Performing Bio-calibrations with Noxturnal and Noxturnal App 

Version 01 - 08 June 2023 

Summary 

This article provides best practices for bio-calibrations with Noxturnal and the Noxturnal App. The recommended bio-calibrations from the AASM are outlined, along with instructions for performing bio-calibrations with Noxturnal and creating custom bio-calibration lists. The Noxturnal App can be used for ambulatory and online recordings, and tips for performing bio-calibrations with the app are provided. The article also includes expected outcomes for bio-calibrations and tips for adjusting equipment if expected results are not achieved.

General instructions for performing bio-calibrations

The sequence of required bio-calibration steps will differ, and your laboratory’s procedures must be followed over this guide. Also, note that different study types will require different checks and bio-calibrations. Refer to your laboratory procedures for the correct instructions. The following are the recommended bio-calibrations from the AASM.

Below are general guidelines to consider when performing bio-calibrations. 

• The Patient should be connected set-up, lying still in the supine position with the lights on.
• Check the patient can hear you if you are not at the bedside or using an intercom system.

💡Tip: A better option is to utilise the Noxturnal App on a tablet, which allows users to be at the patient’s bedside to perform bio-calibrations, impedance checks, and signal adjustments without moving between the control room and the patient. See detailed instructions for performing bio-calibrations with the Noxturnal App in the Performing Bio-calibrations with Noxturnal App section.  

• Perform the bio-calibration steps one at a time, communicating the instructions to the patient and scoring the event when successfully performed. During the patient bio-calibration, observe the recorded signals to ensure the correct responses are evoked for all the defined manoeuvres. 

Performing bio-calibrations with Noxturnal for Online Recordings

To perform bio-calibrations in Noxturnal, enter the bio-calibrations toolbar by clicking the scoring menu (1) and the bio-calibrations at the bottom (2).

The Noxturnal bio-calibration checklist features the standard instructions outlined by the AASM:

• Perform and document an impedance check of the EEG, EOG, and EMG electrodes 
• Record a minimum of 30 seconds of EEG with the patient awake lying quietly with eyes open
• Record a minimum of 30 seconds of EEG with the patient awake lying quietly with eyes closed
• Ask the patient to look up and down without moving head  
• Ask the patient to look left and right without moving head 
• Ask the patient to blink 
• Ask the patient to grit teeth and/or chew 
• Ask the patient to simulate a snore or hum 
• Ask the patient to breathe normally and ensure that airflow and effort channel signals are synchronised 
• Ask the patient to perform a breath hold (10 seconds) 
• Ask the patient to breathe through the nose only (10 seconds) 
• Ask the patient to breathe through the mouth only (10 seconds) 
• Ask the patient to take a deep breath and exhale slowly (prolonged expiration - 10 seconds) 
• Ask the patient to flex the left foot/raise the toes on the left foot 
• Ask the patient to flex the right foot/raise the toes on the right foot  
• Perform and document a repeat impedance check of the EEG, EOG, and EMG electrodes at the end of the PSG recording 
• Repeat physiological calibrations at the end of the PSG recording 

Not included in the AASM recommendations or with the Noxturnal procedure is the EKG. Users should Adjust ECK/ECG signal to provide a clear waveform - the R waveform should deflect upwards to ensure an accurate ECG recorded for analysis. 

⚠️ Note: Noxturnal 6.3.2 and earlier feature a small bug in that the bio-calibrations are inverse to what is made in the settings.

Creating custom bio-calibration lists in Noxturnal

To create custom bio-calibration steps and procedure in Noxturnal, go to Tools (1)… Settings (2)... Bio Calibration (3). 

Custom bio-calibrations are broken down into two parts: 

• Procedure is for naming and organising the sequence of the steps 
• Steps are for creating the individual steps within the procedure 

💡Tip: Create and number your steps before creating and organising the procedure.

Each step has a few options:

1. Step to perform - this is the title of the Step
2. Expected results - this is a description of what should be achieved with the step. This can be left empty if not required.
3. Preferred channels - this is what channels will show on the Noxturnal App when performing the calibration steps. For example, for the instruction blink 5x, the EEG and EOG channels will be shown in the Noxturnal App. This aids users in visualising the important signals when performing bio-calibrations on the smaller screen of a tablet or phone.
   

The next step is to create a procedure and order your steps as outlined in the GIF below. 

Examples of custom bio-calibrations include specific instructions for MSLT and MWTs as outlined by the AASM in Practice Parameters for Clinical Use of the Multiple Sleep Latency Test and the Maintenance of Wakefulness Test. The bio-calibrations for an MSLT are: 

• Keep eyes open for 30 seconds
• Close both eyes for 30 seconds
• Without moving your head; using only your eyes, look right, then left (x5)
• Without moving your head; using only your eyes, look up, then down (x5)
• Slowly and distinctly blink your eyes 5 times
• Clench or grit your teeth.

Users could also add nap start and end instructions to the procedure to help analyse MSLT and MWTs studies. This custom bio-calibration procedure can be downloaded as an attachment at the bottom of this article. Note that the instructions will be inversed in Noxturnal due to a current limitation. 

Performing Bio-calibrations with Noxturnal App

The Noxturnal App can perform bio-calibrations and view signals at the patient's bedside or during set-up. The app can be used for both ambulatory and online recordings. 

 

The app has two modes, Ambulatory Mode, and Online Mode. The Ambulatory Mode allows the Noxturnal App to connect directly to the Nox A1 and A1s recorders for ambulatory studies, and the Online Mode allows the Noxturnal App to connect to the Nox A1 and A1s recorders via the Nox C1 Access Point for online recordings. For ambulatory studies, the app must be set to Ambulatory Mode. The app must be set to Online Mode for connections to online recordings. The mode can be changed in “Settings” under “App Mode”, as shown below.

Note: Custom bio-calibration procedures can be utilised from the app. The custom templates must be created in Noxturnal and transferred to the Noxturnal App. This is done by configuring an ambulatory PSG with Noxturnal by connecting the Nox A1 and A1s recorders to the PC. The custom bio-calibrations will be included in the configuration and stream to the tablet when connected. Custom bio-calibrations will stream to the Noxturnal App via the connection to the Nox C1 for online records. To change the bio-calibration set used, click on the icon below: 

A recording needs to be started to perform bio-calibrations with the Noxturnal App. Bio-calibrations are performed in the BIOCAL tab of the app (see image below). 

Follow the steps outlined in the procedure set. For more details on the standard AASM bio-calibration procedures, see this article's Performing bio-calibrations with Noxturnal for Online Recordings section. To note the bio-calibration event performed, click “Score event (1)” when performing that task. The user can switch between tasks at the top of the tab by clicking the left or right arrows (2) or the bio calibration drop-down menu (3).  

For every bio-calibration step scored in the Noxturnal App, a green vertical line will appear on the signal sheet. The event will be simultaneously scored in the Noxturnal software for online recordings. For ambulatory recordings, the events can be viewed when the recording has been downloaded to Noxturnal.

​​Signals can be viewed on the “BIOCAL” and “TRACES” tabs on the app. To add or remove signals, click the “Select Signal” (1 in the image above) icon at the bottom of the screen and select or de-select signals. In addition, the time frame viewed can be adjusted by clicking the “Time scale” icon (2 in the image above). 

The image below highlights the traces selection and options available. (1) shows a visible signal and (2) shows a signal that is not visible. Visible signals can be toggled on and off in this view. 

Bio-calibration Tips from the Field

Below are the expected outcomes for bio-calibrations performed on a healthy male subject (the author). Note that chin EMG wasn’t used in this example due to the author’s large beard. 

1. The first half of the example below features low-amplitude, mixed-frequency EEG of the user awake with eyes open. A few rapid eye blinks can be seen in the EEG. Halfway through, the instruction is given to close eyes. The EEG pattern slows to show a prominent alpha waveform (awake with eyes closed) in all EEG channels. In most people, when the eyes are closed, alpha waves become prominent in the EEG, especially in the occipital channels. When the eyes are opened, the EEG resumes its original waking activity, which consists of a mixture of fast, low-frequency waves, primarily alpha and beta waves (the latter of which have a frequency of 18 to 30 Hz) This provides important information that the person undergoing the sleep study does produce alpha rhythm or what is now termed posterior dominant rhythm (AASM, 2023), which will be important and helpful for scoring sleep onset and arousal length during analysis. 

If prominent ECG or 50/60Hz is seen in the EEG, EOG, or EMG channels, now is the time to adjust the electrodes to remove this artifact. Reducing and removing artifacts is always better than relying on the ECG and Notch filter. 

To better understand eye movements in bio-calibration commands and how they appear on the EOG and EEG channels, it's important to know that the eyes act as a dipole. This means that the front of the eye has a positive charge in relation to the back of the eye. 

When the eyes move in the same direction, one electrode records a negative charge while the other records a positive charge. When using the AASM-recommended placement for EOG, horizontal eye movements cause greater out-of-phase deflections than vertical movements. For these channels, the eye signals of each channel must deflect in opposite directions during conjunctive eye movements. To simulate these movements during REM sleep, ask the patient to look left and right five times, then up and down five times. This should result in one eye channel deflecting upward and the other deflecting downward due to the eye dipole, but this may vary if alternate EOG electrode placement is used.

 

2. As noted, chin EMG was not used in this example. But notice the strong muscle artefact in the EEG, particularly the frontal and central (Fs and Cs) channels from the patient gritting their teeth. Like limb EMG, the user should expect a sharp increase in chin EMG tone when this command is performed, or the electrodes should be adjusted. 

3. This example shows the detection of limb movements via the limb EMG channels. These channels are important to measure the leg movements that occur with periodic leg movement (PLM) disorder or restless legs syndrome. The example below clearly shows increased leg EMG activity with toe flexes.

Now we will look at several respiratory-related bio-calibrations. The example below is from a real PSG performed in a sleep laboratory. 

Points 1-3 show the breath-hold manoeuvre. This mimics a central apnoea and allows users to check the cannula, RIP Belts, calibrated RIP Flow, and thermistor, if used, do not detect respiratory effort and flow. These sensors must detect the lack of airflow during an apnea episode. 

To ensure that the airflow sensors detect nasal and oral breaths, ask the patient to breathe through the nose only for 10 seconds and then to breathe for 10 seconds through the mouth only. As the person inhales and exhales, the cRIP Flow, thermistor, or thermocouple should generate sinusoidal signals on the airflow channel. Nasal pressure from the nasal cannula only measures nasal airflow; therefore, the signal in this channel should be reduced or flat during oral breathing maneuvers (small changes in nasal pressure do occur with oral breathing and can be detected).

As you can see in this example, all signals are flat, indicating a pause in breathing. The user can adjust the gain to represent the central respiratory event better or adjust the equipment for a better recording of these events.  

Point 4 shows nasal-only breathing. The cannula features good detection of the changes in pressure with inspiration and expiration. The RIP bands also respond to the changes in volume with inspiration and expiration. 

Points 5 and 6 show the patient breathing through their mouth only. The nasal cannula only detects a few small changes in pressure and is mostly flat. The RIP bands show normal respiration, reflected in the calibrated RIP Flow (point 6), which shows the changes in respiratory flow from mouth-only breathing. 

Users should adjust the pressure and airflow sensors if these expected results are not achieved. The nasal cannula should be repositioned if nasal inhalation and exhalation produce no or low flow. Likewise, if the RIP bands produce minimal to no change in respiratory effort when the patient breathes, the bands need to be adjusted. More information on the RIP technology and Nox Medical’s specialised RIP belt technology can be found on the support page. Note the correct sizing is critical, and Nox Medical recommends that minimal resizing of the bands should be required if the correct size is used on the patient. The sizing guide for the Nox RIP belts can be found on the support page. 

Another respiratory feature usually tested during physiologic calibrations is paradoxical breathing, the movement of the abdomen and thorax in opposite directions when the upper airway is fully blocked but the person continues to make respiratory efforts to breathe. Paradoxical breathing can occur during an obstructive sleep apnea (OSA) episode. To mimic paradoxical breathing, ask the person to hold the breath while making breathing movements. An alternative is to ask the person to pant. Either of these actions will cause the thoracic and abdominal RIP channel signals to move in opposite directions.

Note that when viewing the nasal cannula on the Noxturnal App via a tablet, the Flow channel from the nasal cannula with be represented by the Resp. Pressure Raw (cmH2O). This is the unfiltered recording of pressure from the Nox recorder. In the example below, you can see nasal breathing (1) and mouth-only breathing clearly with the changes in pressure. 

Below is another example of respiratory bio-calibrations from Noxturnal for a diagnostic sleep study. 

1. This example shows the patient mimicking snoring. Notice the snore detected in the Audio channel (Db) and the slight flow limitation of the nasal flow channel depicting mild snoring. 
2. Point two shows a breath hold with the Cannula Flow, thermistor, cRIP Flow, and the RIP bands all being flat and showing no breathing detection. This is the expected result. 
3. Sinusoidal detection of nasal-only breathing in Cannula Flow, thermistor, cRIP Flow, and the RIP bands
4. Shows a reduced flow in the Cannula Flow as the patient is breathing through their mouth only. Notice the thermistor, cRIP Flow, and RIP bands still show ongoing respiratory flow and effort. This provides a baseline for scoring when mouth breathing is in question and is an important consideration for the analysis of PSGs. 

This next example is bio-calibrations performed with a full-face (nasal and mouth) PAP mask. Here we see the patient performing a breath hold with the PAP mask on and set to 6cmH2O. The pneumotach of the PAP device and calibrated RIP Flow detected no flow that matches the RIP belts showing no respiratory effort. The user now has baseline information for apnoeas during the study.