Cellular mechanisms of ventricular bipolar electrograms showing double and fractionated potentials

Objectives. This study sought to determine the types of transmembrane action potentials associated with bipolar electrograms that show double and fractionated potentials. Background. The cellular correlates of ventricular bipolar electrograms showing double potentials and fractionated low amplitude potentials remain poorly defined. Methods. bipolar electrogram (1-cm interelectrode distance [6F, USCI]) and two transmembrane action potentials (within 1 mm of each pole) were recorded simultaneously in 12 isolated canine right ventricular endocardial preparations (2 × 1 cm, 2 mm thick). The long axis of the bipolar electrode was parallel to the long axis of the superficial endocardial fibers, and the recordings were made at 40 to 500 Hz. Results. The following phenomen were associated with double potentials: 1) an increase in conduction time between the two poles of the bipole during a) the propagation of premature action potentials (7 of 12 tissues in 4 mmol/liter extracellular potassium ion concentration [K+]o); b) rapid pacing and premature stimuli (3 of 6 in 9 mmol/liter [K+]o); and c) the propagation of slow responses induced by barium chloride (4 mmol/liter). There was positive correlation between conduction time (CT) and interspike interval (IPI) of the double potential (IPI [ms] = 0.5 × CT [ms]+ 35) during early afterdepolarizations induced by barium chloride (4 mmol/liter) superfusion (three of six tissues). The following events were associated with fractionated electrograms: 1) propagation of induced graded responses (six tissues) in 4 mmol/liter [K+]o; 2) induced reentry at cycle lengths of 140 to 170 ms in 9 mmol/liter [K+]o (four of six tissues); and 3) asynchronous afterdepolarizations induced by 4 mmol/liter barium chloride (four of six tissues). Conclusions. Endocardial double potentials and fractionated electrograms seen on clinically used bipolar electrodes occur under conditions of slowed or discontinuous conduction and induced reentry and during asynchronous automatic firing initiated by afterdepolarizations. Caution must be exercised in interpreting such bipolar electrograms because more than one type of cellular action potential may cause these abnormal electrographic results.