Effects of Expiratory Muscle Activation via High Frequency Spinal Cord Stimulation

In persons with spinal cord injury, lower thoracic low frequency spinal cord stimulation (LF-SCS, 50Hz,15mA) is a useful method to restore an effective cough. Unfortunately, the high stimulus amplitude requirements and potential activation of pain fibers significantly limits this application in persons with intact sensation. In this study, the mechanism of the expiratory muscle activation, via high frequency SCS (HF-SCS;500Hz,1mA) was evaluated in dogs. In group one, the effects of electrode placement on airway pressure generation (P) was evaluated. Maximal P occurred at the T9-T10 level with progressive decrements in P at more rostral and caudal levels for both LF-SCS and HF-SCS. In group two, EMG latencies of internal intercostal muscle (II) activation were evaluated before and after spinal root section and during direct motor root stimulation. Onset time of II EMG activity during HF-SCS was significantly longer (3.84±1.16ms) than obtained during direct motor root activation (1.61±0.0.1ms). In group three, P and external oblique (EO) EMG activity, before and after sequential spinal section at the T11/T12 level were evaluated. Bilateral dorsal column section, significantly reduced EO EMG activity below the section and resulted in a substantial fall in P. Subsequent lateral funiculi section completely abolished those activities and resulted in further reductions in P. We conclude that: 1) activation of the expiratory muscles via of HF-SCS is dependent entirely upon synaptic spinal cord pathways and 2) HF-SCS at the T9 level produces a comparable level of muscle activation to that achieved with LF-SCS, but with much lower stimulus amplitudes.

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