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Long QT Syndrome Part II

Posted Oct 04 2009 11:13pm

What is Long QT Syndrome?

Long QT Syndrome (LQTS) is a condition purely diagnosed by an electrocardiogram (ECG). This condition is represented by a long QT interval, hints the name. This represents delayed ventricular repolarization. This condition can be genetic or induced by certain prescribed medications.

How do I detect it?

Easy, as the name implies, measure the QT/QTc interval. The image below shows that the QT interval begins at the very end of the PR Interval, where your isoelectric line will begin another positive or negative deflection, and it ends at the very end of the T wave. Lead II is generally a good place to make your measurement. This may change depending on the R axis.

Normal QT intervals are between 300 & 450ms (0.30 - 0.45 seconds)

An easier way to measure is by looking at your 12-lead diagnostics. While rhythm interpretation may not always be up to par with your monitor; the measurements are usually pretty accurate.

What is the difference between QT & QTc?

The c in QTc stands for corrected. The QTc is the QT interval with a formulated correction based on the patient's heart rate (HR). The faster the HR the shorter the QT interval. The QTc may not always be accurate, but is commonly used for diagnostic purposes. Bazett's formula is used to determine QTc (well according to wikipedia it is).

Why is LQTS bad?

Remember the term refractory period?

Here is your refresher:

The absolute refractory period is the interval during which a second action potential absolutely cannot be initiated, no matter how large a stimulus is applied.

The relative refractory period is the interval immediately following during which initiation of a second action potential is inhibited but not impossible.[1]

Well people with LQTS have a prolonged refractory period, this includes a prolonged relative refractory period. As the definition explains, the relative refractory period tries not to but may invite pre-excitation. What this means is that another beat, from another part of the heart, may occur.

One example of aberrancy due to abnormal refractory period is Ashman's phenomenon. This is an arrhythmia with what is called a "long-short cycle". In atrial fibrillation, you may see a string of QRS complexes with longer R-R intervals (long cycle) followed by a PVC or a run of what looks to be PVCs. This will be followed by QRS complexes with shorter R-R intervals (short cycle). This is caused by pre-excitation during the relative refractory period, and the "PVC" is actually an aberrantly conducted supraventricular complex (bundle branch block pattern). The conduction was delayed due to a busy electrical pathway. Picture a traffic jam in the hearts one lane electrical highway. Below is an example, look at the R-R intervals pre & post premature beat.

Another complication of a longer refractory period is what is known as R on T Phenomenon. This condition may be more severe because it may interfere with hemodynamic stability. A premature complex finds it's home on the tail end of a supraventricular complex and takes over. While the ventricles are attempting to repolarize another focus decides he wants to take over. The best way to do this is by depolarizing while the other pacemaker is repolarizing. Like attacking a sleeping enemy. Below is another image stolen from Google. Those complexes are ventricular in origin.

Another risk associated with LQTS is sudden cardiac arrest. That one doesn't need much explanation. The cause is most likely an arrhythmia brought on by the R on T phenomenon.

More on LQTS to come. If any information I have provided is thought to be inaccurate, please let me know (Tom). I have no problem with being wrong, there is a first time for everything (hopefully sarcasm was detected).

*Once again, all images stolen from Google searches.
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