Mechanism of Expiratory Muscle Activation During Upper Lumbar Spinal Cord Stimultion (SCS)

Abstract

Combined lower thoracic (T9) and upper lumbar (L1) SCS may be a useful method to restore an effective cough mechanism in tetraplegic patients. In previous studies in dogs, we demonstrated that SCS at the T9 level resulted in direct activation of the spinal cord roots in the vicinity of the stimulating electrode and also more caudal roots via spinal pathways. The purpose of the present study was to investigate the mechanism of expiratory muscle activation during upper lumbar (L1) SCS. In 3 anesthetized dogs, expiratory muscle activation was monitored by evoked compound action potentials (CAP) from the intercostal muscles in the 7th and 9th interspaces (T7, T9) and portions of the external oblique innervated by the T13 and L2 motor roots. SCS applied with single shocks resulted in short latency CAP at T13 and L2 but not more rostrally. SCS resulted in long latency CAPs at T7 and T9, suggesting activation via spinal cord pathways. SCS at L1 was also applied before and after sequential section of the dorsal columns (DC), lateral (LF) and ventral funiculi (VF) at the T11 spinal cord level while monitoring airway pressure under conditions of airway occlusion. DC section did not affect short latency CAPs but abolished long latency CAPs at T9 and T7 and also resulted in a fall in airway pressure generation from 26 ± 6 cmH2O to 18 ± 5 cmH2O (NS). Section of the LF and VF resulted in further decrements in airway pressure (17 ± 6 and 11 ± 2 cmH2O respectively, NS). We conclude that SCS at the L1 level results in direct activation of the spinal roots in the vicinity of stimulating electrode and more rostral roots via spinal pathways. (HL34143).