Respiratory tract fluids: analysis of content and contemporary use in understanding lung diseases

Respiratory tract fluid, produced from an enormous area spanning the mucosa of the nose to the alveolar surface, is a complex mixture of serum transudate and locally secreted proteins and glycomucoproteins and of inflammatory and immune effector cells intermingled. Its analysis is important in understanding the pathogenesis of respiratory diseases and remains essential for the clinical diagnosis of most lung disorders. Many basic facts about the formation and composition of this fluid remain unknown, and little information exists about absorptive mechanisms along the airways. Respiratory fluid is not homogeneous but has unique regional characteristics that are becoming better appreciated as more selective sampling methods are devised. Above all, it is a dynamic substance in healthy airways and diseased ones, and any specimen is just a point-in-time sample that can change in composition, often making serial analysis and comparisons necessary. Nasal fluids currently have limited diagnostic application except in allergic rhinitis. Expectorant (sputum) telescopes fluid and cells from all areas and is not from a specific locale, so the trend is to retrieve more selective and regional specimens of airways fluids. Technology largely does not exist to collect area specimens, except for bronchoalveolar lavage, which generally samples the alveolar surface. Clearly, bronchoalveolar lavage fluid analysis has been the favored way to characterize the peripheral air-spaces for the past 10 years or so, and most of this monograph has been devoted to normal data derived from lavage specimens and to a few examples of lung disease that reflect this burgeoning application. In many respects, results obtained from lavage fluid are virtually in catalogue form at present, and it remains to the future to know how some of the observations will help make diagnosis better or elucidate pathogenic mechanisms. Generally, bronchoalveolar lavage fluid analysis has led to better concepts of immunopathology of many diseases and provided new ways to monitor the evolution of certain diseases, especially the diffuse interstitial lung disorders, but development of specific criteria for diagnosis has been less rewarding. However, certain patterns of lymphocyte-predominant alveolitis, suggesting sarcoidosis or hypersensitivity diseases, recognition of specific T lymphocyte defects and opportunistic microorganisms as in AIDS, and the use of microprobe electron analysis to identify cellular particulates all point to more precision of diagnosis. Alveolar proteinosis and histiocytosis X may be detected from lavage fluid components. The prospects are truly exciting.(ABSTRACT TRUNCATED AT 400 WORDS)