Neuromechanical regulation of respiratory motor output in ventilator-dependent high-level (c1-c3) quadriplegics. Simon, Peggy M., Ann M. Leevers, Jenny L. Murty, James B. Skatrud, and Jerome A. Dempsey. Medical Research Service, William S. Middleton Memorial Veterans Hospital, Madison, WI 53705, John Rankin Laboratory of Pulmonary Medicine, Departments of Medicine and Preventive Medicine, University of Wisconsin-Madison, Madison, WI 53706, Mayo Clinic & Foundation, Division of Pulmonary & Critical Care Medicine, Rochester, MN 55905
APStracts 2:0085A, 1995.
To evaluate the role of phrenic and sternocleidomastoid afferents as alternate sources of inhibitory feedback during mechanical ventilation, we studied five C2-C3 quadriplegics with sensory denervation of the rib cage and diaphragm, six C1-C2 quadriplegics with additional loss of sensory feedback from the neck muscles, and seven normal subjects. We compared the return of inspiratory muscle activity (the recruitment threshold, PCO2RT) during mechanical ventilation between subject groups following step-wise increases in PETCO2 either by increasing inspired CO2 (FICO2), decreasing tidal volume (50 cc/min, VT), or decreasing frequency (one breath/2 min, ). Normal subjects were mechanically hyperventilated via a nasal mask until inspiratory activity was undetectable. Efferent input to the sternocleidomastoid was intact at both levels of spinal cord injury, but phasic activity was not evident at the quadriplegics' baseline resting ventilation. The PCO2RT was defined as the level of PETCO2 at which phasic activity of the diaphragm in normal subjects and of the sternocleidomastoid in C1-C2 and C2-C3 quadriplegics reoccurred. The mean PCO2RT (in response to raising PETCO2 via increased FICO2 while maintaining a high VT and ) was not significantly different (p = 0.6) between normal subjects (43 3 mm Hg) and C2-C3 quadriplegics (38 +/- 5 mm Hg). Both subject groups demonstrated a frequency- and volume -related inhibition as evidenced by a substantially lower PCO2RT when PETCO2 was raised by either reducing VT or . In contrast to the C2-C3 quadriplegics, the C1-C2 quadriplegics responded with a similar PCO2RT among the three different mechanical ventilation trials, independent of whether PETCO2 was raised with high VT and , with reduced VT, or with reduced . We conclude that feedback from at least some part of the chest wall is required to produce a volume- and frequency-dependent inhibition of inspiratory muscle activity observed during mechanical ventilation. 

Received 28 April 1994; accepted in final form 14 February 1995.
APS Manuscript Number A394-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 10 March 1995.