Assessment of airway caliber and bronchodilator responsiveness in subjects with spinal cord injury

Study objectives: Previous spirometric findings among subjects with chronic tetraplegia that reduction in FEV₁ and maximal forced expiratory flow, mid-expiratory phase (FEF_25-75%) correlated with airway hyperresponsiveness to histamine, and that many of these subjects exhibited significant bronchodilator responsiveness, suggested that baseline airway caliber was low in this population. To better evaluate airway dynamics in patients with spinal cord injury, we used body plethysmography to determine specific airway conductance (sGaw), a less effort-dependent and more reflective surrogate marker of airway caliber. Design: Cohort study. Setting: Veterans Affairs medical center. Participants: Thirty clinically stable subjects with chronic spinal cord injury, including 15 subjects with tetraplegia (injury at C4-C7) and 15 subjects with low paraplegia (injury below T7), participated in the study. Fifteen able-bodied individuals served as a control group. Interventions: Subjects underwent baseline assessment of spirometric and body plethysmographic parameters. Repeat measurements were performed among subjects with tetraplegia and paraplegia before and 30 min after receiving aerosolized ipratropium bromide (2.5 mL 0.02% solution; 12 subjects) or normal saline solution (2.5 mL; 6 subjects). Measurements and results: We found that subjects with tetraplegia had significantly reduced mean values for sGaw (0.16 cm H ₂O/s), total lung capacity, FVC, FEV₁, and FEF _25-75% compared to subjects in the other two groups. Subjects with tetraplegia who received ipratropium bromide experienced significant increases in sGaw (135%), FEV₁ (12%; 260 mL), and FEF_25-75% (27%). Significant, though far smaller, increases in sGaw (19%) were found among subjects with paraplegia. No discernable change in any pulmonary function parameter was found following the administration of normal saline solution. Conclusions: Subjects with tetraplegia, as opposed to those with low paraplegia, have reduced baseline airway caliber due to heightened vagomotor airway tone, which we hypothesize is the result of the interruption of sympathetic innervation to the lungs, and/or from low circulating epinephrine levels.