Strength-Duration Characterisation of Subcutaneous Pacing: A Preclinical Study

Subcutaneous and extravascular pacing systems represent an emerging class of cardiac rhythm management devices that deliver therapy without direct myocardial contact. Despite their clinical potential, the excitability characteristics of subcutaneous stimulation are not well defined.

Acute preclinical studies were conducted in four swine (n = 4) using a coil-to-can subcutaneous pacing configuration. Monophasic and biphasic pulses ranging from 1 to 10 ms and 1 to 200 mA in amplitude were delivered asynchronously at rates 20-30 bpm above the intrinsic heart rate. Myocardial capture was confirmed via high-fidelity intra-aortic pressure. Strength-duration curves were generated, and rheobase and chronaxie values were derived using Lapicque's model.

All subjects exhibited a hyperbolic strength-duration profile. Rheobase ranged from 19.54 to 77.58 mA, chronaxie varied from 0.70 to 2.94 ms. An inverse relationship was identified between rheobase and chronaxie, suggesting that effects of electrode geometry, tissue impedance, or lead placement may influence the excitability profile. Biphasic waveforms tended to lower capture thresholds versus monophasic stimuli; this difference did not reach statistical significance (p = 0.161). Inter-subject variability was consistent with differences in tissue impedance and electrode-myocardial distance.

Subcutaneous pacing exhibits classical excitability consistent with Lapicque's theory. Thresholds follow predictable dependencies on pulse width and waveform, indicating opportunities to optimize energy delivery safety margins. These data provide a basis for programming strategies and future device designs targeting lower energy use and extended battery life in subcutaneous and extravascular pacing systems.