EFFECT OF CALCIUM-CHANNEL BLOCK ON THE WALL-MOTION ABNORMALITY OF THEIDIOPATHIC LONG QT SYNDROME

Background We recently showed the frequent occurrence of an unusual ve ntricular wall motion abnormality, assessed by echocardiography, in pa tients with the idiopathic long QT syndrome (LQTS). Two new quantitati ve indexes were developed: Th-1/2 (time needed to reach half of the ma ximal systolic thickening), which was smaller in LQTS patients than in controls; and TSTh (time spent at a very low thickening rate before r apid relaxation), which was much greater in LQTS patients, indicating the presence of a slow contraction in the late thickening phase. This marked late systolic ''plateau,'' either rectilinear or with a peculia r double peak pattern, was significantly more frequent in patients wit h a history of syncope or cardiac arrest. The mechanism underlying thi s puzzling phenomenon remained unexplained. Methods and Results The pr esent study assessed the effects of the calcium channel blocker verapa mil on the contraction pattern in 10 LQTS patients (9 females and 1 ma le; mean age, 19+/-7 years) with a marked plateau pattern and in 6 hea lthy controls (4 females and 2 males; mean age, 28+/-5 years). Either verapamil (0.1 mg/kg) or saline was randomly injected over 2 minutes. Saline had no effect. In LQTS patients, verapamil increased Th-1/2 by 27%, from 16.9+/-3.2% to 21.4+/-3.9% of the cardiac cycle (P=.005), an d dramatically reduced TSTh by 92%, from 13.7+/-5.3% to 1.08+/-0.6% of the cardiac cycle (P<.00001). At the peak effect of verapamil, the co ntraction pattern of all patients was normal. In healthy control subje cts, verapamil did not significantly change either Th-1/2 (from 17.6+/ -2.5% to 18.5+/-3.5% of the cardiac cycle) or TSTh (from 0.921+/-0.47% to 1.17+/-0.74%). Conclusions This study demonstrates that the wall m otion abnormality of LQTS is completely abolished by verapamil. These results suggest that symptomatic LQTS patients may have an abnormal in crease in the intracellular calcium concentration before relaxation ha s completed, possibly linked to an early afterdepolarization, and that the contraction abnormality may be the mechanical equivalent of an ea rly afterdepolarization.