The fix is not yet in: recommendation for fixation of lungs within physiologic/pathophysiologic volume range in preclinical pulmonary structure-function studies

Quantitative characterization of lung structures by morphometric or stereologic analysis of histologic sections is a powerful means of elucidating pulmonary structure-function relations. The overwhelming majority of studies, however, fix lungs for histology at pressures outside the physiologic/pathophysiologic respiratory volume range. Thus valuable information is being lost. In this perspective article, we argue that investigators performing pulmonary histologic studies should consider whether the aims of their studies would benefit from fixation at functional transpulmonary pressures, particular those of end-inspiration and end-expiration. We survey the pressures at which lungs are typically fixed in preclinical structure-function studies; provide examples of conditions that would benefit from histologic evaluation at functional lung volumes; summarize available fixation methods; discuss alternative imaging modalities; and discuss challenges to implementing the suggested approach and means of addressing those challenges. We aim to persuade investigators that modifying or complementing the traditional histologic approach by fixing lungs at minimal and maximal functional volumes could enable new understanding of pulmonary structure-function relations.