HYSTERESIS OF THE RT INTERVAL WITH EXERCISE - A NEW MARKER FOR THE LONG-QT SYNDROME

Background The diagnosis of the long-QT syndrome (LQTS) may be difficu lt to establish in patients with normal or borderline prolongation of the QT interval. Noninvasive markers are needed to identify patients w ith LQTS. Methods and Results Fourteen patients with known LQTS, 9 una ffected family members, and 40 control subjects underwent modified Bru ce protocol exercise testing. The RT interval (peak of R wave to peak of T wave) and rate-corrected RT interval (RTc) were measured during e xercise and recovery. The RT interval at 1 minute into recovery was su btracted from the RT interval at a similar heart rate during exercise (ART). The RTc shortened by 61 milliseconds (ms) in the LQTS patients compared with 23 to 26 ms in the other two groups (P=.003 by ANOVA). T he RT interval shortened in a linear fashion in all patients but demon strated persistent shortening during recovery in the LQTS patients. Th is was manifested as a hysteresis loop in the curve relating the RT in terval to cycle length. The hysteresis loop was present in 13 of 14 LQ TS patients and only 4 of 40 control subjects. Delta RT >25 ms had a s ensitivity of 73%, a specificity of 92%, a positive predictive value o f 79%, and a negative predictive value of 90% for LQTS. Conclusions Hy steresis of the RT interval with exercise may be useful for the diagno sis of LQTS.