Sustained ventricular tachycardias (VT) often degenerate into ventricular fibrillation (VF). In the present study, the impact of VT on mean arterial blood pressure (MAP), myocardial blood now (MBF), and myocardial oxygen consumption (MVO2) was assessed. In addition, the degeneration of sustained VT into VF was analysed with respect to MAP. MBF was measured in 48 anesthetized rats with colored microspheres; arterial catecholamine levels were measured by HPLC in 16 additional rats during control conditions and VT. MBF (4.66 +/- 1.29 ml/g/min; mean +/- S.D.) did not change with the onset of VT (5.37 +/- 1.92 ml/g/min, N.S.). Epinephrine (0.22 +/- 0.13 ng/ml) and norepinephrine (0.37 +/- 0.12 ng/ml) increased during VT (3.55 +/- 2.68 ng/ml, P<0.01; 0.88 +/- 0.44 ng/ml, P<0.05), respectively. VF was more frequent when MAP remained normal (MAP>80 mmHg: 26%) than with hypotension (MAP<80 mmHg: 2%, P<0.05). Mechanical failure was observed in 10% of rats with severe hypotension (MAP<60 mmHg), and 2% with moderate hypotension (MAP 60-80 mmHg). The endo-epicardial MBF ratio in the VF group was significantly lower than that in the non-VF group (0.94 +/- 0.17 v 1.11 +/- 0.24, P<0.05). Conclusions: severe hypotension predisposes to the occurrence of acute mechanical failure during VT; moderate hypotension during VT, however, serves as a protective mechanism against VF in structurally normal hearts. Subendocardial hypoperfusion in the presence of an increased energy demand during VT is suggested to be responsible for the initiation of VF. (C) 1997 Academic Press Limited.