AMSIMP is a program that is part of the software package that comes with the VU University Ambulatory Monitoring System (VU-AMS). It can be used to view and analyze ICG data that is recorded with VU-AMS devices, that are equipped with thorax impedance measurement facilities (serial numbers 44XX and 46XX). The program contains an automatic scoring algorithm for detecting the (dZ/dt)min, the upstroke (B-point) and incisura (X-point) in each ICG complex. The automatic scoring algorithm is used as a guideline for choosing these three specific locations manually.
We recommend to read Sherwood et al.'s "Methodological Guidelines for Impedance Cardiography" Published in Psychophysiology 27,1, 1990. before starting with scoring and analyzing the AMSIMP impedance cardiogram.
We have composed a document in which we explain the best way to deal with icg scoring. To see this document go to: ICG Scoring tips
Data can be loaded in 2 ways, depending whether you want to score a new VU-AMS file or load a previously scored ICG data file.
Importing VU-AMS data files (.ams)
Use the 'Import AMS File ...' command from the 'File' menu to import your VU-AMS registration. The program will first extract all recorded ICG complexes from the binary VU-AMS file (.ams). When this is done an automatic scoring algorithm is started, which tries to detect 3 specific locations in each complex: Upstroke (B-point), (dZ/dt)min and Incisura (X-point). These 3 points are shown as vertical (movable) lines in the ICG-graph and are used to calculate several physiological variables: Stroke Volume, Heather Index, Pre Ejection Period (PEP), and Left Ventricular Ejection Time (LVET).
Loading (partially) scored ICG data files (.icg)
As soon as you save a file from AMSIMP, a binary ICG-file (with extension .icg) is created to store all (manual) scoring on the ICG complexes thus far. Use the 'Open...' command from the 'File' menu to load previously scored ICG data files.
There are two parameters that influence the calculation of the stroke volume: the distance between the measuring (white) ICG electrodes, and the specific blood resistance (rho). These variables can be changed by clicking the 'Set parameters…' command in the 'Edit' menu.
By default the following values are used:
|electrode distance||Le||20.0 cm|
Viewing and scoring complexes
After loading your ICG data the following window is displayed.
This window is used to display the following information:
Graphical representation of the current ICG complex.
A graphical representation of the current ICG complex is depicted in the upper right of the window. The graph shows the 128 dZ/dt samples (every 4 msec) that constitute a single ICG complex. Since the VU-AMS device uses a pre-trigger buffer, the starting point of this graph lies 12 milliseconds before the R-wave (3 dZ/dt samples). Note that each complex is the result of ensemble averaging several successive ICG complexes. The VU-AMS device measures 128 dZ/dt samples (with a sampling rate of 250 Hz) around each R-wave, giving a dZ/dt time series of 512 milliseconds for each heartbeat. To reduce the amount of data, the VU-AMS device ensemble averages all measured complexes that fall within a specified period of time, typically between 15 to 60 seconds (see the AMSCOM manual for information on how to change this setting). This averaged time series is then stored as one complex (i.e. one per period).
The graph also shows three vertical lines, representing: Upstroke (B-point), (dZ/dt)min and Incisura (X-point). You can use the mouse or the keyboard (left or right arrow in combination with the shift key) to move the lines. The Page Up and Page Down key's allow you walk through all recorded complexes.
The black spikes on the bottom of the screen represent the candidate upstroke points that where calculated during automatic scoring. The highest spike is selected as the most likely Upstroke point. This scoring algorithm is far from perfect. Visual inspection of each complex is always necessary. See scoring tips for some guidelines.
Next to the graph, this window contains several frames.
In the upper left of the window the following information is given:
- complex#: Indicating which ICG complex is currently available for scoring. The total number of complexes is indicated as well.
- Date & Time: Date and time of measurement of this complex
- Label: Optional label attached to this complex. Use AMSGRA software to label time fragments.
The middle panel on the left side of the window indicates the parameter settings.
- rho: Specific blood resistance in [ohm.cm]
- Electrode distance (Le): Distance between measuring electrodes in [cm].
- Z0: Average thorax impedance in [ohm] during current period.
- Heartrate avg.: Average heart rate in beats per minute [bpm] during current period.
The lower panel on the left side of the window shows the parameters that are calculated by AMSIMP.
Stroke volume: Stroke volume in [cc]. Calculated with the following formula (1):
, where tlve = tincisura - tupstroke
Heather index: Header index in [ohm/sec^2]. Calculated with the following formula (2):
, where tR->min is the time between the R-wave and (dZ/dt)min.
- Minute volume: SV*HRA in [l/min]
- PEP: Pre Ejection Period: the time between Q-onset and upstroke in [msec].
- R - (dZ/dt)minimum: tR->min the time between the R-wave and (dZ/dt)min in [msec].
- LVET: Left Ventricular Ejection Time in [msec], i.e. tlve = tincisura - tupstroke
In this panel the scoring of the current ICG complex, as indicated by the three vertical lines, is given. All times are relative to Q-onset
- Upstroke (B-point): Upstroke point in [msec]
- (dZ/dt)minimum: Minimum dZ/dt of complex in [msec]
- Incisura (X-point): Incisura point in [msec]
In the lower right panel the user can indicate whether the current ICG complex is accepted (black) or rejected (red). Rejected complexes can be discarded from the output file (.irp), which will be used for further statistical analysis.
Once your ICG data is scored you can save all information (ICG complexes and scored points) to a new binary ICG file. The ICG file can be opened again by AMSIMP, preserving all scoring information.
When saving the binary ICG file, AMSIMP also creates an ICG report file; This is an ASCII file (with the extension .irp) that contains a single line for each ICG complex.
Example of an ICG report file (.irp)
Each line contains the following information:
|1 - 8||ppnr||string|
|9 - 15||ICG complex number, starting at 0||integer|
|16 - 22||beat-to-beat block number, starting at 0||integer|
|24 - 31||date||dd-mm-yy|
|33 - 40||time||hh:mm:ss|
|41 - 46||PreEjection Period (PEP)||[msec]|
|47 - 52||(dZ/dt)min time||[msec]|
|53 - 58||incisura (X-point)||[msec]|
|59 - 65||average thorax impedance (Z0)||[ohm]|
|66 - 71||average inter-beat-interval (IBI)||[msec]|
|72 - 77||amplitude upstroke (B-point)||[ohm/sec]|
|78-83||amplitude (dZ/dT)min value||[ohm/sec]|
|84-89||amplitude incisura (X-point)||[ohm/sec]|
|90-95||Left Ventricular Ejection Time (LVET)||[msec]|
|96-101||R peak to (dZ/dT)min in time||[msec]|
|102-109||stroke Volume (SV)||[cc]|
|119 -107||judgement: A(=Accepted) or R(=Rejected)||'A' or 'R'|
|120 +||optional label description (N/A if not available)||label codes|
Note that, since the lines in these files are relatively long (>80 characters), some programs wrap the lines at the end when there is not enough room to print them on one line. However, most data processing software and statistical packages read the lines correctly.
This report file can be used as input for statistical programs for further analysis.
Kubicek WG, Karnegis JN, Patterson RP, Witsoe DA, Mattson RH (1966) Development and evaluation of an impedance cardiac output system. Earospace Med 37: 1208-1212
Heather LW (1969) A comparison of cardiac output values by the impedance cardiograph and dye dilution in cardiac patients.
Progress Report Contract No NAS9-4500: 247-258 National Aeronautics and Space Administration, Houston, Texas, USA