Evoked Potential Procedure

Patient preparation

• The technician explains the procedure to the patient.
• The patient is asked to use the bathroom prior to beginning the study so they will be comfortable during the test
• Technician obtains all patient and clinical data needed for the report and billing.
• Patient information includes correct spelling of patient's name, medical record number, date of birth, referring physician, patient location, procedure number.
• Clinical information includes type of procedure requested, reason for evoked potential, relevant symptoms and current medications.
• Medications are listed and spelled correctly.
• The patient data are logged in correctly.
• The technician verifies the physician signature on the request form

Machine operation

• The technician adjusts evoked potential machine settings to obtain optimum accurate recording. This includes calibration and correct montages.
• Technician applies electrodes correctly within 30 minutes. This includes 10/20 electrode placement.
• The technician verifies that the electrodes are functioning correctly.
• Technician identifies & responds appropriately to significant evoked potential patterns.
• Significant evoked potential patterns include normal variants, artifact patterns and abnormal evoked potential patterns.
• Appropriate responses include: label the correct type of pattern, eliminate artifacts when present, apply additional electrodes, add additional channels to montage or change the recording method so that the pattern may be interpreted most accurately.
• The technician obtains two reproducible responses that are appropriate for interpretation
• Technician removes electrodes correctly, cleans the patient's scalp, cleans the electrodes and sterilizes electrodes when appropriate.
• Technician completes all needed forms and transfers the evoked potential data for interpretation.

Maintenance

• Technician archives files on CD-ROM correctly.
• The technician records any adjustments for inventory and orders supplies as needed.

Managing artifacts or non-reproducible evoked responses

• View responses in single sweep mode.  Determine if artifact is present.   Determine if artifact is time locked, random signal, rhythmic signal or time locked to the stimulus.
• Random artifacts are due to external random noise or physiological noise and are reduced by signal averaging.
• Rhythmic artifacts suggest unclean power supply, external machine generated artifact 60 Hz artifact or internal machine malfunction. 
  • The power supply is improved with UIPS and isolation transformers. 
  • External machine artifacts are improved by turning off the machine generating the artifact.  This may require trial and error or observing the artifact only when the machine is turned on or used.  Artifacts may also be improved by moving recording cables away from machines that generate artifacts.
  • 60 Hz artifact occurs when the time interval between sinusoidal waves is 16.7 milliseconds.  This artifact occurs with high electrode impedance.  Electrodes may need to be refilled.
  • Internal machine malfunction requires machine repair, swapping machine components or machine replacement.
  • twisting the recording wires together may reduce the external noise artifact
  • mechanical vibrations of the machine or recording cables may also cause artifacts
• Machine generated artifacts may occur from
  • ventilator tubes
  • IV pumps
  • cooling devices
  • suction tubes
  • stethoscopes
  • ECG cables
  • power cords
  • electrocautery
• Time locked artifacts may arise from excessive stimulus or inappropriate use of 60 Hz notch filter.  Excess stimulus current occurs when stimulus electrodes have high impedance.
• Absent responses may occur because of absent or low intensity stimulus, sensory impairment in the modality tested, incorrect connections, machine settings or montage.   The sensory impairment may occur from underlying disease, the effects of surgery, ischemia, cooling or low body temperature.  For example, a cooling blanket placed on the leg stimulated during a tibial nerve evoked potential.

Additional actions taken for other specialized studies are listed below:

Operative procedures

• The technician records: 
  • The type, time and method of administration of all drugs given during surgery.
  • Method of administration refers to bolus injections, inhalation or continuous intravenous infusion
  • Periodic blood pressure measurements
  • Oxygenation
  • Keys steps in the surgical procedure
• Redundant electrodes are applied in the event that one comes loose.
• Cables are placed and immobilized to reduce the risk of disconnection during surgery.
• Do not use a ground to the machine chassis.  The inputs from the machine are electrically isolated and come with current protection devises to protect the patient.   Multiple grounds may cause ground loop currents that may be dangerous to the patient who may be at risk for microshock.
• Stretching, compression, direct injury and cooling may all cause evoked potential changes during surgery.
• Bolus drug injections are more likely to cause evoked potential changes during surgery.
• Interpretation is based on significant reductions in amplitude (> 50% reduction) or delays in latency (> 10%).

Auditory evoked potential: Machine settings are:

• filter: 100 - 3000 Hz
• time base 10 milliseconds
• averages 2000
• rate 11.1 Hz
• duration 100 microseconds
• intensity 70 dB above threshold
• stimulus polarity rarefaction
• noise masking contralateral ear 20 dB below stimulus intensity

Visual evoked potentials: Machine settings are:

• filter: 5 - 100 Hz
• time base 200 milliseconds
• averages 2000
• rate 4.1 Hz

Median nerve evoked potentials: Machine settings are:

• filter: 100 - 1500 Hz
• time base 40 milliseconds
• averages 2000 (500 - 2000 responses used for operative monitoring)
• rate 4.7 Hz
• duration 100 microseconds

Tibial nerve evoked potentials: Machine settings are:

• filter: 100 - 1500 Hz
• time base 100 milliseconds
• averages 2000 (500 - 2000 responses used for operative monitoring)
• rate 4.7 Hz
• duration 100 microseconds