Observing the QT interval when a patient stands up quickly can provide a simple and quick method to identify patients who may have Long QT syndrome (LQTS), according to Sami Viskin and colleagues in an expedited study in JACC.
The researchers studied 68 patients with LQTS and 82 controls. Each subject had a baseline ECG while lying down and was then asked to stand up quickly. Both groups had a similar heart rate acceleration in response to standing, but significant differences were found in the response of the QT interval.
In controls the QT interval shortened by 21 ms but increased by 4 ms in the LQTS subjects. As the RR interval shortened more than the QT interval, the corrected QT interval increased by 50 ms in controls and by 89 ms in the LQTS patients. According to the authors, this difference was highly significant (p<0.001) and the receiver-operating characteristic curves “showed that the test adds diagnostic value.”
The authors conclude that their “test is easy to perform and should be used in addition to more accepted tests when necessary,” although they note that “it is important to avoid overdiagnosis ” based on the test.
The concept is a “clever observation that can be used at the bedside,” said Wesby Fisher, an EP and author of the Dr. Wes blog. But he also noted that the “standard deviation of QT interval changes will limit its negative predictive value.”
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