- The Cardea SOLO™ ADX S400 software supports interactive and automated determination of QT and RR values from median averaged beats.
- Additional tools support interactive correction of QT for changes associated with heart rate, thus deriving QTc.
- The QTc correction formulas include Bazett, Fridericia, Framingham, Hodges and a personalized correction derived from the patient’s QT-RR data.
- Corrected or uncorrected QT data can also be displayed as a function of time, highlighting any temporal changes in QTc over the recording period.
- Images of the QT vs RR and QT vs Time can be captured and included in the Report.
Measuring QT
The below figure shows the interactive and automated tool for determining the QT-RR values for the patient.
Figure 8.1. QT-RR evaluation.
- Waveform: The displayed waveforms are computed from a selection of 30 beats centered on the time point of interest. Within this interval, beats are selected that are all within the same Beat Class, are normal sinus beats, and have valid leading and following RR intervals. Each beat is time aligned with the onset of the QRS (Isoelectric point). To derive the waveform value for each sample the amplitude values from all beats, at the selected sample, are sorted largest to smallest and the middle half are averaged (Median Averaged Beat). This method is quite robust in the presence of noise.
- Q Onset, S End and T End (Q, S and T): The software automatically picks the onset and offset for these three phases. The vertical lines in the above figure show the automated picks. The end of T phase is highlighted in Blue, indicating that it is currently selected for interactive editing. Position the cursor at any point and click – the end of T will move to the click point and the measurements (top of figure) will automatically update. The Radio Buttons below the graph support changing which phase is selected for editing. You can also select a phase for editing by right-clicking in the vicinity of the phase.
- Interval: The time interval, on the right side just above the graph, defines the time interval between selected analysis points. The default is 10 minutes. Clicking either the Back or Forward button will select and process the ECG data for the updated analysis point. The entire record can be manually reviewed by successively selecting Forward.
- Recording Heart Rate Overview: The colored ribbon at the bottom the screen represents the average heart rate over the entire record. Cool colors are low heart rates and hot are high heart rate. Moving the cursor along the strip displays the date and time of the point on the ribbon. Clicking anywhere on the ribbon will update the screen with the QT analysis for the selected point.
- Start Button: The Start button will start processing the entire record, beginning from the currently displayed waveform. Each waveform analyzed is presented, results saved, time index is incremented, and so on until the end of the record. The Start button changes to Stop, supporting termination of the automatic processing.
- Exit: The Exit button closes the QT analysis window.
QT Review
Following the QT Analysis step, the associated beat waveforms and end of T picks cam be reviewed and edited using the QT Review tools located on the second tab of Figure 8.1.
Figure 8.2. QT Review.
- The red ticks are the picked end of T locations.
- Clicking on a trace will reposition the pick point to the click point. Both traces will be updated with the new pick.
- Right clicking on a trace will display a pop-up menu showing the trace number with an option to delete the two traces and associated QT value from the collection.
- Zoom increases the trace amplitudes and
- Next / Last Page moves the display to the next or previous 50 traces.
QT Versus RR
Following QT Review (second Tab in figure 8.1 above) the QT vs RR graph displays all the measurements.
Figure 8.3. QT vs RR graph (Uncorrected for heart rate).
- Invariably there is some noise in the determination of QT associated with noisy sections of the ECG record.
- These noisy points can be quickly removed from the data set.
- To select a region of the plot for editing, click down on the left mouse button and drag the mouse down and to the right.
- A box will track the mouse and all points within the box are selected for editing.
- Releasing the mouse button displays the selection box. The selected data points can be:
- Deleted from the record (permanently deleted);
- Include in the analysis (restored if the points have been previously
- Excluded;
- Exclude from the analysis; and,
- Cancel.
- Excluded data points are displayed in Gray.
- This same functionality is available in the QT versus Days plot. When looking for temporal changes in QT it might be useful to Include / Exclude time segments for independent analysis. The Restore button sets all points to Include.
- The QT data displayed on figure 8.3 can be corrected for heart rate using the standard equations via the drop-down menu at the lower left (Bazett, etc).
- The applicability of the selected equation can be judged by how well the resulting QTc values are flattened around the QT value at RR = 1 second (HR: 60, QTc).
- Often these population-based equations do a poor job of correcting QT for an individual.
- The “Personalized” option fits a quadratic curve through the raw QT-RR data points and uses the resulting equation to heart rate correct the data.
Figure 8.4. QTc using a Personalized QT-RR correction curve.
The Save button creates and saves a JPG image of the graph. You can specify a title for the image and note that the image should be included in the updated report.
QT Versus Time
The QT values, corrected or raw, as shown on the QT-RR graph, can also be displayed as a function of time.
Figure 8.5. QTc versus time.
- The daily average QTc value is shown at the bottom of the graph.
- The color coding of the QT data points can also be displayed with a time based gradient spectrum. Clicking back to the QT-RR plot continues the color spectrum. This can help visually identify QT changes that occur over time.
- The red trace is a smoothed fit to the QT data. The duration of the smoothing window, in hours, can be changed using the input box at the lower left.
- The last tab, QT Images, displays the images saved during the analysis, and a designation of which images should be included in the updated final report.
Figure 8.6. Image summary.