EFFECTS OF HEART-RATE ON VULNERABILITY TO FIBRILLATION IN A COMPUTER-MODEL

Effects of heart rate on refractory period (RP) duration and disparity have opposing actions on vulnerability to fibrillation. Both bradycar dia and tachycardia have been reported to increase vulnerability to fi brillation, and the role of their effects on RP duration and disparity in producing that effect is uncertain. That role has been investigate d with a computer model of propagated excitation having nonuniform cyc le length-dependent refractoriness and slow propagation during incompl ete recovery of excitability. Vulnerability was assessed as fibrillati on threshold (FT), defined as the duration of train stimulation requir ed re initiate simulated fibrillation. When measured as a function of train onset time during a cycle, FT initially decreased to a minimum a nd then increased to the original level. Slower rates shifted that cur ve upward and to the right, so that the FT was higher during early por tions of the cycle but lower in later portions. Longer mean duration o f RPs increased FT during all portions of the cycle, increased the dif ference of FT at various rates during early portions of the cycle, and decreased differences later in the cycle. Greater RP range reduced th e FT and decreased the difference of FT with varied rare in early port ions oi the cycle, while increasing the difference in later portions. Accelerating rate had additional effects on FT-related to nonuniform p ropagation of responses prior to train stimulation. The findings defin ed mechanisms based on established effects of rate on RP, by which eit her tachycardia or bradycardia could increase vulnerability to fibrill ation, and demonstrated the effects of RP range and duration on the me chanisms.