Fig. 4.

In vitro recordings from an excised endocardial site of origin of focal VT. Horizontal lines mark the zero voltage. Voltage (65 mV) and time (2 s) calibrations are shown. Upward arrow points to the last paced action potential. Downward arrow points to delayed afterdepolarizations (DADs). A: with isoproterenol (Iso), pacing at 2.5 Hz produces three DADs, two of which reach threshold and result in triggered activity (TA). B: TA is blocked by addition of PD-123319 (PD), but a DAD remains, not reaching threshold. C: TA is again induced after PD washout, stopping spontaneously with a single DAD. Note that none of the tracings shows spontaneous phase 4 depolarization, so this cell is endocardial muscle.