Impedance cardiography

What does this channel measure?

Electrical impedance changes in the thoracic cavity are largely dependent on the movement of blood. The largest contributor is the blood that is pumped vigorously by the left ventricle into the aorta with every heartbeat. The impedance cardiography (ICG) dZ/dt signal captures the velocity changes of the blood allows estimating pre-ejection period (PEP), left-ventricular ejection time (LVET), and stroke volume, among other cardiovascular parameters. PEP measures the latency between the onset of electromechanical systole, and the onset of left-ventricular ejection.

Interest in PEP springs largely from studies suggesting it is most heavily influenced by sympathetic innervation of the heart. Particularly in combination with the parasympathetic marker of cardiovascular activity RSA, PEP can be used to partition components of autonomic activation in a study of cardiovascular reactivity. PEP is noninvasively measured for any given beat as the time between the Q-point in an electrocardiogram (ECG) signal and the B-point in the derived impedance signal, dZ/dt.

The B-point in the ICG represents the opening of the aortic valve, when the blood suddenly shoots out of the already contracted left ventricle into the aorta. The B-point dZ/dt value is usually around 0, corresponding to very low velocity of the blood. The X-point represents the closing of the aortic valve to prevent the blood from the aorta streaming back into the left ventricle. Since the direction of blood flow at this point has typically already reversed (because of the 'cardiac afterload', the blood pressure the heart has to pump against), the X-point dZ/dt value is usually somewhat negative. The Z-point (dZ/dtmax) represents the maximal speed of the blood ejection. From these 3 points in relationship to the Q-point in the ECG, a variety of meaningful parameters can be estimated:


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Data preparation

Because the ICG signal is highly susceptible to even subtle movement artifacts and isometric muscular contraction near the thorax, it is typically necessary to average the wave forms across many beats to overcome the noise confound and assure reliable detection of the B-point and other inflection points in the ICG curve.

This averaging relies on the times specified in a special ICG timing file, which has the same type as standard ANSLAB timing files, except that it is named 'MyFileName.icg.m' instead of 'MyFileName.m' (see timing files for more information): segments found in this file will be used for beat averaging. You can create such a timing file by running marker analysis. You can additionally subdivide intervals created with the marer analysis using timing file modification from the tools menu.

Information from the analyzed ECG file is also needed for performing averaging of the wave forms, so make sure to have run the ecg analysis beforehand. If beat averaging is not synchronized as shown in the picture below, sampling rate information that was used for ecg analysis is likely to be incorrect: times of R-waves are interpreted based on the sampling rate information and beat epochs are extracted according to these times. Therefore, if the ecg-sampling rate is incorrect, beat epochs are badly selected.

Thirdly, for each subject the main impedance level and the sensor distance is required. You can supply this information manually by entering the corresponding values in the dialog forms shown below:

Less laborious is however to collect these values for all subjects in a textfile and have ANSLAB read the values from this textfile automatically. This textfile must located directly in the icg-subfolder of your study folder (not in a subfolder of it) and must be called 'icgparam.txt'. This file must contain three tab delimited columns of only numbers, the first column beeing the subject number, the second column sensor distance given in centimeters and the third the the impedance given in ohm.

An example content of 'icgparam.txt' is shown below (subject 19 - 32): 
    19    15      24
    20    15      28.5
    21    8.35    21.2
    22    6.9     31
    23    14.35   28
    24    12.4    27
    25    15.75   23.6
    26    17.1    27
    27    15      30
    28    13.8    32
    29    8.25    33
    30    20.35   30.5
    31    18.15   32
    32    13.75   31
Regarding the impedance, ANSLAB also offers the option to use a channel stored in the raw file. If this option is chosen, ANSLAB uses the mean impedance level within a segment based on the respective channel values inside the segment (or, if beat-by-beat analysis is enabled, the mean value of impedance values across a single beat).

Depending on the way you want to use to pass the needed information into ANSLAB, be sure to select the corresponding options in the ICG-options dialog.

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ICG analysis options

The ICG options dialog provides various different options:
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Analysis procedure

Segment-based vs. beat-by-beat analysis

Starting with ANSLAB 2.4, you can choose to either perform the ICG analysis based on timing file segments, on a beat-by-beat basis, or both (the analysis mode can be set in the options).

Considerations for batch mode

The ICG analysis may also be run in batch mode. However, since there may be high inter-subject differences with respect to the location of the characteristic points in the ICG waveform (i.e. B-, Z-, and X-point) it is recommended to carry out a manual analysis.

This is especially true in cases where the ICG signal has a low signal quality. In such a case it is crucial to set the detection windows right. You may run a batch analysis in this case, followed by batch plotting and make a judgement on the right detection windows for all subjects.

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Editing of ICG data

As with the other variables, select the file you want to look at. ANSLAB preprocesses the dZ/dt signal and in the first display (axis A) shows the dZ/dt ensembles synchronized by the ECG Q-point of each heart beat. In the second axis, an ensemble average across the shown beats is displayed, with standard error margins and the automatically detected B-, Z-, and X-point, as shown below.

You can display the corresponding piece of raw signal by selecting the 'see raw signal' button:

Outlier exclusion and autoexclusion

You can manually exclude outlier curves using the 'exlusion box' outlier rectangular function in the left axis. Moreover, if autoediting is activated in the ICG options, ANSLAB automatically exludes outlier curves that are above or below the mean +/- 2 standard deviations in the B-point window (shaded in light red).

Special emphasis is put on the B-point window, as outliers distort this point most heavily, although you can extend the sensitive window to cover the entire beat. Exluded curves are plotted in light red. You can adjust the auto-editing parameters (sensitive window and standard deviations factor) in the ICG-options. The auto-editing outlier criterion is calculated statically using all beats in a segment, whereas the +/-1 standard deviation range in the average plot is updated automatically based on remaining valid beats. Hitting the 'clear seg'-button will undo all exclusions for the current segment (including auto-exclusions).

Adjusting the B-, Z- and X point

You can drag the B-, Z,- and X-point in the right axis. You can also drag the detection windows. Dragging the left line of a detection window will move the window to a new position. Dragging the right line allows to change the window size. Number of beats, number of excluded beats, segment number, PEP and LVET values are displayed and updated according to editing steps in the data window.

The detection window positions and widths are also shown in the dynamic section and can be changed by entering new values (values are given in milliseconds and relative to the Q point position). The detection window settings can be saved for use with other files by pressing the 'save'-button. Selecting a point (X, Z or B) with the mouse also activates that point for a precise adjustment with the arrow keys (left and right). This is indicated by the 'arrow key setting active for:'-radiobuttons:

The activated option shows which point can be moved by pressing the left and right arrow key. Using this option, the points are moved by a very small amount (1 ms per keystroke). Therefore, adjusting the points roughly with dragging first is advisable. ANSLAB will remember exluded beats for a segment and point position changes you performed with dragging. If you whish to reset these choices for a given segment, press the 'clear segment'-button. Pressing the 'clear all'-button will reset all editing steps performed so far and restart with the first defined segment.

Pressing 'accept' will save the current averaged waveform along with the the B-, Z- and X-points and continue with loading the beats of the next segment. You can set a segment to missing data by choosing the 'set missing'-button. Hitting the '<<<'-button allows you to go back to the previous segment and continue editing there. You can also jump to a segment of your choice by entering a number in the 'jump to segment'-editbox, and you can save editing results to the current point by hitting the small 'save'-button.

If loading of previous analysis results is activated in the ICG options, these editing results are loaded when reopening the file for analysis. You can then use the 'last edited'-button to jump to last segment for which editing results can be found. After the all segments have been processed, extracted parameters are plotted over the duration of the file and you can choose to save the reduced data to file or discard analysis results. Note that extracted calculated traces are plotted as 'event'-type traces and can be directly aligned and compared with raw ICG and ECG signal by switching from 'event' to 'raw'-display-mode.

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Guidelines for setting characteristic points

The Z-point is almost always easy to identify and the detection algorithm hardly makes a mistake here. The B- and X-points can be more problematic in some subjects during certain tasks, especially if there is much movement artifact. And of course, analyzing ICG is much easier in young and healthy subjects than in older adults with cardiac disease.

If you see a distinct B-point in the raw data but not in the ensemble average this indicates that the ensembles are not aligned well and that the inflection point is 'washed out'. This could be because of a noisy ECG resulting in errors in the Q-point detection (in the order of a few milliseconds). But the Q-point times are the basis for the alignment of the ICG ensembles. If you are facing this problem you could use enable the option to use the q_fixed Q-point estimation. This forces ANSLAB to align the ICG at a Q-point estimated from the more reliable R-waves.

Another reason could be that the period you are averaging over does not represent a steady state. If the PEP changes considerably across your averaging period, this would also wash out the B-point inflection from the ensembles. In this case it is recommended that you define smaller segments in the ICG timing files. A reasonable estimation of ICG parameters can be based on as few as 15 beats. Note that PEP and other ICG parameters depend to some degree on the filling of the lungs, so it is important to average across several breaths.

If you can not see a distinct B-point in both the ensemble averages and raw data, this indicates that the subject has a cardiac and thoracic morphology that makes estimation of PEP with just spot electrodes difficult. You could exclude this subject from the ICG statistics or use the zero crossing mode. You may also manually reset the B-point to the zero dZ/dt line. This needs to be done across all tasks to be consistent.

The X-point can sometimes be ambiguous. There can be two or even three dips after the peak in the ICG signal. As a general rule it is then the second or third dip. Among these two it is the one followed by a consistently steeper immediate increase, which indicates the closing of the aortic valve and sudden stop of reflux of blood into the left ventricle. Another guideline is that typical LVET values for young healthy subjects range between 300 and 350 ms, but it also depends much on various factors like physical fitness and body mass index. It is important to be consistent with the identification of the X-point within subjects.

Sometimes excluding a problematic subject from the analysis of ICG derived parameters is the best option and helps to not distort group statistics.

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