NGF and Apnea: Piedimonte and colleagues19 have demonstrated in animal models that RSV via nerve growth factor increases nociceptive fibers; substance P; and 1 of its receptor subtypes, neurokinin 1.20 Of particular relevance to the association between albumin and apnea is that, with the use of labeled albumin in RSV-inoculated F-344 rats, substance P mediated the extravasation of albumin into the airways.19,21,22 What remains unclear is why neurogenic inflammation and albumin extravasation exist in the rare cases of apnea compared with other infants with severe bronchiolitis.