Version 1 - September 2023 LBL-0436
This Customer Service Document explains how clinicians can ensure reliable readings from the impedance values provided by the Nox A1s PSG System.
Introduction to Impedance
The ExG signals in PSG are small voltages recorded from the skin’s surface. Creating the optimal conditions for recording these signals is essential for the best results. One of the tools available to the user is the impedance view within Noxturnal and the Noxturnal app.
For PSG, impedance is the resistance to the current flow from the electrode to the skin surface. Higher signal impedance can increase the noise in the recorded signals (Kappenman & Luck, 2010). As a result, the AASM states the goal of electrode application is to obtain the lowest impedance possible without compromising patient comfort or skin integrity. The AASM outlines the standard values to achieve for impedance for specific signals:
- EEG and EOG signals are 5 kOhm (unipolar)
- EMG and ECG signals are 10 kOhm (bipolar)
However, it could be argued that this limit is unnecessarily strict with modern biopotential amplifiers with high input impedances (>5 MΩ in Nox A1s device). While this is true, adhering to the AASM standard values or your local protocol for site-specific impedance values is recommended. It is also essential to ensure that the measurement going into the device is as clean as possible. Low impedance values will help achieve high-quality signals.
Impedance in Noxturnal
Noxturnal and the Noxturnal app feature a colour code system for impedance values. Green impedance values mean the signals meet the AASM recommendations. The colour code for impedance views in Noxturnal and the Noxturnal app is shown below. Note that the Nox A1s has a method for detecting disconnected signals. The impedance measurement on Nox A1s requires at minimum the PGND, M1 and M2 to be attached to the patient and connected to the device. Signals will show as off if they are disconnected. For bipolar channels, if one electrode is off, both will show as off. This is because the impedance of bipolar channels is measured between the electrodes.
Evaluating Impedance Status in Noxturnal
There are two methods to generate/view the impedance of signals in Noxturnal:
- Impedance View
- Impedance Check
The Impedance View widget can be added to a signal sheet in Noxturnal through View > Impedance View in the Noxturnal Toolbar. Another impedance indicator is on the left axis of the ExG signals, which receive a coloured “squiggly line” for signals which may have high impedance values. The screenshot below shows the impedance view widget in the toolbar (1) and the continuous impedance view on a signal (2).
The signals from the Nox A1s that receive impedance checks are broken down into unipolar and bipolar. These two categories perform impedance checks in a slightly different manner.
- Reference electrodes (M1 and M2)
- Chin EMG
- Limb EMG
An Impedance Check can be performed by clicking the impedance icon in the Noxturnal toolbar (1 in the screen below). The advantage of the active impedance check is that it provides impedance values for the individual EEG electrodes (2). The continuous impedance view on the signals shows information for both the recording and reference signal (e.g. F4 - M1). The active view provides tabled data on the signal (2), impedance value (3), and the status (4).
Noxturnal Impedance Worksheet
Noxturnal provides a default impedance sheet for checking the measured signals' impedance. This worksheet view can help users notice patterns in particular signals when determining the high-impedance source or artefact. This sheet can be added to the workspace from the worksheet menu below. This worksheet will provide continuous impedance measurements as signals for the individual unipolar electrodes and the bipolar channels.
Using impedance in PSG with and the Noxturnal App
To check impedance in the Noxturnal app, follow these steps:
1. Connect from the Noxturnal app to the Nox A1s
2. Go to the sensors page
3. Go to the sensor overview (icon at the bottom of page)
4. Click start impedance check
5. The EEG, EOG, and chin EMG impedance values will be shown. Tip - the colour labels under the EEG are the colours of the cables in the Nox EEG bundle and NOT a representation of impedance value.
6. Click the impedance icon at the top of the app to end the impedance check.
Minimum required signals for impedance measurements
The Nox A1s requires three signals to be attached to the recorder and the patient to measure impedance accurately. These three signals are:
- Patient Ground (PGND)
A stable PGND is required for accurate impedance measurements as the resistance (impedance) is measured between the signals and the PGND. This forms a critical concept for understanding and fixing impedance issues when performing PSG with the Nox A1s.
All impedances for the Nox A1s are performed in relation to the PGND. Because of this, a stable PGND connection is required. The first step of troubleshooting high impedance is to ensure that the PGND has a clean connection site and that the electrode is securely connected to the skin. However, it is important for the technician to keep in mind that it will not always be obvious that the PGND has come loose or fallen off, as the recorded signals can look normal. The impedance values can be reported as normal. This is because the device can compensate for the PGND being off and the impedance values are not accurate without the PGND. Often, patient movement will result in artefact and noise in all signals, indicating that the PGND has poor contact with the patient or is off.
In the example below, the PGND is disconnected (off), and then connected with very bad impedance (300kohm). At this point there was a small jump across all channels. PGND off can result in a drifting pattern or noise over all channels. For this reason, detecting it can be challenging, so it must be well-prepared at the start of the study.
Error Situation – M1 or M2 fall off
Likewise, M1 and M2 EEG signals are required for accurate unipolar measurements with the Nox A1s. If either M1 or M2 is off, the other reference will show as off. We can also see that the individual unipolar signals in the active impedance view are good. This is an erroneous measurement because, without the M1 and M2 connected, the impedance measurement is not accurately performed.
- The M1 is off, but both M signals show high impedance. As the PGND, M1, and M2 are required for accurate impedance measurement, these values do not reflect the actual impedance.
- The active impedance check shows high values for both M signals due to the M1 being off.
- The M1-referenced signals feature significant noise due to the electrode being off.
Error Situation – M1 or M2 with high impedance
If the reference electrodes (M1 or M2) feature high impedance, only the signals that are referenced to the trouble electrode will show high impedance. In this example, the M1 signal features high impedance.
1. The M1 signal shows high impedance in the active check
2. The M1 shows high impedance in the active check table
3. All unipolar signals that use M1 as a display reference show high impedance.
Consideration - PGND, M1 and M2 only connected
Another important detail is that when only PGND, M1 and M2 are connected, M1 and M2 will display as (M1+M2)/2. When you attach another unipolar (EEG) signal with good impedance, you will get the correct impedance values for the individual M1 and M2 values. In the example below, all EEG channels are off, and M1 and M2 have differing impedance values but have an impedance of 6.5k ohm combined. When another EEG signal is attached to the body, the M1 and M2 are correctly measured.
Consideration - Bipolar Channels
An important consideration is for bipolar channels. Each channel has two inputs: positive and negative. The measured signal is the voltage difference between the two inputs. The impedance of the bipolar connection is the sum of two electrodes. The impedance information for the entire bipolar connection is provided and not the individual positive and negative electrodes. As a result, it is best to check and fix both the positive and negative connections when performing corrective action.
Another important consideration is that bipolar signals have the impedance measured across the positive and negative electrodes of the channel. If one side is OFF, the channel will display as bad with an impedance over 140k ohm. To get the combined impedance calculated as OFF (set value as over 200k in the software), the other signals must be over 60kohm. That is why high impedance values on the bipolar channels, it is best to re-prep both negative and positive electrodes. >This is highlighted in the example below. Signal one of the right leg shows increased impedance on the channel. It is recommended to check and re-prep both bipolar electrodes for this signal. Signal two is a very high impedance because one of the electrodes is off, but the impedance measure is below the 140k ohm set to display as off in the software. The final signal features both electrodes of the bipolar signal off and displays as off because the impedance is measured as over the 200k ohm threshold set in Noxturnal. For both of these examples, both electrodes for the signal should be fixed.
Understanding a little about how Nox A1s take the tiny voltages on the body’s surface and turn them into signals in Noxturnal will help determine the source of high impedance in sleep study recordings.
What causes high impedance?
The cause of high impedance in PSG can often come down to a few factors:
- Poor site preparation
- Dirty cables
- Dry electrodes
- Loose or off electrodes
The dead cells on the surface of the skin act as a barrier to transmitting the tiny currents from the body to the electrodes. Scrubbing the skin to remove all dirt and oils and exfoliating as many layers of dead skin without impacting patient comfort is essential for high-quality signal measurement. Uniform site preparation, particularly for PGND, M1, and M2 regions, will usually help achieve high-quality signals with low impedance.
Sweat can produce an electrical bridge that creates significant noise in the channels impacted. If it’s PGND or a reference channel, all signals will feature sweat artefact. Sweat artefacts can occur when there is excessive sweating during a PSG recording. This can lead to distorted or noisy signals in the EEG readings. Sweat on the scalp can cause electrical interference, making it difficult to interpret the EEG patterns accurately. Keeping the scalp clean and dry before the recording is essential to minimise sweat artefacts during an EEG.
When the contact between the scalp and the electrode is disrupted, an open channel forms and the electrode is more prone to pick-up noise. This can happen because of a loose-fitting electrode, dry electrodes, body movement, or hair pushing the electrode. It is vital to minimise the movement of the PGND signal. To do this, avoid areas of the forehead with creases or wrinkles as best as possible. If using adhesive electrodes for the PGND, ensure it is applied on a flat site on the forehead. Avoiding the forehead muscles can help to maintain a stable PGND throughout the study. Attaching the PGND just below the hairline works best.
PSG Examples of Impedance Readings:
This section discusses scenarios for when specific interventions will help with lower impedance and improving signal quality.
Reducing impedance is important for obtaining high-quality EEG signals. Some common techniques used to reduce impedance include:
- skin preparation (e.g., cleaning the skin, shaving hair),
- ensuring electrodes are clean and free of damage,
- Replace adhesive electrodes (especially PGND and bipolar with the Nox A1s) as they could be dried out, which increases contact impedance to the skin.
The starting point for improving signal quality and reducing impedance with the Nox A1s is to ensure that the PGND, M1, and M2 references have stable connections with low impedance. If not, begin by cleaning and reapplying these electrodes.
Here is a common scenario for high impedance on a single EEG (unipolar) signal. In this example, O1-M2 is showing a black continuous impedance. A quick check shows that all other EEG signals have a suitable impedance. We can isolate the cause of the issue to the O1 cable as none of the other EEG signals are impacted, which rules out the shared reference electrode (M2) and the PGND.
In the final example, the high impedance is shared by EEG signals that reference M1. Notice how none of the M2 referenced EEG signals feature high impedance or signal artefact.
Kappenman ES, Luck SJ. The effects of electrode impedance on data quality and statistical significance in ERP recordings. Psychophysiology. 2010 Sep;47(5):888-904. doi: 10.1111/j.1469-8986.2010.01009.x. Epub 2010 Mar 29. PMID: 20374541; PMCID: PMC2902592.