T-Wave Alternans and Electrocardiology

[Oliver Lang]

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Howdy! I am currently looking to enroll in an Electrophysiology graduate-level course. I have done some prior Undergraduate research in the field. I mostly dealt with algorithms and the computer programming aspects. I am trying, now, to learn the science behind the approach.

Does anyone care to give me a minute and explain to me the basis of T-wave alternans, how it is used to predict arrythmias like LQTS? Do Sodium-Potassium channels play a major role in ventricular repolarization or is there another mechanism at work?

 

[Nik]

Cardiac Action potential and ionic currents was the subject of my senior research. I blieve the sodium/calcium exchang also helps brings back these slements back to resting potential. In addition, the delayed rectifer potassium currents (composed of the rapidly activating Ikr and the slowly activating IKs) are the principal repolarization forces helping to establish resting membrane potential. Finally, that potential is maintained by the inward rectifier potassium cuurent (Ik1).

Hope this helps. i have a cardiology text used by cardiology fellows that I used for my research on this subject. If you want the title, let me know and I will dig it out.

 

[Nik]

Cardiac Action potential and ionic currents was the subject of my senior research. I blieve the sodium/calcium exchang also helps brings back these slements back to resting potential. In addition, the delayed rectifer potassium currents (composed of the rapidly activating Ikr and the slowly activating IKs) are the principal repolarization forces helping to establish resting membrane potential. Finally, that potential is maintained by the inward rectifier potassium cuurent (Ik1).

Hope this helps. i have a cardiology text used by cardiology fellows that I used for my research on this subject. If you want the title, let me know and I will dig it out.

 

[Oliver Lang]

I would like to check some of those books out. I have access to a medical library and it could make for fun reading. To me, this is the most interesting part of physiology.

 

[Hopkins2010]

Can you explain why T wave alternans is effective in predicting LQTS? From what I understand, TWA is just a very small microvolt amplitude deviation between alternating T waves.

But why does the amplitude of the T wave indicate anything about the duration of the QT segment? All of the technical papers I've read just mention on the side that TWA has been shown to be a good predictor of LQTS, but they dont state the electrophysiologic principles on why this is so. Instead, they just look for after the fact clinical correlations for this assessment. Can somebody clear up this connection for me?

 

[Oliver Lang]

Originally posted by baylor21:
•Can you explain why T wave alternans is effective in predicting LQTS? From what I understand, TWA is just a very small microvolt amplitude deviation between alternating T waves.

But why does the amplitude of the T wave indicate anything about the duration of the QT segment? All of the technical papers I've read just mention on the side that TWA has been shown to be a good predictor of LQTS, but they dont state the electrophysiologic principles on why this is so. Instead, they just look for after the fact clinical correlations for this assessment. Can somebody clear up this connection for me?•

Hopefully so! I had asked a similar question a while back. 😉:

"T wave Alternans and Variability - Rationale, Methods, and Clinical Experience"

Wojciech Zareba, MD, PhD
University of Rochester, Rochester, NY, USA

"T wave alternans and T wave variability are new noninvasive ECG parameters used in the risk stratification process. The electrophysiologic justification for the use of repolarization alternans and variability parameters is supported by studies showing the association between those ECG phenomena and arrhythmogenic conditions present in damaged (or genetically altered) ventricular myocardium. Both, T wave alternans and T wave variability reflect various degree of regional/transmural heterogeneity of repolarization in myocardium, which is a critical component of arrhythmic cascade. New computerized methods are being developed to quantify microvolt level beat-to-beat changes in repolarization duration and morphology. T wave alternans analysis is being performed using spectral method (most widely used), complex demodulation and correlation methods. T wave variability analysis is being done suing time-stretch methods, correlation or wavelet-based methods. Although the

diagnostic and prognostic significance of those methods is not yet well established, data from recent clinical studies indicate that especially exercise- or pacing-induced T wave alternans might have better prognostic significance than other noninvasive tests used in risk stratification. Data on prognostic significance of T wave variability are limited, nevertheless, very encouraging especially since this phenomenon occurs much more frequently than T wave alternans. Application of those novel methods quantifying repolarization alternans and variability in large populations of patients with various disorders is needed to determine the practical usefulness of these newly developed noninvasive ECG indices."

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I hope this helps, I read this a little while back on a net-surf when I was doing my research and working on a computer program for T-Wave Alternans analysis. We did a lot of visualization plotting to try to decipher and detect all this mumbo jumbo from standard EKG. We got to use some pretty neat equipment!

 

[Hopkins2010]

OK, that helps some. When they refer to spectral methods I wonder what they mean specifically. Are they talking a simple 256 or 512 point fast fourier transform or Dirichlet wavelet transforms?

 

[MacGyver]

I'd use a digital peaking filter instead of FFTs or wavelets.

Peaking filters would isolate the particular TWA frequency you are looking for, whereas FFT essentially does the same thing while also giving you a bunch of extraneous info about all frequencies that you really dont need.

Using the peak digital filter output in a DSP board, you should be able to isolate even the smallest TWA peaks of 1-5 uV.

Before you run the input ECG signal to the DSP you will need heavy amplification using a low THD preamplifier. Most DSP chips (TI and Motorola) require a minimum signal amplitude of 200-300 mV for good signal capture.

Dirichlet wavelets are also overkill, just like FFTs. You can use them to get the same result, but the effort in programming is a waste of time.