QT interval variability

QT interval variability (QTV) refers to the physiological phenomenon of beat-to-beat fluctuations in QT interval of electrocardiograms. Increased QTV appears to be a marker of arrhythmic and cardiovascular death; it may also play a role for noninvasive assessment of sympathetic nervous system activity.^[1]

Other terms used include: "QT variability", "beat-to-beat variability of ventricular repolarization (BRV)"

Beat-to-beat measurement of QT interval using two-dimensional signal warping (2DSW).

QT interval measurement

Under normal resting conditions, beat-to-beat changes in QT interval are very small with a standard deviation typically below 5 ms. Digital high resolution ECG sampled at 300 Hz or higher and dedicated computer algorithms are required for QTV assessment.^[2] Template-based algorithms that use parts of, or the entire ECG waveform usually deliver good results;^[3] template stretching or warping techniques^[4] perform comparably well in the presence of noise.

QTV Analysis

A number of metrics have been proposed for QTV quantification. The QT variability index (QTVi) has been most frequently reported in the literature: ${\displaystyle QTVi=\log {\frac {SDQT^{2}/(QT_{mean})^{2}}{SDHR^{2}/(HR_{mean})^{2}}}}$ , where ${\displaystyle SDQT}$ , ${\displaystyle SDHR}$ , ${\displaystyle QT_{mean}}$ , and ${\displaystyle HR_{mean}}$  denote standard deviation and mean of QT interval and heart rate time series, respectively.^[5]

More advanced approaches that take into account the relationship between QTV and heart rate variability include vector autoregressive process models^[6] and information domain approaches.^[7]

 

Example traces of heart rate and QT interval variability in a normal heart and after myocardial infarction.

References

1. Baumert, Mathias; Porta, Alberto; Vos, Marc A.; Malik, Marek; Couderc, Jean-Philippe; Laguna, Pablo; Piccirillo, Gianfranco; Smith, Godfrey L.; Tereshchenko, Larisa G.; Volders, Paul G.A. (June 2016). "QT interval variability in body surface ECG: measurement, physiological basis, and clinical value: position statement and consensus guidance endorsed by the European Heart Rhythm Association jointly with the ESC Working Group on Cardiac Cellular Electrophysiology". Europace. 18 (6): 925–944. doi:10.1093/europace/euv405. PMC 4905605. PMID 26823389.
2. Baumert, Mathias; Schmidt, Martin; Zaunseder, Sebastian; Porta, Alberto (2016-03-01). "Effects of ECG sampling rate on QT interval variability measurement". Biomedical Signal Processing and Control. 25: 159–164. doi:10.1016/j.bspc.2015.11.011. ISSN 1746-8094.
3. Baumert, Mathias; Starc, Vito; Porta, Alberto (2012-07-30). "Conventional QT Variability Measurement vs. Template Matching Techniques: Comparison of Performance Using Simulated and Real ECG". PLOS ONE. 7 (7): e41920. doi:10.1371/journal.pone.0041920. ISSN 1932-6203. PMC 3408402. PMID 22860030.
4. "2DSW – Two-Dimensional Signal Warping (2DSW)". 2dsw.com. Retrieved 2017-12-01.
5. Berger, Ronald D.; Kasper, Edward K.; Baughman, Kenneth L.; Marban, Eduardo; Calkins, Hugh; Tomaselli, Gordon F. (1997-09-02). "Beat-to-Beat QT Interval Variability: Novel Evidence for Repolarization Lability in Ischemic and Nonischemic Dilated Cardiomyopathy". Circulation. 96 (5): 1557–1565. doi:10.1161/01.cir.96.5.1557. ISSN 0009-7322. PMID 9315547.
6. Porta, A.; Baselli, G.; Caiani, E.; Malliani, A.; Lombardi, F.; Cerutti, S. (1998-01-01). "Quantifying electrocardiogram RT-RR variability interactions". Medical and Biological Engineering and Computing. 36 (1): 27–34. doi:10.1007/bf02522854. ISSN 0140-0118. PMID 9614745. S2CID 26095975.
7. Porta, Alberto; Bari, Vlasta; Maria, Beatrice De; Baumert, Mathias (2017). "A network physiology approach to the assessment of the link between sinoatrial and ventricular cardiac controls". Physiological Measurement. 38 (7): 1472–1489. doi:10.1088/1361-6579/aa6e95. PMID 28430108. S2CID 3435105.