More Effective and Safer Cardiac Electric Stimulation Using Multidirectional and Biphasic Stimuli

Abstract

Because the ability of electric fields to excite cardiac cells depends on stimulus direction, effective in situ cardiac stimulation requires relatively high stimulus amplitude. However, high-strength fields may cause electroporation and cell injury. In this study, we compared the effectiveness of unidirectional (US) and multidirectional stimulation (MS) in 16 populations of isolated, randomly-oriented cardiomyocytes. MS was achieved by automatically switching stimulus delivery among 3 electrode pairs oriented at 0, 60 and 120° with a reference axis. Stimuli were triplets of 5-ms voltage pulses applied 5 ms apart (total duration < refractory period). For US, single pulses were applied at only one direction at each run. Using US (monophasic pulses) for successive runs at all directions, mean threshold field (ET) was 3.8±0.1 V/cm. US with 1.2xET at a single direction recruited 38±1% of cells, whereas total US recruitment (the sum of recruitment at the 3 directions without intersection) was 83±2%. With MS (1.2xET), recruitment reached 90±2% (p<0.05 vs. single direction US). With biphasic pulses, ET and the stimulus amplitude required for ∼90% recruitment were 20-25% lower than with monophasic stimuli (p<0.05). Thus the greater efficiency of MS was further enhanced by using biphasic stimuli. Experiments with high-strength pulses at a single direction showed that the field required for lethal injury in 50% of the tested cells (LE50) was 70±2 (N=12) and 81±1 V/cm (N=9) for monophasic and biphasic waveforms, respectively (p<0.05). Considering the safety index of electric stimulation as LE50/ET, we conclude that biphasic stimuli are safer (index ∼26 vs. 18 for monophasic) because of both lower ET and potency of lethality (CNPq, CAPES, FAPESP).