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IBI Sequence Analyzer

The IBI Analyzer module performs heartbeat locked analysis around user-specified events, which can be used to calculate stimulus specific heart rate acceleration and deceleration.


Table of contents
  1. Introduction
  2. User Corrections
  3. Processing and Analysis Pipeline
  4. Settings
  5. Metrics

Introduction

The IBI Sequence Analyzer performs event-based IBI sequence extraction using the IBIs generated and corrected in an ECG Signal Analyzer, and events defined using Event Definition Rules. Per event, the neighboring IBIs are detected and indexed.

This module requires that an ECG Signal Analyzer be present in the PhysioAnalyzer file. The IBIs used in the IBI Sequence Analyzer are automatically updated when the linked ECG Signal Analyzer changes.

The output of this module consists of the IBIs surrounding events, and their delays relative to those events. As such, with some post processing, this module allows anticipatory and evoked IBIs to be analyzed both as a function of their beat index (i.e., beat -1, beat 0, beat 1, beat 2, etc. around a stimulus); and, through interpolation, as a function of relative time (i.e., IBI at -1, 0, 1, 2 etc. seconds around a stimulus).

IBI Sequence Analyzer
Figure 1: The IBI Sequence Analyzer, showing IBIs -5 to 5 surrounding an event (red triangle). The size of the IBI circles increase with their index, which allows the user to detect when IBIs from one event overlap IBIs from another event. Note that IBI(-1) is the first IBI to occur after the event, however, it is not characterized as IBI(0) due to the threshold being set at 0.5.

User Corrections

The IBI module itself does not allow users to correct artifacts, this must instead be done in the linked ECG module. Any changes made to the IBIs in the linked ECG module are automatically transmitted to the IBI module, and the graphs and results table are automatically updated.


Processing and Analysis Pipeline

When indexing the IBIs around events, the IBI corresponding to the event itself, IBI(0), is first identified. IBI(0) is the first IBI after an event, unless the event occurs after a certain fraction of that IBI, in which case the next IBI becomes IBI(0). The time before which the event must occur so that a given IBI remains IBI(0) is calculated using Eq 1.

\[\begin{equation} t_{cutoff} = t_{IBIstart} + (threshold \cdot IBI) \tag{Eq. 1} \end{equation}\]

After IBI(0) is defined, the IBIs spanning from the setting Lower Index to the setting Upper Index, relative to IBI(0), are extracted; e.g., if Lower Index = -2 and Upper Index = 5, two IBIs before IBI(0) and five after are considered, for 8 in total.

Note, only events where all neighboring IBIs are present are analyzed.


Settings

The IBI module must be linked to an ECG module in order for it to function. This is done by filling in the tag of the ECG module in the Tag of the ECG Analyzer field.

The auto-generated list below shows the settings available in the IBI Sequence Analyzer module:

  • General Settings:
    Name, source and event settings for this IBI Sequencer. An ECG Analyzer must exist, so that its (corrected) IBIs can be used for sequencing.

    • Analyzer prefix (tag):
      The tag (name) of this PhysioAnalyzer. The tag must be unique and start with a letter, and may only contain alphanumeric characters.

    • Tag of the ECG Analyzer:
      The tag of the ECG Analyzer from which the (corrected) IBIs are to be used.

    • Generate events from:
      Specifies how events are generated.

  • Sequencing Settings:
    This analyzer extracts the IBI values around predefined events. First, the IBI corresponding to the event–IBI(0)–is detected; then, the IBIs spanning from ‘Lower Index’ to ‘Upper Index’, relative to IBI(0), are extracted. IBI(0) is the first IBI after an event, unless the event occurs after IBIstart+(threshold*IBI), then the next IBI becomes IBI(0).

    • Lower index:
      The lower index of the IBIs to be analyzed.

    • Upper index:
      The upper index of the IBIs to be analyzed.

    • IBI(0) threshold:
      The threshold for classifying IBI(0).


Metrics

The output of the IBI Sequence Analyzer is determined by the settings; for each IBI index, two variables are produced. See table below. Empty rows in the output may indicate that that event features missing IBIs.

The auto-generated table belows lists all the metrics produced by the IBI Sequence Analyzer module.

Table 1: The metrics calculated by the IBI Sequence Analyzer module.

Variable: Unit: Description:
IBI_index s The value of the IBI with the given index, per event.
Delay_IBI_index s The time of the IBI with the given index, relative to the event.

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