Last Updated 20-08-2024

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 View

There are three methods for viewing the impedance of the EEG, EOG, ECG, and EMG signals of Nox SAS. 

• Continuous impedance signal view
• Impedance View (requires Noxturnal 7.1)
• SAS Impedance Signal sheet (requires Noxturnal 7.1)

Continuous impedance view (1) provides the real-time impedance value of the channel during an online recording or when reviewing the study data. 

Impedance View (2) provides the individual signal impedance value as a diagram and also has the option for a table view (3) of all of the ExG signals that have impedance measured. 

The final option (4) is the built in SAS Impedance Signals worksheet that shows the ExG signal impedance in a time series. This view can helpful for getting an overall picture of the signal quality during an online recording. 

During Online recordings, users can add the SAS impedance view widget (5) to the toolbar for a quick visual guide to the individual SAS head cable signal impedance measurements. 

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. With Nox SAS, the recording references are AF7 and AF8. These three electrodes - PGND, AF7, and AF8 - must be connected to the client 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 signal impedance in the table view or the SAS Impedance Signals worksheet. 

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

Note that the individual signal impedance for the ECG and limb EMG can only be viewed in the SAS Signal Impedance worksheet. Only the SAS face cable (EEG, EOG, facial EMG) signals are available in the Impedance View. Continuous impedance measurements of the bipolar ECG and limb EMG channels are available in the worksheet view. 

Known Issues for Impedance Measurements with Nox SAS

• The SAS Impedance View table will show no data when reviewing the data after a recording has finished e.g. when scoring or reporting. Click on a signal and move forward in time (one epoch) and the table will generate impedance data. 

 

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