Reentry and fibrillation in the mouse heart - A challenge to the critical mass hypothesis

The idea that fibrillation is only possible in hearts exceeding a critical size was introduced by W. Garrey >80 years ago and has since been generally accepted. In ventricular tissue, this critical size was originally estimat ed to be 400 mm(2). Recent estimates suggest that the critical size require d for sustained reentry is approximate to 100 to 200 mm(2), whereas 6 times this area is required for ventricular fibrillation. According to these est imates, fibrillation is not possible in the mouse heart, where the ventricu lar surface area is approximate to 100 mm(2). To test whether sustained ven tricular fibrillation could be induced in such an area, we used a high-spee d video imaging system and a voltage-sensitive dye to quantify electrical a ctivity on the epicardial surface of the Langendorff-perfused adult mouse h eart. In 6 hearts, measurements during ventricular pacing at a basic cycle length (BCL) of 120 ms yielded maximum and minimum conduction velocities (C Vmax and CVmin) of 0.63+/-0.04 and 0.38+/-0.02 mm/ms, respectively. At a BC L of 80 ms, CVmax and CVmin changed to 0.55+/-0.03 and 0.34+/-0.02 mm/ms. A ction potential durations (APDs), measured at 70% repolarization at those p acing frequencies were found to be 44.5+/-2.9 and 40.4+/-2.6 ms, respective ly. The wavelengths (CV x APD) were calculated to be 28.6+/-3.4 mm in the C Vmax direction and 16.8+/-1.5 mm in the CVmin direction at BCL 120 ms. Wave lengths were significantly reduced (P<0.05) at BCL 80 ms (CVmax, 22.2+/-1.8 mm; CVmin, 13.7+/-0.9 mm). In 5 hearts, stationary vortex-like reentry org anized by single rotors (4 of 5 hearts) or by pairs of rotors (1 of 5 heart s) was induced by burst pacing. In the ECG, the activity manifested as sust ained monomorphic tachycardia. Detailed analysis showed that the local CVs were reduced in the vicinity of the rotor center, which allowed the reentry to take place within a smaller area than was calculated from wavelength me asurements during pacing. In 4 of 7 hearts, burst pacing resulted in a poly morphic ECG pattern indistinguishable from ventricular fibrillation. These data challenge the critical mass hypothesis by demonstrating that ventricul ar tissue with an area as small as 100 mm(2) is capable of undergoing susta ined fibrillatory activity.