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.

electrical stimulation; cough; respiratory muscles

This article has been cited by other articles:

				A. F. DiMarco and K. E. Kowalski Effects of chronic electrical stimulation on paralyzed expiratory muscles J Appl Physiol, June 1, 2008; 104(6): 1634 - 1640. [Abstract] [Full Text] [PDF]

				G. Chawla and G. B. Drummond Fentanyl decreases end-expiratory lung volume in patients anaesthetized with sevoflurane Br. J. Anaesth., March 1, 2008; 100(3): 411 - 414. [Abstract] [Full Text] [PDF]

				K. E. Kowalski, J. R. Romaniuk, and A. F. DiMarco Changes in expiratory muscle function following spinal cord section J Appl Physiol, April 1, 2007; 102(4): 1422 - 1428. [Abstract] [Full Text] [PDF]

				J. Lim, R. B. Gorman, J. P. Saboisky, S. C. Gandevia, and J. E. Butler Optimal electrode placement for noninvasive electrical stimulation of human abdominal muscles J Appl Physiol, April 1, 2007; 102(4): 1612 - 1617. [Abstract] [Full Text] [PDF]

				A. F. DiMarco, K. E. Kowalski, R. T. Geertman, and D. R. Hromyak Spinal Cord Stimulation: A New Method to Produce an Effective Cough in Patients with Spinal Cord Injury Am. J. Respir. Crit. Care Med., June 15, 2006; 173(12): 1386 - 1389. [Abstract] [Full Text] [PDF]

				A. F. DiMarco, K. E. Kowalski, G. Supinski, and J. R. Romaniuk Mechanism of expiratory muscle activation during lower thoracic spinal cord stimulation J Appl Physiol, June 1, 2002; 92(6): 2341 - 2346. [Abstract] [Full Text] [PDF]

				A. F. DiMarco, J. R. Romaniuk, K. E. Kowalski, and G. Supinski Mechanical contribution of expiratory muscles to pressure generation during spinal cord stimulation J Appl Physiol, October 1, 1999; 87(4): 1433 - 1439. [Abstract] [Full Text] [PDF]