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We used optical mapping to characterize the reentrant circuit of ventricular tachycardia (VT) during acute myocardial infarction (MI) in isolated canine left ventricular preparations.The nature of the reentrant circuit that underlies VT during acute MI is not well understood.Using optical mapping in isolated canine left ventricular preparations, we characterized the reentrant circuit of monomorphic VT (mean cycle length 245.3 +/- 15.6 ms, n = 7) induced by programmed stimulation during acute MI.Optical mapping during VT revealed a functional reentrant circuit consisting of four components: (1) a protected isthmus located between the infarction area and the functional line of block; (2) an entrance site located at one end of the isthmus; (3) an exit site located at the other end of the isthmus; and (4) an outer loop consisting of nonischemic normal tissue, connecting the exit and entrance sites. Rate-dependent slow conduction within the border zone was associated with significant changes (n = 6) in action potential amplitude (99.1 +/- 0.4 vs 71.4 +/- 0.6 mV, P.01), maximal diastolic potential (-80.6 +/- 0.2 vs -65.4 +/- 0.6 mV, P.05), action potential duration at 90% repolarization (APD(90); 188.4 +/- 1.0 vs 164.3 +/- 3.1 ms, P.05), and dV/dt (302.4 +/- 7.9 vs 168.5 +/- 3.6 V/s, P.05). Compared to preparations with no inducible VT (n = 7), formation of a functional line of block was the key mechanism for initiation of functional reentry in preparations with VT. When comparing preparations with sustained and nonsustained VT, preservation of slow conduction over the isthmus was the key component for maintenance of sustained VT.The reentrant circuit of monomorphic VT in the setting of acute MI involved both the infarction border zone and nonischemic normal tissue. The underlying mechanism is related to the presence of rate-dependent slow conduction and the development of a functional line of block in the border zone.