A previous study in this laboratory examined the effect of micropore ultrafiltration of blood products on pulmonary gas exchange and subsequent pulmonary dysfunction--related morbidity and death. Morbidity and death from pulmonary failure was not affected; however, gas exchange was improved following ultrafiltration with 40 micrometers filters, as reflected by lower Bohr dead-space fractions. This difference might be explained by reduction of the microaggregate load seen in the pulmonary microvasculature. The purpose of this study was to examine in more detail these gas exchange alterations, paying particular attention to the correlation of changing Bohr dead-space ventilation detected with multiple inert gas analysis with direct determinations of microaggregate size and number. Fourteen patients with isolated cutaneous thermal injury scheduled for major early burn would excision were selected for study. Following transfusion with homologous blood products, the ventilation/perfusion ratio (Va/Q) distributions determined by inert gas analysis remained essentially unchanged except for subtle changes in both high VA/Q and dead-space compartments, resulting in significantly increased Bohr dead-space fractions (P less than 0.05). This combination of gas exchange alteration is consistent with vasoactive and occlusive changes in the pulmonary microvasculature following microaggregate infusion. The correlation of changing dead-space ventilation with the total microaggregate load was poor (r = 0.15) but was significant when compared with counts of microaggregates greater than 90 micrometers in diameter (r - 0.85). These findings suggest that gas exchange alterations following blood transfusion are primarily reflected by increased dead-space ventilation secondary to vasoconstriction and occlusion of the pulmonary microvasculature with microaggregates greater than 90 micrometers in diameter.