed from user area


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Device Immunity to RF

Hospital devices have conductors ofvarious lengths.

Many of these devices are designedto amplify very weak signals.

Many of these devices haveconnecting wires that can act asantennas.


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EMI Recognition

Device works in shop but is erraticon floors.

Patients condition does not match

data provided by device.

Intermittent malfunction duringcertain weather conditions or whentransmitter is nearby.

Inexplicable alarms

03 Electromagnetic Interference

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