This document explains the most commonly used detector settings within the Noxturnal software that are not self-explained.
No settings available (trained neuronal network)
Minimum Interval: An arousal event is not scored if the time interval between the start time of the event and the stop time of the last event is less than the Minimum Interval.
Join Interval: If the time interval between two arousal events, from stop time to start time, is less than the Join Interval, these two events are merged together. The new event has a quality value equal to the higher quality value of the two original events.
Quality Threshold: A model, trained with a machine learning algorithm, is used to detect arousals. Each arousal event is assigned a quality value, based on how confidently the model can detect it. An arousal event is not scored if the quality value is lower or equal to the specified Quality Threshold.
Minimum Upright Angle: If the upright angle of the patient (derived from the elevation signal) is higher or equal to the Minimum Upright Angle, the patient is considered upright. The upright angle is defined as the deviation of the patient’s upper body from being horizontal, where an upright angle of 0° represents a horizontal upper body, and 90° represents a vertical upper body.
Why 53.13°? The upright angle signal is derived from the measured elevation signal. In the previous versions of the algorithm any elevation values higher or equal to than 0.8 were interpreted as an upright, which corresponds to the current default value of the upright angle, ~53.13°. This value is used in order not to change the behavior of the detector from previous versions. The user can now edit this parameter when creating the position detector protocol. Any values between 0° and 90° are allowed.
Amplitude Threshold: Minimum amplitude to create an activity event.
Join Interval: If the time interval between two activity events, from stop time to start time, is less than the Join Interval, these two events are merged together.
Drop calculated against: See info on Desaturation detector
Max duration to have an Arousal following: 15 s
Lowest Saturation value: The saturation values below the specified Lowest Saturation Value are marked as an artifact and not used for hypopnea detection.
Minimum Desaturation Drop: A drop is defined to start wherever the signal starts dropping. The starting value is the last one in the plateau before the drop, but the rest of the plateau itself is not included. The end of the drop is defined as the start of the any of the following events:
- A rise that is >= 2%
- A plateau longer than the Longest Plateau Duration
- The signal has risen once (1%) and a plateau afterwards duration is > 3 s
- The number of consecutive rises (1%) is >= 2
- The signal drops below the Lowest Saturation Level
The drop in the signal from the start value to the stop value shall be equal to or higher than the Minimum Desaturation Drop for a desaturation event to be scored.
Shortest Desaturation (sec): The time interval from the start point to the stop point as defined above shall be equal to or longer than the Shortest Desaturation specified for a desaturation event to be scored.
Longest Plateau Duration: Maximum duration of a flat interval in the signal in to be eligible to be included in a drop event.
Lowest Saturation value: The saturation values below the specified Lowest Saturation Value are marked as an artifact. Desaturation events that overlap an artifact, are not scored.
Please see this article about Flow Limitation:
Threshold: The standard deviation is calculated for the flow signal during inhalation using a Blackman window (signal processing) to exclude the edges of the inhalation period. If the standard deviation is lower or equal to the specified Threshold, the breath is categorized as limited, otherwise as normal
Test for center frequency: The center frequency is the frequency that divides the spectrum power equally in two parts. If Test center frequency is chosen, a snoring is only scored if the center frequency is below 500 Hz.
- Thresholds: The threshold type can be either Relative or Absolute.
- Relative: A limb movement (LM) is detected if the amplitude of the leg signal being scored exceeds 5 times the background noise which is present during resting periods. The LM event is however defined from the point in time where it exceeds 3 times the background noise, and to the point where it goes below that limit again. This description fits the settings as pictured above, but both thresholds can be edited by the user.
- Absolute: Works in the same way as the relative threshold, except the thresholds are now the exact number of µV that the signal has to rise above the background noise.
- Time Constraints: The settings in this chapter apply to the detection of LMs (as opposed to PLMs)
- Minimum Duration: The minimum duration for an LM to be scored.
- Maximum Duration: The maximum duration for an LM to be scored.
- Join Interval: If two LMs are separated by an interval smaller than the specified Join Interval they are merged together
- Minimum Interval: An LM event is not scored if the time interval between the start time of the event and the stop time of the last event is less than the Minimum Interval. This parameter is effectively not used when it the same or lower than the Join Interval.
ECG: Brady and Tachycardia
Minimum Interval: A bradycardia or tachycardia event is not scored if the time interval between the start time of the event and the stop time of the last event is less than the Minimum Interval.
Pulse Wave Analysis (Autonomic Activity)
Minimum Drop: Similar to the desaturation drop detector, a drop is defined to start wherever the signal starts dropping. The starting value is the last one in the plateau before the drop, but the rest of the plateau itself is not included. The end of the drop is defined as the start of the any of the following events:
- A plateau longer than the Maximum event duration
- The signal has risen once and a plateau afterwards duration is > 3 s
- The number of consecutive rises is >= 2
Moving Average Duration: The size of the window used for estimating the baseline (from which the drop is estimated).
- Minimum and Maximum Duration: These refer to the minimum and maximum timeframe for the event duration.
- Detection Threshold: the EMG needs to be at (x) times of the background EMG activity. The EMG value is allowed to go below the threshold for a maximum of 0.25 seconds without ending an event.
- Minimum and Maximum Interval: These refer to the allowed interval between bursts to be counted together in a phasic event. If two bursts are closer than the minimum interval, the latter is not included in the phasic event (and does count towards the minimum number of events). It is not removed either, so if the third burst fits the criteria, the second is included by default. If the interval between two bursts is larger than the maximum interval, they cannot both be included in the same phasic event. There is not a maximum number of bursts specified.
- Minimum number of Events: In order to score a Phasic event there need to be minimum (x) burst events to correspond meeting the minimum and maximum interval conditions.
This detector scores first scores all tonic and all phasic events and combines these accordingly with potential mixed events. The rules for these events are slightly modified from the AASM guidelines (detection threshold for phasic is different) but for more details please also see: https://support.noxmedical.com/hc/en-us/articles/201604873-Sleep-Bruxism
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