Pattern of expiratory muscle activation during lower thoracic spinal cord stimulation

Abstract

Large positive airway pressures (Paws) can be generated by lower thoracic spinal cord stimulation (SCS), which may be a useful method of restoring cough in spinal cord-injured patients. Optimal electrode placement, however, requires an assessment of the pattern of current spread during SCS. Studies were performed in anesthetized dogs to assess the pattern of expiratory muscle recruitment during SCS applied at different spinal cord levels. A multicontact stimulating electrode was positioned over the surface of the lower thoracic and upper lumbar spinal cord. Recording electromyographic electrodes were placed at several locations in the abdominal and internal intercostal muscles. SCS was applied at each lead, in separate trials, with single shocks of 0.2-ms duration. The intensity of stimulation was adjusted to determine the threshold for development of the compound action potential at each electrode lead. The values of current threshold for activation of each muscle formed parabolas with minimum values at specific spinal root levels. The slopes of the parabolas were relatively steep, indicating that the threshold for muscle activation increases rapidly at more cephalad and caudal sites. These results were compared with the effectiveness of SCS (50 Hz; train duration, 1-2 s) at different spinal cord levels to produce changes in Paw. Stimulation at the T9 and T10 spinal cord level resulted in the largest positive Paws with a single lead. At these sites, threshold values for activation of the internal intercostal (7-11th interspaces) upper portions of external oblique, rectus abdominis, and transversus abdominis were near their minimum. Threshold values for activation of the caudal portions of the abdominal muscles were high (>50 mA). Our results indicate that 1) activation of the more cephalad portions of the abdominal muscles is more important than activation of caudal regions in the generation of positive Paws and 2) it is not possible to achieve complete activation of the expiratory muscles with a single electrode lead by using modest current levels. In support of this latter conclusion, a two-electrode lead system results in more uniform expiratory muscle activation and significantly greater changes in Paw.

Comparison of wire and disc leads to activate the expiratory muscles in dogs

“…[23][24][25][26] Importantly, the results of these studies translated well into clinical trials in which SCS resulted in near maximum expiratory muscle activation and was successful in restoring an effective cough mechanism in subjects with SCI. [20][21][22] Airway pressures generated with the disc leads, therefore, represented our gold standard with which other leads could be compared.…”

“…[23][24][25][26] The disc lead was an eight-electrode assembly with 4-mm-diameter platinum electrode contacts embedded in polyurethane plastic with 10 mm contact spacing (disposable subdural eight contact strip electrode (1 × 8), Ad-Tech Medical Instrument Corp, Racine, WI, USA). In separate trials, wire leads with four platinum-iridium (90%/ 10%) 6 mm electrode contacts were inserted onto the dorsal epidural space in the midline.…”

“…A remote ground electrode was positioned in the back musculature. Since we have previously demonstrated, both in animal 24,25 and human 20,21 studies, that any combination of two of the three spinal cord levels tested (T9, T11, and L1) resulted in maximum pressure generation, we focused our evaluation to stimulation of each site individually and the simultaneous stimulation of various combinations of two sites. The effects of both monopolar and bipolar stimulation were also evaluated.…”

“…Employing a stimulus frequency of 50 Hz, airway pressure generation rose progressively with increasing stimulus amplitude until a plateau was reached at ∼15 mA. 25,26 With SCS at maximum amplitude (15 mA), increases in stimulus frequency also resulted in progressive increases in pressure generation Figure 4 Mean relationships between stimulus amplitude and airway pressure generation during monopolar stimulation with the disc leads (T9 + T11 and T9 + L1) and bipolar stimulation with the parallel wire configuration (T9-T11 and T9-L1). *P < 0.05 compared to monopolar disc.…”

“…Using 4 mm disc electrode contacts, we found that maximum positive airway pressures occurred with stimulation applied at the T9-T10 dorsal epidural spinal cord level. [23][24][25][26] Stimulation above and below this region resulted in progressive reductions in airway pressure generation. Employing a stimulus frequency of 50 Hz, airway pressure generation rose progressively with increasing stimulus amplitude until a plateau was reached at ∼15 mA.…”