Role of action potential shortening in arrhythmia induction: implications for external defibrillation

The electrode configuration plays an important role in the efficient use of energy during a defibrillation shock. Also, failure of many defibrillation shocks can be attributed to postshock induction of arrhythmias. It has been suggested that juxtaposition of regions with long and short action potential duration (APD) can induce cardiac arrhythmias. The effect of different strength shocks of either polarity, delivered from different electrode configurations, on action potential propagation and duration was studied in intact rabbit hearts using video imaging technology. Shocks were delivered during the early plateau stage of the action potential after a pre-shock depolarized the entire heart rapidly. The frequency of extra beats observed reduced as shock strength was increased. Calculations of APD over the entire heart revealed that stronger shocks resulted in increased areas of APD prolongation and reduced regions of APD shortening. The results indicate that efficient use of shock energy depends on the electrode configuration and that APD shortening plays an important role in the induction of post-shock arrhythmias.