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Nox SAS Signals and Impedance Measurements

Version 01 - LBL-0438

Last Updated 26-02-2024

Nox SAS Head Cable

The Nox SAS head cable features signals for the EEG, EOG, and facial EMG to replace chin EMG. The image below guides you to the new electrodes and their locations on the Nox Head Cable.

Nox SAS EEG 

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

Nox SAS EOG

Nox SAS EOG features a different reference to the EEG - AFz. The EOG channels are unipolar and are E2-AFz and E3-AFz. Again, this differs from the mastoid reference for EOG in standard PSG. electrode.

Nox SAS Facial EMG 

The final signal from the SAS head cable replaces chin EMG. The Nox SAS head cable measures EMG for sleep staging from the forehead muscles (temporalis muscles). To achieve this, the signals for EOG are utilized to record the muscle tone of the facial muscles. 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

Nox SAS Impedance Measurements

The core principle for impedance measurements with the Nox SAS cable is that a stable PGND and recording references are required for accurate impedance measurement. In the case of standard PSG, these recording references are M1 and M2. With Nox SAS, the recording references are AF7 and AF8. These three electrodes - PGND, AF7, and AF8 - must be connected to the users and the Nox A1s for the impedance measurements displayed in Noxturnal to be accurate.

Patient Ground Electrode

The Patient Ground (PGND) electrode is required for accurate voltage and impedance measurements. If the PGND is off or features high impedance, all unipolar and bipolar signals and their impedance measurements can appear normal but, over time, will drift, and small movements can cause signal noise.

Noxturnal cannot determine if the PGND is off. Users will need to be vigilant for global signal issues. When users notice global signal issues, they should check to ensure the PGND is securely attached to the patient skin. In the case of unsupervised sleep studies, analysis can proceed if the user determines the signals are clinically acceptable. 

Scenario: Recording Reference (AF7 or AF8) Off

If one of the Nox SAS head cable recording reference signals (AF7 and AF8) is OFF, Noxturnal will show high or OFF for the impedance on 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.

Scenario: Continuous Impedance Measure 

The Nox SAS head cable is different from standard PSG because the references used for impedance measurements are not the same as the display references used for the EEG. The EEG display reference takes the average of E3 and E4. 

The continuous impedance monitor on each channel will highlight when the impedance values between the signals are greater than the set values for the unipolar and bipolar channels.

In the example below, the individual signals have impedances greater than 5kOhm (E2 and AFZ). Still, the difference between the signals that make up the EOG channel E2-AFZ is not greater than 5kOhm, so no impedance warning is shown. For this reason, users will want to use the continuous impedance measurements on the channels and the SAS impedance worksheet to maintain low impedances for in-lab recordings and when reviewing HSAT performed with Nox SAS. 

This is how the continuous impedance measurements work for the unipolar and bipolar channels. Users looking for a direct application of impedance from the AASM Scoring and Reporting Manual should reference the individual SAS signal impedance measurements in the worksheet attached to this support article. Users can download the worksheet at the bottom of this support article and import it into Noxturnal to use during in-lan recordings and when reviewing Nox SAS studies. 

Note: Refer to this separate support article for more information on impedance measurements with the Nox A1s.

Nox SAS Body Cable

The Nox SAS body cable records the ECG/EKG and limb EMG signals. They are both bipolar channels. 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.

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 attached to the user. Over time, the device will drift with the PGND, and eventually, users will 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 ECGs 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. This will be fixed in a later release of Noxturnal. 

Appendix: Nox SAS Signals

Signal Name Recording Signals PSG Equivalent Type Signal Type
AF3-E3E4

AF3

E3

E4

~F4 EEG Unipolar
AF4-E3E4

AF4

E3

E4

~F3 EEG Unipolar
AF7-E3E4

AF7

E3

E4

Frontal EEG EEG Unipolar
AF8-E3E4

AF8

E3

E4

Frontal EEG EEG Unipolar
E2

E2

AFz

E2 EOG Unipolar
E3

E3

AFz

E3 EOG Unipolar
EMG Frontalis - left

E1

E3

Chin EMG EMG (forehead) Unipolar
EMG Frontalis - right

E2

E4

Chin EMG EMG (forehead) Unipolar

LF-RA

 

LF

RA

  ECG Bipolar
LA-RF

LA

RF

ECG ECG Bipolar
LF-LA

LF

LA

  ECG Bipolar
LF-RA

LF

RA

  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 support@noxmedical.com

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