Airway hyperreactivity in subjects with tetraplegia is associated with reduced baseline airway caliber

Objectives: We administered aerosolized histamine to 32 subjects with tetraplegia to determine whether there were differences in spirometric and/or lung volume parameters between responders and nonresponders. Results: Baseline pulmonary function parameters revealed mild to moderate restrictive dysfunction. We found that 25 subjects (78%) were hyperreactive to histamine (mean provocative concentration of a substance causing a 20% fall in FEV₁ [PC₂₀], 1.77 mg/mL). Responders (PC₂₀, < 8 mg/mL) had significantly lower values for forced expiratory flow between 25% and 75% of the outflow curve (FEF_25-75), FEF_25-75 percent predicted, and FEF_25-75/FVC ratio. Among all 32 subjects, the natural logarithmic transformation performed on PC₂₀ values (lnPC₂₀) correlated with FEF_25-75 percent predicted, FEV₁ percent predicted, and FEF_25-75/FVC ratio but not with FVC percent predicted. Responders with PC₂₀ values < 2 mg/mL (n = 13) had significantly reduced values for FVC, FVC percent predicted, FEV₁, and FEV₁ percent predicted compared to those with PC₂₀ values between 2 mg/mL and 8 mg/mL. In addition, among responders, there was a significant correlation between lnPC₂₀ and FVC percent predicted. A significant relationship was found between maximal inspiratory pressure (PImax) and both FEV₁ percent predicted and FEF_25-75 percent predicted, but not between lnPC₂₀ and either PImax or maximal expiratory pressure (PEmax). Conclusions: These findings demonstrate that subjects with tetraplegia who exhibit airway hyperreactivity (AHR) have reduced baseline airway caliber and that lower values for lnPC₂₀ are associated with parallel reductions in surrogate spirometric indexes of airway size (FEV₁ percent predicted and FEF_25-75 percent predicted) and airway size relative to lung size (FEF_25-75/FVC ratio). The absence of an association between lnPC₂₀ and FVC percent predicted for the entire group or between lnPC₂₀ and either PImax or PEmax indicates that reduced lung volumes secondary to respiratory muscle weakness cannot explain the mechanism(s) underlying AHR. Among responders, however, a possible role for reduction in lung volume, as it pertains to increasing AHR, cannot be excluded. Proposed mechanisms for reduced baseline airway caliber relative to lung size in subjects with tetraplegia include unopposed parasympathetic activity secondary to the loss of sympathetic innervation to the lungs and/or the inability to stretch airway smooth muscle with deep inhalation.