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 to approximately 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 micros, 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 (Paw) 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 +/- 0.8, 12.2 +/- 1.0, 14.6 +/- 1.4, 17.8 +/- 1.8, 23.0 +/- 1.8, 27.7 +/- 2.2, 35.2 +/- 2.7, and 40.4 +/- 2.9 cmH2O, respectively. Bilateral stimulation of seven (from T8 to L1) and eight spinal nerve levels (from T7 to L1) produced the highest changes in(Paw). Stimulation of six or less spinal nerve levels resulted in significantly lower (Paw). We conclude: 1) lower thoracic spinal nerve stimulation near the neuroforamen using microstimulators produces significant expiratory pressure, 2) percutaneous placement of the microstimulators near the neuroforamen is effective in producing expiratory pressure, and 3) percutaneous placement of the microstimulators for restoring cough may potentially be used as a relatively noninvasive clinical tool for patients with spinal cord injury, or other neurological or respiratory disorders. Further studies will be needed.