Positive effects of prophylactic ventilator treatment on gas exchange and extravascular lung water in a porcine model of adult respiratory distress syndrome induced by endotoxaemia

Abstract

The influence of prophylactic ventilator treatment was evaluated in a porcine model of early adult respiratory distress syndrome (ARDS) induced by endotoxaemia. Sixteen animals, controls, under continuous i.v. ketamine anaesthesia were either mechanically ventilated using intermittent positive pressure ventilation (IPPV; n = 6) with air or breathed air spontaneously (n = 10). Twenty animals under continuous i.v. ketamine anaesthesia and spontaneously breathing air were infused i.v. with E. coli endotoxin (10 micrograms X kg-1 X h-1) over 6 h. Fifteen animals under continuous i.v. ketamine anaesthesia were given IPPV with air and were infused i.v. with E. coli endotoxin in the same dosage regimen. In the controls, cardiac output decreased slightly. Otherwise there were no changes in pulmonary gas exchange, pulmonary haemodynamics or extravascular lung water. In spontaneously breathing and IPPV animals given endotoxin there was a profound deterioration in pulmonary gas exchange, a marked rise in pulmonary vascular resistance and a moderate increase in extravascular lung water. Animals given IPPV showed a significantly less pronounced impairment in pulmonary gas exchange and a significantly smaller increase in extravascular lung water than in animals breathing spontaneously, whereas the changes in pulmonary haemodynamics were fairly similar in both groups. Animals with IPPV also had an improved survival rate. The beneficial effects of mechanical ventilation on pulmonary gas exchange are not due to changes in extravascular lung water, but are caused by its influence in counteracting terminal airway and alveolar closure. These results indicate that mechanical ventilation, when instituted early in the course of human ARDS induced by septicaemia, might be of potential value in the prevention of severe pulmonary failure and death.