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Figure 2 Identification of sites of endocardial activation 0–30 msec preceding the onset of the QRS complex during ventricular tachycardia (VT). The VT depicted in this example had a cycle length of 240 ms, was associated with severe hypotension and loss of consciousness and, therefore, was induced only once. The data provided by the mapping system were based on a single 3 s sample of the VT. The top panel shows an open view of the left ventricular reconstruction, with the septum, base and apex as labeled; the inflated noncontact balloon catheter is shown only in the top panel. The letters A, B, C and D in the left ventricular reconstruction designate the sites at which the virtual electrograms in the top panel were computed. The instantaneous isopotential maps are shown in panels 1–4 at the time points designated by the vertical lines through the electrograms. This VT was no longer inducible and did not recur spontaneously after delivery of radiofrequency energy at sites A–D, as guided by the locator signal.