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Premature stimulation is used to characterize the reentrant circuit during ventricular tachycardia (VT) in patients. The goal of this study was to compare the effects of premature stimulation on functional and anatomic reentrant VT.In 18 Langendorff-perfused rabbit hearts, thin layers of anisotropic left ventricular subepicardium were created by a cryoprocedure. In 8 hearts, rapid pacing induced reentry around a line of functional conduction block; in 10 hearts, reentry occurred around a fixed epicardial obstacle created by a cryoprobe. The cycle lengths (CL) of functional and anatomic VT were 110±10 and 167±17 milliseconds, respectively. During anatomic VT, the excitable gap measured 43% of the CL and premature stimuli could always reset VT (44±12 milliseconds). During early premature beats, conduction of the orthodromic wave was slightly depressed, but anatomic VT was never terminated. Reset curves at different sites in the ventricle revealed three different response types, both determined by and characterizing the spatial and temporal relation between pacing and recording sites. Premature stimulation during functional VT revealed a local excitable gap at the pacing site measuring 27% of the cycle length of VT. However, in only 3 of 8 hearts, premature stimuli could reset functional VT by 8%. In 5 VTs, advancement of the paced activation was fully compensated by prolongation of the return cycle, and VT was not reset. Due to slow conduction both toward and inside the circuit, the paced orthodromic wave lost its prematurity already within a distance of 6 to 10 mm from the pacing site.Both during anatomic and functional reentry, an excitable gap is present in the reentrant circuit. Three different response curves reveal the localization of the pacing and recording sites in the circuit. Anatomic VT can always be reset by premature stimuli, whereas in 5 of 8 hearts, functional VT could not be reset. In the other 3 hearts, VT could only be reset for less than 7% to 11% of the VT interval. Therefore, it seems very unlikely that clinical VT based on functional reentry can be reset.