Inspiratory Muscle Activation via Ventral Lower Thoracic High Frequency Spinal Cord Stimulation.

In animals, high frequency spinal cord stimulation (HF-SCS) applied on the ventral epidural surface at T2 level results in negative airway pressure generation consistent with inspiratory muscle activation. In the present study, in anesthetized dogs, we found that ventral HF-SCS (500Hz) applied at all thoracic levels resulted in negative airway pressure generation. In the region of the lower thoracic spinal cord, negative airway pressure generation was most pronounced at the T9 level. At this spinal level, airway pressure generation was monitored: 1) during ventral HF-SCS over a wide range of stimulus amplitudes (0.5-15mA) and frequencies (50-1000Hz) and 2) following spinal sections at the C8 (to assess potential diaphragm activation) and subsequently at the T6 (to assess potential intercostal muscle activation) levels. The application of low stimulus currents between 1-2 mA and high stimulus frequencies (>300Hz) resulted in the development of large negative airway pressure generation. Stimulation with 1mA, 500Hz resulted in a highest negative airway pressure generation of 47±2cmH2 O. Increasing stimulus current was associated with progressive reductions in the magnitude of negative airway pressure generation. HF-SCS (500Hz) with 15mA resulted in a negative airway pressure generation of 7±3cmH2 O. C8 section markedly reduced negative airway pressure generation and subsequent T6 section resulted in positive airway pressure generation. Our results indicate the existence of pathways with connections to both the phrenic and inspiratory intercostal motoneuron pools in the ventral part of the lower thoracic spinal cord. We speculate that the circuits mediating the previously described excitatory intercostal-to-phrenic reflex mediate the observed responses.