Expiratory Muscle Activation via High Frequency Spinal Cord Stimulation

Abstract

BackgroundLack of an effective cough may result in the frequent aspiration of airway secretions and foreign material and the development of recurrent respiratory tract infections. In persons with spinal cord injury, lower thoracic low frequency spinal cord stimulation (conventional LF‐SCS, 50Hz, 15mA) results in large positive airway pressure generation (P), and is a useful method to restore an effective cough mechanism. Unfortunately, activation of the expiratory muscles via LF‐SCS requires high stimulus amplitudes, which may also cause unwanted side effects including stimulation of sensory fibers and therefore cannot be applied in patients with intact sensation.ObjectiveTo evaluate the effectiveness a novel method of expiratory muscle activation, which involves the application of high frequency spinal cord stimulation with low stimulus amplitudes (HF‐SCS; 500Hz, 1mA).MethodsStudies were performed on 13 anesthetized dogs. Multi contact stimulating electrodes were positioned over the dorsal surface of the lower thoracic and upper lumbar spinal cord. In one group of animals the effects of HF‐SCS and LF‐SCS on P were evaluated. SCS was applied at functional residual capacity (FRC) and also over a wide range of lung volumes. Given our previous success with LF‐SCS, these values were used as our gold standard to which all comparisons were made. In a second group of animals, the mechanism of expiratory muscle activation during HF‐SCS was evaluated. P and EMG recordings from the external oblique (EO) before and after spinal cord section at the T12 level were evaluated. Internal intercostal muscle (II) activation at the level of the stimulating electrode was also assessed via II EMG recordings, before and after T9 root section.ResultsFor both HF‐SCS and LF‐SCS, the optimum site for pressure generation was in the region of T9 spinal level (62±5 and 66±8cmH2O). P declined progressively at more rostral and caudal levels. In response to increasing lung volume, P increased in linear fashion during both HF‐SCS and LF‐SCS over the entire vital capacity range. For both HF‐SCS and LF‐SCS, bilateral dorsal column section significantly reduced EO EMG activity recorded below the level of spinal section and also resulted in a substantial fall in P (from 56±6 to 28±1cmH2O, p<0.05). Subsequent lateral funiculi section completely abolished those activities and resulted in further reductions in P. However, onset time of II EMG activity during HF‐SCS was significantly longer than obtained during direct T9 motor root activation (p<0.05).ConclusionThese results suggest that HF‐SCS produces a comparable level of expiratory muscle activation to that achieved with conventional stimulus parameters, but with much lower stimulus amplitudes and is more dependent upon synaptic pathways rather than direct motor root activation.Support or Funding InformationSupport: VA RR&D A1488‐R and NIH‐NINDS R01NS064157