123I-MIBG radioaerosol lung clearance in COPD patients with fixed and partially reversible obstruction to evaluate the functional status of pulmonary adrenergic innervation

In order to test the hypothesis that a greater sympathetic activity underlies partial reversibility of the bronchial obstruction in selected patients with chronic obstructive pulmonary disease (COPD), we assessed the pulmonary clearance of inhaled I-meta-iodobenzylguanidine (I-MIBG) radioaerosol in 10 patients with fixed and nine with reversible obstruction. The clearance of inhaled I-MIBG is known to be inversely related to I-MIBG uptake by adrenergic terminals. Groups were matched for age and judged free from confounding comorbidity. The penetration index did not distinguish between the groups (fixed obstruction, 97.39±14.59%; reversible obstruction, 89.09±19.95%; P = 0.659); this excludes the possibility that the inequality of tracer penetration could affect I-MIBG clearance. The I-MIBG clearance was 140.92±7.67 min in patients with fixed obstruction and 151.08±31.54 min in patients with reversible obstruction (P = 0.604). In conclusion, COPD patients with fixed and reversible obstruction show comparable binding of the tracer to adrenergic pulmonary receptors. Thus, a greater receptor responsiveness or post-receptor mechanism probably underlies the partial reversibility of bronchial obstruction.