Expiratory muscle activation by functional magnetic stimulation of thoracic and lumbar spinal nerves

This study was conducted to stimulate respiratory muscles by functional magnetic stimulation (FMS) of the spinal nerves (T1-L5) to obtain maximum expiratory function.

A prospective before and after trial.

Functional Magnetic Stimulation Laboratory, Spinal Cord Injury Service, VA Palo Alto Health Care System, Palo Alto, CA.

Twelve normal able-bodied subjects.

A commercially available magnetic stimulator with a round magnetic coil (MC) was used. Respiratory muscle activation was achieved by placing the MC at each spinous process ranging from T1 to L5 vertebral levels.

The planned major outcome was to determine the optimal MC placement for producing maximal expiratory pressure (MEP) and expiratory reserve volume (ERV) by FMS. These measurements were compared with the subjects' voluntary maximal efforts. A profile with varying stimulation intensities was also obtained in select individuals for determining the highest expiratory pressure.

Stimulation at the T9 spinal level resulted in the highest mean MEP and ERV. Stimulation between T8 and L5 produced similar MEP and ERV as obtained from the T9 MC placement. The mean maximum MEP and ERV produced by FMS were 76.8 +/- 6.4 cm H2O (7.52 +/- 0.62 kPa) and 1.28 +/- 0.15 L, which were 67% and 79% of the subjects' voluntary maximal efforts, respectively. A stimulation intensity of 80% resulted in the highest expiratory pressure.

FMS of lower thoracic and lumbar regions produced significant expiratory pressures and volumes. FMS of the expiratory muscles may prove to be a valuable technique for restoring cough in patients with spinal cord injury or other neurologic diseases, and in critical care or perioperative settings.