The team then induced electrical activity in these engineered tissues as would occur in ventricular tachycardia and attempted to halt it with pace-setting pulses.
"In the beginning, there is a single rotating wave -- the 'culture dish' analog of ventricular tachycardia," Bursac said. "After a short period, we applied trains of pulses in an attempt to terminate this wave."
The pulses successfully halted the wave 80 percent of the time, they reported. In the remaining cases, however, the pulses converted the single wave into multiple waves that continued to activate the cardiac cells at an accelerated rate.
"In other words, instead of terminating it, the pacing actually perpetuated and worsened the initial condition in about 20 percent of the cases," Bursac said. "That percentage approximates the frequency with which this is thought to occur in patients with implanted defibrillators."
The researchers' analysis revealed a possible explanation for the two responses. They found significant differences in propagation of electrical activity through the engineered tissues between those that responded to pace-setting shocks by halting their aberrant rhythm, and those that responded by accelerating their rhythm.
Should such characteristic patterns hold in patients, physicians could potentially use them to identify those people for whom defibrillators are more likely to worsen abnormal heart rate, Bursac said.
So-called implantable cardioverter defibrillators (ICD) monitor the timing of heartbeats and can deliver small or large shocks to correct arrhythmias. Such devices have become increasingly common for patients who have experienced arrhythmias and are therefore at increased risk for future rhythm abnormalities, Bursac said.
In patients with implanted defibrillators, the devices often attempt to terminate ventricular tachycardia by delivering electrical pulses to a single site at a rate slightly higher than