Impedance is the measure of electrical resistance to flow. In electrophysiology, measuring input impedance is often used as a predictor of the signal quality. Lower impedances mean a better connection to the patient. 

Place the Electrodes

Skin Electrodes

Use alcohol wipes and/or skin prep solutions to clean the skin where electrodes will be placed. If using gold cup electrodes, overfill with conductive paste to ensure there are no air bubbles in the cup of the electrode.

Dead skin cells, oils, and hair can act as insulators and prevent making a good electrical contact with the patient. Make sure that you prepare the skin extremely well, and that you place the electrode in the exact same place you prepared!

If the electrode is placed on the scalp for a VEP test, prepare a small landing pad. Place a small, fingertip-sized pad of electrode paste onto the scalp at the electrode site prior to electrode placement. Gently rub the paste into the hair using a circular motion, making sure to keep the pad very small (no larger than the cup of the electrode itself). This will coat the strands of hair, making for better electrical contact with the scalp.

Corneal Electrodes

See our article on Applying DTL electrodes for tips on DTL placement.

For all corneal electrode types, it is important that the eye be wet. If the patient has dry eyes, try instilling a drop of artificial tears. 

For DTL electrodes:

• Make sure that the electrode is making contact along the full width of the patient’s eye. Note
  how the thread enters the eye at the inner corner and exits at the outer corner.
• Make sure that the inner sticky pad is positioned very close to the inner corner of the eye so that the thread naturally falls into the inner corner. For patients with deeply-set eyes, pushing on the sticky pad at the nasal canthus can help pull the thread in towards the pad and help it cling to the full width of the eye.

Additional Notes for Testing Children

Ideally, impedances will be as low as possible, and values will be closely matched. However, when testing young children, it may be necessary to make compromises in preparation quality in order to retain patient compliance during testing. Aim to have impedances below 15k as much as possible. Values under 5kΩ are optimal.

Check Impedance

From the test window, click on IMPED (short for Impedance).

Clicking this opens up a secondary window which depicts an image of the amplifier with the active inputs displayed, as well as the current mode. 

Continuity Mode

Continuity Mode is an indication of whether you are connected to the patient. As soon as all connections are detected, the amplifier will move into Impedance Mode.

This means that if you are connected to the patient, and the amplifier remains in Continuity Mode, you have a problem with the connection to the patient!  

FAIL = no connection detected at this input

OK = connection detected at this input.

Important note: a notation of "Ok" simply means a connection was recognized. It is not a determination of connection quality. Connection quality is only measured in Impedance Mode. 

Impedance Mode

Impedances are measures of electrical resistance. Impedance mode measures how well you are connected to the patient.

Ideal impedances are low and as close to each other in value as possible. Diagnosys uses traffic light coloring, as well as visual indicators on the amplifier itself, for ease of use.

Key