Stimulation of the Expiratory Muscles Using Microstimulators

Respiratory complications constitute a major cause of morbidity and mortality in patients with spinal cord injury. These complications arise in part due to the loss of supraspinal control over the expiratory muscles and the resultant difficulties in clearing airway secretions effectively. The purpose of the present study is to evaluate the efficacy of lower thoracic spinal nerve stimulation using wireless microstimulators in activating the expiratory muscles. Studies were performed on nine anesthetized dogs. A thoracic laminectomy was performed on each dog, and was followed by spinal cord transection at T2. A total of 16 microstimulators (supplied by the Alfred Mann Foundation, Santa Clarita, CA) were inserted percutaneously into the bilateral intercostal nerves approximately 1 $\sim$ 3 cm distal to the neuroforamen from T7 to L1 in each dog. The stimulation parameters were: frequency of 20 Hz , pulse width of 200 $\mu{\rm s}$, and stimulation burst of 2 s. The stimulation intensities were 3.78, 5.4, 8.1, and 10.8 mA. The pressure-generating capacity of the expiratory muscles was evaluated by the change in airway pressure $({\rm P}_{\rm aw})$ at functional residual capacity, which was produced by the microstimulators during airway occlusion. As a general trend, the expiratory pressure generated using the microstimulators increased with increasing intensity and the number of spinal nerves recruited. The maximal expiratory pressures generated from one, two, three, four, five, six, seven, and eight pairs of spinal nerves were 8.4 $\pm$ 0.8, 12.2 $\pm$ 1.0, 14.6 $\pm$ 1.4, 17.8 $\pm$ 1.8, 23.0