Long QT Syndrome other diagnostic studies
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]; Associate Editor(s)-in-Chief: Charmaine Patel, M.D. [3]
Overview
The diagnosis of long QT syndrome can be difficult when abnormalities on electrocardiogram are borderline or intermittent. In cases where the history and symptoms are suggestive of long QT syndrome, but few or no abnormalities are seen on electrocardiogram, further testing can be done to unmask long QT syndrome with the use of exercise treadmill testing, and catecholamine provocation testing.
Non-exercise Catecholamine Stress Testing
During this type of test, an EKG is performed while the patient is given an infusion of epinephrine. The epinephrine challenge is a useful test to establish electrocardiographic diagnosis in silent LQT1 mutation carriers, thus allowing implementation of prophylactic measures aimed at reducing sudden cardiac death. This test can unmask what is known as concealed long QT syndrome, which shows a normal QT interval on EKG at rest. This test can show a prolonged QT interval in persons who have a history of fainting spells in response to intense exercise or emotional upset, and therefore point to the diagnosis of long QT syndrome.
Exercise Treadmill Testing
Similar to catecholamine provocation testing, an exercise treadmill test with EKG, can unmmask concealed long QT syndrome. This is also used as a provocation test for persons who have a pertinent history suggestive of long QT syndrome, or a family history of long QT syndrome, but do not have any abnormalities on resting EKG. Studies have shown not only the usefulness of exercise stress testing in the diagnosis of long QT syndrome, but also it's utility in determining the subtype of long QT syndrome that the patient has. Krahn et al. showed that the hysteresis of the QT segment observed during exercise may be helpful in the diagnosis of long QT syndrome. They also showed that differences in the QT interval that were greater than 21ms between 1 min into recovery and early exercise may be indicative of long QT syndrome [1] . A study done by Takenaka et al. demonstrated that the QTc in LQT1 patients was significantly prolonged during exercise, whereas in LQT2 patients, exercise did not cause any significant changes in the QTc, however did show a prominent notch on the descending limb of the T wave. They also showed that there is a steeper QT/R-R slope in LQT2 patients than in LQT1 patients during exercise [2] .
References
- ↑ Krahn AD, Klein GJ, Yee R (1997). "Hysteresis of the RT interval with exercise: a new marker for the long-QT syndrome?". Circulation. 96 (5): 1551–6. PMID 9315546.
- ↑ Takenaka K, Ai T, Shimizu W, Kobori A, Ninomiya T, Otani H; et al. (2003). "Exercise stress test amplifies genotype-phenotype correlation in the LQT1 and LQT2 forms of the long-QT syndrome". Circulation. 107 (6): 838–44. PMID 12591753.