Mechanism of expiratory muscle activation during lower thoracic spinal cord stimulation

Lower thoracic spinal cord stimulation (SCS) may be a useful method to restore an effective cough mechanism. In dogs, two groups of studies were performed to evaluate the mechanism of the expiratory muscle activation during stimulation at the T9-T10 level, which results in the greatest changes in airway pressure. In one group, expiratory muscle activation was monitored by evoked muscle compound action potentials (CAPs) from the internal intercostal muscles in the 10th, 11th, and 12th interspaces and from portions of the external oblique innervated by the L1 and L2 motor roots. SCS, applied with single shocks, resulted in short-latency CAPs at T10 but not at more caudal levels. SCS resulted in long-latency CAPs at each of the more caudal caudal recording sites. Bilateral dorsal column sectioning, just below the T11 spinal cord level, did not affect the short-latency CAPs but abolished the long-latency CAPs and also resulted in a fall in airway pressure generation. In the second group, sequential spinal root sectioning was performed to assess their individual mechanical contribution to pressure generation. Section of the ventral roots from T8 through T10 resulted in negligible changes, whereas section of more caudal roots resulted in a progressive reduction in pressure generation. We conclude that 1) SCS at the T9-T10 level results in direct activation of spinal cord roots within two to three segments of the stimulating electrode and activation of more distal roots via spinal cord pathways, and 2) pathway activation of motor roots makes a substantial contribution to pressure generation.