Closed-loop electrical epidural stimulation restores ipsilesional in-phase diaphragm activity in spontaneously breathing rats after acute C2-hemisection
Authors: Mickle Alyssa , Coffey Richard , Hall Natale , Peñaloza Jesús , Dale Erica 
Published: May 2023

Epidural electrical stimulation has great promise for recovering many functions after spinal cord injury, including breathing. High-frequency stimulation of the thoracic [DiMarco and Kowalski, 2009] and cervical cord [Gonzalez-Rothi et al. 2017] recovers some phrenic and diaphragm activity. However, open-loop stimulation paradigms induce tonic activity and, thus far, have not elicited recovery of breathing function independent of the stimulation period. We have shown in C2-hemisected rats, closed-loop epidural stimulation (CLES; paced from contralateral diaphragm EMG activity) increases excitability in the respiratory motor network [Malone et al. 2022]; still, the ability of CLES to recover respiratory function following spinal cord injury has not been explored. Here, we hypothesized that CLES can elicit phasic ipsilesional diaphragm activity in C2-hemisected rats without tonic output. To test this, 3-5 month female Sprague-Dawley rats were anesthetized via inhaled isoflurane anesthetic, implanted with stimulating wire electrodes on the dorsal dura at C4 and recording electrodes bilaterally on the diaphragm, and C2-hemisected. After injury, a 15-30 minute recovery to ensure abolishment of ipsilateral diaphragm activity was followed by measurement of spinal motor evoked potentials of the diaphragm, and a baseline recording of diaphragm EMG activity. Biphasic CLES (n=3), set to an amplitude of ¼ the motor threshold, or a sham waiting period (n=3) was delivered for 20 minutes. EMG post processing was done in Spike2 using custom scripts. Stimulated animals received on average 101.8 +/- 22.8 Hz of stimulation during breaths. In stimulated animals, phasic ipsilesional diaphragm EMG activity significantly increased during stimulation (23.7 +/- 1.9 spikes per breath vs 4.32 +/- 1.9 at baseline, p < 0.005). Stimulation did not increase tonic activity of the ipsilesional diaphragm, and ipsilesional diaphragm activity stopped with cessation of stim indicating no spontaneous recovery of activity. Additionally, there was no significant recovery of sham animals during the same period (5.2 +/- 1.9 vs 4.5 +/- 1.9 spikes per breath). Peak EMG activity and modulus of the contralesional diaphragm were significantly elevated from baseline during stimulation (peak value 0.19 +/- 0.015 V vs 0.15 +/- 0.005 V baseline p < 0.05, % change modulus 34.9 +/- 5.9%, p < 0.03). This work represents the first report that CLES can increase phasic diaphragm EMG activity both ipsi- and contralaterally after acute C2-hemisection. Further study is necessary to determine if CLES can elicit similar effects in a chronic model of injury. Additionally, work to determine the mechanism underlying these changes is essential to tailor this therapy for improving breathing function in individuals with spinal cord injury. T32HL134621, R01HL153102, SPARC OT2OD023854 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.