Ventilation heterogeneity in excised lobes: effect of tidal volume

Although several factors are known to influence nonuniformity of ventilatio n, including lung mechanical properties (regional structure and compliance) , external factors (chest wall, pleural pressure, heart), and ventilatory p arameters (tidal and preinspiratory volume, flow rate), their relative cont ributions are poorly understood. We studied five excised, unperfused, canin e right-middle lobes under varied levels of tidal volume (VT), thus elimina ting many factors affecting heterogeneity. Multiple-breath washouts of N-2 were analyzed for anatomic dead space volume (VDanat), nonuniformity of N-2 washout, and nonuniformity between joined acinar regions vs. that occurrin g between larger joined regions. Approximately 80% of ventilation heterogen eity was found among joined acinar regions at resting levels of VT, but inc reasing VT reduced intra-acinar heterogeneity to about 25% of that found at resting levels. Increasing VT had essentially no effect on VDanat and hete rogeneity among larger joined regions. The results indicate that the magnit ude of VT is a major influence on the dominant intra-acinar component of ve ntilation heterogeneity and that VT effects on VDanat are likely due to per fusion and/or influences normally external to the lobar structure.