Two types of ventricular fibrillation in isolated rabbit hearts - Importance of excitability and action potential duration restitution

Background-The combined effects of excitability and action potential duration (APD) restitution on wavefront dynamics remain unclear. Methods and Results-We used optical mapping techniques to study Langendorff-perfused rabbit hearts. In protocol IA (n=10), D600 at increasing concentrations was infused during ventricular fibrillation (VF). With concentration increased to 0.5 mg/L, fast VF (dominant frequency, 19.1+/-1.8 Hz) was consistently converted to ventricular tachycardia (VT). However, increasing D600 further to 2.5 or 5.0 mg/L converted VT to slow VF (11.9+/-2.3 Hz, P=0.0011). In an additional 4 hearts (protocol 113), tetrodotoxin converted a preexisting VT to slow VF (11.0+/-1.4 Hz). Optical maps show wandering wavelets in fast VF, organized reentry in VT, and spatiotemporal periodicity in slow VF. In protocol 11, we determined APD and conduction time(-1) (CT-1) restitutions during D600 infusion. CT-1 was used as an estimate of excitability. At 0.1 mg/L, APD and CT-1 restitutions were steep and flat, respectively. APD restitution became flattened when D600 increased to 0.5 mg/L, converting fast VF to VT. Further increasing D600 to 2.5 or 5.0 mg/L steepened CT-1 restitution and widened the range of S-1 pacing cycle lengths over which CT-1 decreased, converting VT to slow VF. Conclusions-Two types of VF exist in isolated rabbit hearts. Fast (type I) VF is associated with a steep APD restitution, a flat CT-1 restitution, and wandering wavelets. Slow (type II) VF is associated with a flat APD restitution, a steep CT-1 restitution, and spatiotemporal periodicity. Both excitability and APD restitution are important in VF maintenance.