(Editor’s Note: An article in the May issue of the Journal of the American College of Cardiology proposes a simple bedside ECG test for better diagnosis of the abnormal heart rhythm known as Long QT Syndrome.)

The Response of the QT Interval to the Brief Tachycardia Provoked by Standing: A Bedside Test for Diagnosing Long QT Syndrome. Viskin, et al. Journal of American College of Cardiology. May 2010. p.1955-1961.

What is long QT syndrome?

Long QT Syndrome (LQTS) is a well known, primarily genetic, repolarization abnormality that increases the risk of SCD. Having a prolonged QT interval (prolonged ventricular action potential) increases the likelihood that the vulnerable component of the T-wave will receive early stimulation from a PVC. Known as the “R-on-T phenomenon,” it can cause a lethal form of ventricular tachycardia known as Torsades de Pointes (TdP). TdP is twisting of the QRS complexes around the baseline (or axis shifting).
It’s also believed the risk of sudden death in individuals with LQTS is increased during increased adrenergic states (i.e. exercise, excitement). Since the QT interval shortens with increased heart rate, using the QTc (corrected QT) becomes crucial. Recall that QTc equations correct or normalize the QT interval relative to heart rate increases by adding the proper amount of time (ms). Additional points to remember:

• The QT duration is normally 220-460ms.
• QTc intervals > 450ms not due to medication should be considered for LQTS.>
• Some intravenous, antibiotics, antipsychotics and antiarrhythmics can cause temporary LQTS.
• Arrhythmic symptoms occur infrequently.
• Diagnosing LQTS can be tricky.

Further complicating the diagnosis, only 2.5% of the healthy population has prolonged QT intervals, and 10–15% of LQTS patients often have normal resting QT interval. Research presented at this year’s ACC Scientific Sessions revealed that 80% of EPs correctly diagnose LQTS, <50% of cardiologists are correct and <25% of primary care docs are correct. Once diagnosed, treatments consist of beta blocker therapy, genetic-guided identification or ICD implantation.

Using ECG for bedside diagnose of LQTS (JACC, May 2010). The physiologic transition from sitting to standing naturally causes brief sinus tachycardia resulting from momentary mild hypotension. Researchers studied 68 patients with a history of LQTS and 82 control patients without it. Each patient rested supine (lying) for 10 min then stood up for 5 min w/ continuous ECG. Using the Bazett’s formula, the resulting QTc intervals differed: at maximal tachycardia the QTc increased by 50ms in the control group and by 90ms in the LQTS group. Thus the response of the QT interval was impaired in patients with LQTS. In summary, although the QTc intervals of the LQTS patient and control subjects were different at baseline, the differences increased during standing.

Talking points

• Accurate diagnosis of LQTS is critical due to its potential lethality.
• The natural QT shortening during brief standing is impaired in LQTS patients (ie greater adjusted increase in QTc).
• Simply observing the QTc interval during brief tachycardia induced by standing provides important information that can help clinicians diagnose LQTS that may not be present at rest.

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