Acute effects of Escherichia coli endotoxin on the pulmonary microcirculation of anesthetized sheep...

Infusion of Escherichia coli endotoxin into sheep causes a syndrome analogous to the adult respiratory distress syndrome. Physiologic measurements show an initial phase of marked pulmonary hypertension followed by a phase characterized by the production of large quantities of protein-rich lung lymph. The present study relates the structural changes that occur during endotoxemia to concomitant functional changes. In five anesthetized open-chest sheep, we monitored pulmonary and systemic artery pressure for a 1 hour baseline period and for 4 hours after the start of E. coli endotoxin infusion (1.25 microgram/kg, intravenously). We also measured cardiac output, arterial blood gases and pH, and number of circulating leukocytes. In addition, we sequentially biopsied random lobes from the lungs of each sheep at baseline and at 15, 30, 60, 120, 180, and 240 minutes after the start of endotoxin. Five control sheep were treated identically except that they received saline instead of endotoxin. By 15 minutes after the start of endotoxin infusion, light microscopy revealed margination and accumulation of leukocytes in the lungs' microcirculation. Counts of the number of peripheral lung granulocytes in biopsy specimens showed a 3-fold increase above baseline by 15 minutes and a 6-fold increase by 4 hours. By electron microscopy, the leukocytes were identified as both granulocytes and lymphocytes, present in approximately equal numbers. Some granulocytes were fragmented, and specific granules were found free in the vessel lumen. By 30 minutes, some leukocytes were migrating into the interstitium. By 60 minutes, interstitial edema was seen, and there was focal endothelial cell damage. Correlation of the structural with the physiologic changes shows that the initial accumulation of leukocytes in the microcirculation occurs when pulmonary hypertension develops. The migration of leukocytes into the interstitium and endothelial cell damage precedes the physiologic changes that we interpret as increased pulmonary vascular permeability. Since gram negative septicemia is a frequent occurrence in the adult respiratory distress syndrome the changes described here may be similar to the alterations that occur early in the development of the syndrome in man.