A Computational Model of Upper Thoracic High‐Frequency Spinal Cord Stimulation to Optimize Inspiratory Muscle Activation

Abstract

BackgroundHigh‐frequency spinal cord stimulation (HFSCS) has been previously shown to elicit physiological activation of the diaphragm and inspiratory intercostal muscles via spinal cord pathways in canines. It was previously hypothesized that during upper thoracic HF‐SCS, the phrenic motoneuron pools are activated via spinal pathways located in the ventro‐lateral funiculus (VLF).ObjectiveThe aim of the present study was to develop a computational model of ventral HFSCS to optimize stimulation parameters and electrode configurations for activation of the inspiratory muscles.MethodsWe created a computer model of an anatomically‐accurate finite element model (FEM) of the canine spinal cord and surrounding anatomy, along with an implanted HFSCS electrode array. From this FEM, we calculated extracellular voltages generated by SCS and applied them to multicompartment cable models of axons within the VLF. Model analysis compared activation thresholds for several unique electrode designs, configurations, and electrode placements in order to optimize activation of these VLF fibers to maximize inspiratory muscle activation.ResultsFor initial model parameterization, we performed experimental measurements testing various electrode configurations and designs. We then used our computer model to evaluate several additional stimulation configurations (e.g. monopolar, bipolar) on VLF fiber selectivity. We then evaluated the effect of wider or narrow lateral lead spacing, contact spacing, and contact size. Finally, we considered several SCS waveform settings, such as rectangular waveforms and ramped waveforms, to potentially generate muscle activation more similar to natural inspiration.ConclusionsWe used our model to optimize electrode configurations, placements and novel stimulation waveforms of HFSCS to target fibers within the VLF. In particular, we found that wider lateral spacing between percutaneous electrodes better targets VLF fibers as opposed to the narrower spacing initially tested. We also show that ramped stimulation slowly recruits VLF fibers over time, which could improve naturalistic inspiratory muscle activation. In future work, we will perform experimental measurements in an animal model of HFSCS to confirm the ability of these optimized electrode designs and waveform parameters to better recreate normal inspiration.DisclosureDr. DiMarco is a founder of and has a significant financial interest in Synapse BioMedical, a manufacturer of diaphragm pacing systems and holds patents for spinal cord stimulation to restore cough and respiration (5,678,535; 5,911,218; 5,999,855; 8,751,004). Drs. DiMarco and Kowalski hold the U.S. patents for technology related to the content of this manuscript: Respiratory Muscle Activation by Spinal Cord Stimulation (8,352,036).Support or Funding InformationThis work is supported by NIH‐NINDS (R01NS064157).