Short-term variability (STV) of QT (STV_QT) and beat-to-beat heart rate variability (STV_RR) has been used as a marker for predicting proarrhythmic risk.

Reference

For more information on this calculated parameter, please refer to the following publications:

Fossa, A. A., Langdon, G., Couderc, J. P., Zhou, M., Darpo, B., Wilson, F., … Davis, J. D. (2011). The use of beat-to-beat electrocardiogram analysis to distinguish QT/QTc interval changes caused by moxifloxacin from those caused by vardenafil. Clinical Pharmacology and Therapeutics, 90(3), 449–454. https://doi.org/10.1038/clpt.2011.139 
Fossa, A. A., Wisialowski, T., Magnano, A., Wolfgang, E., Winslow, R., Gorczyca, W., … Raunig, D. L. (2005). Dynamic beat-to-beat modeling of the QT-RR interval relationship: Analysis of QT prolongation during alterations of autonomic state versus human ether a-go-go-related gene inhibition. Journal of Pharmacology and Experimental Therapeutics, 312(1), 1–11. https://doi.org/10.1124/jpet.104.073288

Varkevisser, R., Wijers, S. C., Van Der Heyden, M. A. G., Beekman, J. D. M., Meine, M., & Vos, M. A. (2012). Beat-to-beat variability of repolarization as a new biomarker for proarrhythmia in vivo. Heart Rhythm, 9(10), 1718–1726. https://doi.org/10.1016/j.hrthm.2012.05.016 

Hinterseer, M., Thomsen, M. B., Beckmann, B. M., Pfeufer, A., Schimpf, R., Wichmann, H. E., … Kaab, S. (2008). Beat-to-beat variability of QT intervals is increased in patients with drug-induced long-QT syndrome: A case control pilot study. European Heart Journal, 29(2), 185–190. https://doi.org/10.1093/eurheartj/ehm586 

Poincare Plots

As mentioned in the publications and other publications pertaining to beat-to-beat variability, a Poincare plot can be used as a tool to visually inspect beats that deviate from it's previous beat.  Please refer to How to create a Poincare Plot using Ponemah Custom Derived Parameter for more information about creating Poincare Plots in Ponemah.  

Setup a QT vs RR Scatter Plot

A RR vs QT cloud plot (aka scatter plot) can be used to inspect and make calculations off of slope.  

Prior to getting the slope and intercept from the scatter plot of the RR vs QT cloud plot, it's important to note that ECG analysis, preferably using ECG PRO, needs to be completed and "Delete marks from Unmatched Cycles" is performed.  

Open the Graph Setup (P3 Setup>Graph setup)

• • Setup the graph type as Scatter
  • Select the input for both the X and Y axes as the same ECG channel
  • Set the X axis Parameter to RR-I
  • Set the Y axis Parameter to QT-I.  Note that in the Bin Options field, QTcH now becomes enabled.

View the QT vs RR Scatterplot

• • Set the logging rate (Press F8) to the appropriate logging rate that should be averaged together for the RR/QT calculations.  Recommended to set to 10 sec or potentially larger.  NOTE:  if the "View Entire Dataset"  icon is red (and not grayed out), click on the icon so that the entire dataset is being displayed.  You may also need to auto-scale the graph page to view all the data for this step and the next few steps.  This can be done by double clicking on the graph page and clicking on the "AutoScale" button or entering appropriate values.

Custom Derived Parameter

Please refer to VIDEO: Ponemah v5.x Custom Derived Parameters to learn how to create customer derived parameters.  Below are steps to create the derived parameter within Ponemah.  Alternatively, the xml files attached to this article (at the bottom of the webpage) can be imported in Ponemah.  

The equation is:

Build your equation by performing the following steps

• Click on "Add New" and enter an appropriate name for STV_QT such as "STV_QT"
• Select ECG as the Parent Algorithm.
• Select Mean as the Averaging Method.
• Enter ms in the Unit field. 
• Build or manually enter the formulas for STV_QT
  • Sqrt(Pow([QT-I(1)]-[QT-I(0)],2))/(Sqrt(2))
• Select "Update".  The formula is created and the OK button is now enabled.  The derived parameter setup should be similar to what is shown in the figure below.  

• Click on "Add New" and enter an appropriate name for STV_QT such as "STV_RR"
• Select ECG as the Parent Algorithm.
• Select Mean as the Averaging Method.
• Enter ms in the Unit field. 
• Build or manually enter the formulas for STV_RR
  • Sqrt(Pow([RR-I(1)]-[RR-I(0)],2))/(Sqrt(2))
• Select "Update".  The formula is created and the OK button is now enabled.  The derived parameter setup should be similar to what is shown in the figure below.

Report the Custom Derived Parameter

Lastly, you will want to select the newly created customer derived parameter by going into the Derived Parameters dialog window (Setup>P3 setup>Channel Input Setup) to enable output of the STV derived parameters on the appropriate ECG channel(s).