Proceedings of The Physiological Society

Physiology 2016 (Dublin, Ireland) (2016) Proc Physiol Soc 37, PCB009

Poster Communications

Expiratory muscle activation via high frequency spinal cord stimulation to restore cough in a dog model

K. E. Kowalski1,2, J. R. Romaniuk1, S. Brose3,4, M. Richmond3,5, T. Kowalski2, A. F. DiMarco4,2

1. Research, Louis Stokes VAMC, Cleveland, Ohio, United States. 2. MetroHealth Research Institute, MetroHealth Medical Center, Cleveland, Ohio, United States. 3. Spinal Cord Injury, Louis Stokes VAMC, Cleveland, Ohio, United States. 4. Physical Medicine and Rehabilitation, Case Western Reserve University, Cleveland, Ohio, United States. 5. Medicine, Case Western Reserve University, Cleveland, Ohio, United States.

Background: Lack of an effective cough may result in the frequent aspiration of airway secretions and foreign material, which could lead to serious recurrent respiratory tract infections. In persons with spinal cord injury (SCI), lower thoracic spinal cord stimulation (conventional SCS, 50 Hz, 15mA) results in large positive airway pressure generation, and is a useful method to restore an effective cough mechanism. Unfortunately, activation of the expiratory muscles via SCS requires high stimulus amplitudes, which may also cause unwanted side effects including stimulation of sensory fibers, and cannot be applied in patients with intact sensation. Objective: The purpose of the present study was to evaluate our hypothesis that lower thoracic SCS with high stimulus frequencies and low stimulus currents will result in sufficient activation of the expiratory muscles to produce large positive airway pressures and high peak expiratory airflow rates sufficient to generate an effective cough. Methods: Studies were performed on 5 dogs, anesthetized with pentobarbital sodium (25 mg/kg IV, initially) and intubated. Additional doses of pentobarbital sodium were given, as required (1-3 mg/kg IV). The effects of varying stimulus amplitudes and frequencies on positive airway pressure generation produced by SCS were evaluated following hyperventilation-induced apnea. SCS was applied after tracheal occlusion, at functional residual capacity (FRC) and also over a wide range of lung volumes (0.3L below to 1.3L above FRC) via a disc electrode positioned epidurally at the T9 spinal level. Given our previous success with conventional stimulus parameters (50Hz, 15mA), these were used as our gold standard to which all comparisons were made. Results: At any given level of stimulus current, at FRC, mean expiratory airway pressure generation was largest at 500Hz, compared to all other stimulus frequencies. For example, with stimulation at 1 mA and frequencies of 50, 200, 300, 500 and 600Hz, expiratory airway pressures were 12±6, 26±2, 39±2, 60±5 and 51±6cmH2O, respectively. In comparison, the mean airway pressure generation measured at FRC with conventional stimulus parameters (50Hz, 15mA) was 66±8 cmH2O. Moreover, airway pressure generation increased in linear fashion in response to increasing lung volume during high frequency spinal cord stimulation (HF-SCS) and during conventional stimulation, over the vital capacity range. Summary: These results suggest that HF-SCS produces a comparable level of expiratory muscle activation and positive airway generation to that achieved with conventional stimulus parameters, but with much lower stimulus amplitudes. Conclusion: HF-SCS may be a useful method to restore an effective cough in patient populations with intact sensation and who would benefit from restoration of an effective cough.

Where applicable, experiments conform with Society ethical requirements