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Nox SAS Signals and Impedance Measurements

Version 01 - LBL-0438

Last Updated 22.09.2023


  1. Features & Signals of Nox SAS:
    • Records EEG, EOG, and facial EMG
    • All signals on the head are unipolar.
  2. Differences from Standard PSG:
    • Nox SAS records EEG from AF8, AF4, AF3, and AF7 electrodes without mastoid references. EEG references are the average of E3 and E4, resulting in different channel labels than PSG.
    • EOG measures eye movements from a different reference, AFz, which differs from the standard PSG that uses mastoid references.
    • Instead of under-the-chin electrodes for chin EMG, Nox SAS derives its EMG from forehead muscles (temporalis). The electrodes around the eyes record this, producing signals EMG.Frontalis-Left (E1-E3) and EMG.Frontalis-Right (E2-E4).
  3. Impedance Measurements:
    • Requires a stable PGND and references (AF7 and AF8) for accuracy.
    • In normal PSG with the Nox A1s, impedance references are M1 and M2, but they are AF7 and AF8 for Nox SAS.
    • If PGND is off, signals might look normal initially, but noise will occur over time. Fixing PGND is necessary, although the signals can be analyzed if deemed clinically acceptable by the user.
    • If references AF7 or AF8 are off, impedance measurements will be inaccurate. They must be securely attached.
  4. Nox SAS Body Cable:
    • Records ECG/EKG and limb EMG signals, both of which are bipolar channels.
    • Impedance measurement requires the PGND, AF7, and AF8 for accuracy.
    • The Nox SAS body cables offer a four-lead ECG: LA-RF, LF-RA, LF-LA, and LA-RA.

Note: Refer to the descriptions and images below for further details. Refer to this separate support article for more information on impedance measurements with the Nox A1s.

Nox SAS Head Cable

The Nox SAS head cable features signals for the EEG, EOG, and the surrogate for the chin EMG derived from the forehead muscles. All of these signals measured on the head are unipolar. Before diving in, let’s consider how the Nox SAS head cable records the EEG, EOG, and EMG signals and how it differs from standard PSG.

EEG is recorded from the Nox SAS cable's AF8, AF4, AF3, and AF7 electrodes. As these are unipolar channels, they require the PGND (GND) and a reference electrode. The Nox SAS cable does not feature mastoid (M1 and M2) references to simplify the PSG process. The references for the EEG are taken from the average of E3 and E4. This is why the EEG signals in the Nox SAS worksheet have different labels compared to standard PSG e.g. AF8-E3E4.

Eye movements are measured from a separate reference - AFz. The EOG channels are unipolar and are E2-AFz and E3-AFz. Again, this differs from standard PSG in that the reference is on the forehead and not behind the ears on the mastoid bone, as with the M2 reference used for PSG EOG.

The final signal from the SAS head cable is the surrogate for chin EMG. Standard PSG places the electrodes under the chin to measure EMG from the chin muscles. The Nox SAS head cable derives its EMG for sleep staging from the muscles of the forehead, namely, the temporalis muscles. To achieve this, the cables that record the eye movements (EOG) are utilized to record the temporalis muscle EMG. The signals use different references that are more suited to detected muscle movements. The EMG signals are:

  • EMG.Frontalis-Left: Corresponds to the E1-E3 signal
  • EMG.Frontalis-Right: Corresponds to the E2-E4 signal

The image below guides you to the new electrodes and their locations on the Nox Head Cable.

Nox SAS Impedance Measurements

The core principle for impedance measurements with the Nox SAS cable is that a stable PGND and references are required for accurate impedance measurement. In the case of normal PSG, these impedance references are the M1 and M2. But with Nox SAS, the impedance references are AF7 and AF8. Without these three electrodes connected to the skin and the device, the impedance measurements displayed in Noxturnal will not be accurate. The second principle is that the Nox A1s measure impedance (or resistance to electrical flow) between the signal and the body (considered the PGND).

With these two principles in mind, let’s consider impedance for a few critical situations.

Scenario: PGND Off

The first is when the PGND is OFF or features an open channel. Perhaps counter-intuitively, when the PGND is off, all signals and impedance can look completely normal. However, the electrodes will drift over time, and small movements will result in signal noise. When this occurs, the PGND needs to be fixed, though it is possible to analyse signals with the PGND off if they are clinically acceptable by the user.

Scenario: Impedance Reference (AF7 or AF8) Off

If one of the Nox SAS head cable reference signals (AF7 and AF8) are OFF, Noxturnal will show high or OFF for the impedance of all signals. This is because the reference signals (AF7 and AF8) are required for correct impedance measurements with the Nox SAS head cable. For this reason, AF7 and AF8 must be correctly and securely attached to the skin.

The Nox SAS head cable differs from standard PSG because the references used for impedance measurements are not the same as the display references used for the EEG. As outlined above, the EEG reference averages E3 and E4. So what happens when one of these is off or features a high impedance? Consider the screenshot taken from Noxturnal below, which features an OFF. As shown in the image, all the signals that feature E3 show as OFF, and the signals without E3 are standard. This highlights why it is critical to know the active and reference signals for the unipolar channels of the Nox SAS head cable and how they differ from standard PSG.

The following screenshot provides an example of impedance reasoning. Firstly, if the AFz is OFF, both EOG signals referenced to AFz feature high impedance and noise in the signals. Secondly, we can see that the EEG signals with AF4 and AF7 feature high impedance but minimal noise. Using reasoning, we can see that the E3E4 references are not the culprit because only two of the four EEG signals feature high impedance. In this example, the signals AF4 and AF7 feature high contact impedance. Users can determine if these are clinically acceptable, but they can also use the other EEG channels, AF3 and AF8, which feature low impedance values.

Nox SAS Body Cable

The Nox SAS body cable records the ECG/EKG and limb EMG signals. They are both bipolar channels. Bipolar channels measure the electrical difference between the positive and negative electrodes, and the PGND creates the electrical zero for the measurement. For the impedance measurement of the bipolar channels in the Nox A1s with the Nox SAS body cable, the resistance is measured between the positive and negative side of the body in relation to the PGND (that is, the positive side to PGND and negative side to PGND).

Like the signals recorded with the Nox SAS head cable, the Nox SAS body cable signals require the PGND, AF7 and AF8 to be attached securely to the person for accurate impedance measurements. Without these three signals, the impedance measurements shown on the signal will not be accurate. It is essential to keep this in mind when recording PSG with the Nox SAS Solution. One significant example is when the PGND is off. When this happens, the impedance can show as expected, and the signals look normal. However, the impedance values are only accurate with the PGND. Also, over time, as the device drifts in regard to the body's electrical potential, you will eventually see signal clipping as the voltages measured are out of range of the amplifier in the Nox A1s device.

The Nox A1s with the Nox SAS body cables can record and view multiple ECG/EKG channels. Instead of the standard rhythm strip, the Nox SAS body cables offer four lead ECG in the form of RA, RF, LA and LF. The screenshot below details the make-up of the four available ECG/EKG channels with Nox SAS Solution. The channels are:

  • LA-RF
  • LF-RA
  • LF-LA
  • LA-RA

Known Issues for Impedance Measurements with Nox SAS

  • The Nox SAS head cables (EEG, EOG, forehead EMG) are not currently shown in the active impedance view in Noxturnal. Only E2 and E3 EOG signals will show in this view. The continuous impedance values will show on the signal during an online recording and when reviewing the study.
  • The Noxturnal app also does not support viewing Nox SAS head cable signals other than E2 and E3.
  • Both issues will be fixed in a future development project.

Appendix: Nox SAS Signals

Signal Name Recording Signals PSG Equivalent Type Signal Type




~F4 EEG Unipolar




~F3 EEG Unipolar




Frontal EEG EEG Unipolar




Frontal EEG EEG Unipolar



E2 EOG Unipolar



E3 EOG Unipolar
EMG Frontalis - left



Chin EMG EMG (forehead) Unipolar
EMG Frontalis - right



Chin EMG EMG (forehead) Unipolar





  ECG Bipolar



ECG ECG Bipolar



  ECG Bipolar



  ECG Bipolar
Left leg   Leg Leg EMG (limb) Bipolar
Right leg   Right Leg EMG (limb) Bipolar


*Please note that not all features or all products are available in all markets. Our products are medical devices and subject to registration/approval in each market area. For more information on product availability please contact

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