Complement activation and its relationship to adult respiratory distress syndrome. An experimental study in pigs.

Pulmonary leucostasis induced by complement activation has been considered an important pathogenic factor in adult respiratory distress syndrome (ARDS). To determine whether complement activation per se could evoke pulmonary dysfunction similar to ARDS, pigs were repeatedly infused with complement-activated plasma (CAP). Complement activation was produced by incubation of plasma with zymosan. Three groups of animals were investigated. Control animals received non-activated plasma. Nine animals (Group II) were given four infusions of CAP at a rate of 7 ml X min-1, and another nine animals (Group III) received two CAP infusions at a rate of 7 ml X min-1 followed by two at a rate of 14 ml X min-1. In the control animals there were no changes in gas exchange or haemodynamic variables and the leucocyte counts gradually increased. Infusion of CAP resulted in transient peripheral leucopenia and a dose-rate-dependent reversible increase in pulmonary vascular resistance in all animals. In one animal of Group II and in six of Group III there was a significant infusion-related decrease in Pao2 due to increased venous admixture. These animals were characterized by an enhanced pulmonary vascular tone before the start of the first CAP infusion. They also displayed a more pronounced pulmonary vascular response to infusion of CAP. The changes in gas exchange variables and pulmonary haemodynamics showed no relation to the degree of leucopenia or decrease in platelet count. The increased venous admixture was caused by "dry" ventilation/perfusion mismatching and not by oedema. These results suggest that additional factors besides complement activation and pulmonary leucostasis are required for the development of increased microvascular permeability and the pulmonary oedema characterizing ARDS.