Activation of the Inspiratory Intercostal Muscles by Electrical Stimulation of the Spinal Cord

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Abstract
Electrical stimulation of the spinal cord was evaluated as a method of activating the inspiratory intercostal muscles. Studies were performed in anesthetized dogs after hyperventilation-induced apnea. A stainless steel electrode, rubberized along its entire length except for 2 to 3 mm at the distal tip, was introduced epidurally onto the dorsal surface of the thoracic spinal cord. Stimulating electrodes were also placed in each hemidiaphragm. Intercostal electromyograms, inspired volume, and thoracoabdominal movements were monitored. The inspiratory capacity was determined in each animal as the volume required to achieve an airway pressure of +25 cm H2O during passive lung inflation. Spinal cord stimulation at the T2-T3 spinal level resulted in maximal inspired volume generation and electrical activation of the parasternal, external, and internal intercostal muscles of the upper and midrib cage regions as determined by electromyograms. Intrathoracic pressure swings increased progressively with increasing stimulus amplitude and frequency until plateaus were reached at 6 mA and 40 Hz, respectively. Postphrenicotomy spinal cord stimulation resulted in expansion of the rib cage and reduction in circumference of the abdominal compartment. Inspired volumes during spinal cord stimulation were 537 .+-. 49 ml (prephrenicotomy, prone), 347 .+-. 19.6 ml (postphrenicotomy, prone), and 303 .+-. 30.6 ml (postphrenicotomy, supine). Bilateral diaphragm activation alone resulted in inspired volumes of 404 .+-. 39 ml. Combined diaphragm and postphrenicotomy spinal cord stimulation (supine) resulted in an inspired volume of 712 .+-. 72 ml, which approximated the inspiratory capacity (803 .+-. 35 ml). Our results suggest that spinal cord stimulation may be a useful physiologic and clinical tool to produce coordinated contraction of the inspiratory intercostal muscles.