News | November 03, 2011

Recovery Phase Of Treadmill Stress Test Unmasks Concealed Long QT Syndrome With High Accuracy

November 3, 2011 – New study indicates strong diagnostic ability of treadmill exercise stress testing to uncover Long QT syndrome (LQTS) in patients.  Here, investigators from the Mayo Clinic show how the recovery phase displays significant and maladaptive changes in the QTc unveiling the most common LQTS type, LQT1. The study, published in the November edition of HeartRhythm, the official journal of the Heart Rhythm Society, is the largest, single-center study of treadmill exercise stress testing in patients with genetically proven LQTS as well as those who were dismissed as normal.

Long QT syndrome affects about 1 in 2,500 individuals, and nearly 40 percent of patients with LQTS can have a nondiagnostic QTc at rest.  However, through treadmill stress testing, this new study reveals significant differences in QTc values for the three primary types of LQTS (LQT1-LQT3) at a specific moment in time during the exercise test.  QTc was calculated at rest, peak exercise and for the first five minutes of recovery.

Between 1998 and 2008, more than 1,000 patients with a referral diagnosis of LQTS were seen at the Mayo Clinic’s Long QT Syndrome Clinic in Rochester, Minn.  Of these patients, an Institutional Review Board-approved, retrospective analysis was performed on the treadmill stress test in 243 individuals including, 82 LQT1, 55 LQT2, 18 LQT3, and 88 genotype-negative patients (control group) dismissed as normal.   

Results of the treadmill exercise stress test reveal the following:

  • Maladaptive QTc prolongation during the recovery phase distinguishes LQTS, particularly LQT1 patients, from the control group. 
  • Specifically, patients with LQT1 most distinguished themselves from the other genotypes and the controls at 2 and 3 minutes of recovery.
  • Beta-blockers did not significantly obscure the response noted during the recovery phase, which eliminates the need for a beta-blocker washout.

Overall, even with nondiagnostic QTc at rest, a prolonged QTc (> 470 ms) seen in patients at 3 minutes of recovery provides at 75 percent pre-genetic test probability for LQT1.

“Our results refine and improve the diagnostic ability of treadmill stress tests during the evaluation of patients with possible long QT syndrome by showing precisely where the greatest diagnostic separation resides, the recovery phase,” stated lead author Michael J. Ackerman, M.D., Ph.D., director of Mayo Clinic’s Long QT Syndrome Clinic and the Windland Smith Rice Sudden Death Genomics Laboratory in Rochester, Minn.  “We hope that this targeted approach to reading and measuring the QTc will not only help to catch long QT syndrome when it is there, but to also help decrease the current rate of overdiagnosis of Long QT, which has resulted from misinterpretations of stress tests, and ultimately improve patients’ quality of life.”

Over the past two decades, other diagnostic tools used in the evaluation of LQTS have increased dramatically including genetic testing and provocative testing with catecholamines, like the epinephrine QT stress test.  However, unlike exercise stress tests, the epinephrine QT stress test is not available at many centers and without appropriate experience in detecting the LQT1 response during epinephrine, it can be challenging to interpret.  In contrast, exercise stress tests, such as treadmill testing, are more readily available at cardiology centers around the country.  As shown through this study, evaluating patients during the recovery phase of treadmill exercise testing can result in an accurate diagnosis of LQTS, and offers clinicians another approach to consider.

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