The association of circulating endotoxin with the development of the adult respiratory distress syndrome.

Despite extensive investigation, the pathogenesis of the adult respiratory distress syndrome (ARDS) remains uncertain. As yet, there is no clear explanation of why some patients at risk for ARDS develop the syndrome, whereas others do not. Neutrophils and complement fragments have been implicated in the acute lung injury, but it is clear from published data that evidence of complement activation alone predicts neither the development nor the severity of ARDS. We investigated whether the combination of endotoxin, a leukocyte-priming agent, and complement fragments, leukocyte-stimulating agents, was associated with the development of ARDS. Ninety-eight patients were identified as being either at risk for the development of ARDS or having ARDS, and serial blood samples were obtained. There was no correlation between C5 fragments and the development of ARDS. C3 fragment levels were increased in 89% of the patients with ARDS, but they were also increased in 62% of patients at risk. Endotoxin was detected in 74% of the plasma samples obtained from patients at risk who subsequent developed ARDS and in 64% of the plasma samples obtained from the patients with ARDS. In contrast, only 22% of the plasma samples obtained from the patients at risk who did not develop ARDS had measurable endotoxin. We suggest that the combination of endotoxin and complement fragments may be one mechanism involved in the development of ARDS.

High-dose corticosteroids in patients with the adult respiratory distress syndrome.

Corticosteroids are widely used as therapy for the adult respiratory distress syndrome (ARDS) without proof of efficacy. We conducted a prospective, randomized, double-blind, placebo-controlled trial of methylprednisolone therapy in 99 patients with refractory hypoxemia, diffuse bilateral infiltrates on chest radiography and absence of congestive heart failure documented by pulmonary-artery catheterization. The causes of ARDS included sepsis (27 percent), aspiration pneumonia (18 percent), pancreatitis (4 percent), shock (2 percent), fat emboli (1 percent), and miscellaneous causes or more than one cause (42 percent). Fifty patients received methylprednisolone (30 mg per kilogram of body weight every six hours for 24 hours), and 49 received placebo according to the same schedule. Serial measurements were made of pulmonary shunting, the ratio of partial pressure of arterial oxygen to partial pressure of alveolar oxygen, the chest radiograph severity score, total thoracic compliance, and pulmonary-artery pressure. We observed no statistical differences between groups in these characteristics upon entry or during the five days after entry. Forty-five days after entry there were no differences between the methylprednisolone and placebo groups in mortality (respectively, 30 of 50 [60 percent; 95 percent confidence interval, 46 to 74] and 31 of 49 [63 percent; 95 percent confidence interval, 49 to 77]; P = 0.74) or in the reversal of ARDS (18 of 50 [36 percent] vs. 19 of 49 [39 percent]; P = 0.77). However, the relatively wide confidence intervals in the mortality data make it impossible to exclude a small effect of treatment. Infectious complications were similar in the methylprednisolone group (8 of 50 [16 percent]) and the placebo group (5 of 49 [10 percent]; P = 0.60). Our data suggest that in patients with established ARDS due to sepsis, aspiration, or a mixed cause, high-dose methylprednisolone does not affect outcome.

Oxygen metabolites stimulate thromboxane production and vasoconstriction in isolated saline-perfused rabbit lungs.

Generation of reactive oxygen metabolites, thromboxane increases, and vasoconstriction have been implicated in the pathogenesis of acute edematous lung injury, such as that seen in patients with the Adult Respiratory Distress Syndrome (ARDS), but their interactions are unknown. We hypothesized that reactive O2 products would stimulate arachidonic acid metabolism in lungs and that vasoactive products of arachidonate, such as the potent vasoconstrictor thromboxane A2, might then mediate O2-metabolite-induced pulmonary vasoconstriction. We found that O2 metabolites generated by injection of purine plus xanthine oxidase caused increases in mean pulmonary artery perfusion pressures (27 +/- 4 mmHg) in isolated perfused lungs. In addition, purine plus xanthine oxidase also caused 30-fold increases in perfusate levels of thromboxane B2 (the stable metabolite of thromboxane A2) compared with only twofold increases in 6-keto-PGF1a (the stable metabolite of prostacyclin). Moreover, prior addition of catalase inhibited both vasoconstriction and the thromboxane B2 production seen in isolated lungs following injection of purine plus xanthine oxidase. Similarly, pretreatment with cyclooxygenase inhibitors, either aspirin or indomethacin, also completely blocked thromboxane generation and markedly attenuated pressor responses usually seen after purine plus xanthine oxidase (increase in mean pulmonary artery perfusion pressures, 4.4 +/- 1.5 mmHg). Furthermore, imidazole, a thromboxane synthetase inhibitor, also decreased O2-metabolite-induced thromboxane generation and vasoconstriction. These results suggested that thromboxane generation might participate in O2-metabolite-induced vasoconstriction. However, since a significant correlation between thromboxane levels and the degree of vasoconstriction could not be demonstrated, and since addition of superoxide dismutase reduced thromboxane generation but did not affect the intensity of vasoconstriction, it is possible that thromboxane is not the only vasoactive mediator in this model. We conclude that exposing lungs to O2 metabolites results in thromboxane generation and that thromboxane is a major mediator of oxidant-induced vasoconstriction.

Staphylococcus aureus and human platelets cause pulmonary hypertension and thromboxane generation in isolated saline-perfused rabbit lungs.

The potential contributions of bacterial-platelet interactions to the development of acute edematous lung injury, such as that seen in the adult respiratory distress syndrome (ARDS), remains unknown. We found that the addition of Staphylococcus aureus, 502A to isolated rabbit lungs perfused with saline, and human platelets rapidly decreased the number of circulating platelets, increased pulmonary artery perfusion pressures, and generated thromboxane B2, the stable derivative of thromboxane A2. In contrast, increases in perfusion pressures or thromboxane levels did not occur when platelets treated with acetylsalicylic acid (ASA) were used, even though ASA-treated platelets disappeared from the perfusates. The results suggest that activation of platelets by bacteria may account for thrombocytopenia, platelet microemboli, and/or contribute to increases in pulmonary artery pressures seen in some patients with ARDS.

Prevention of thiourea-induced pulmonary edema by hydroxyl-radical scavengers.

Thiourea (TU), a very effective hydroxyl radical (.OH) scavenger, has little value as a probe of .OH in vivo because it causes fatal pulmonary edema. To test the hypothesis that TU-induced lung injury results from .OH-mediated oxidation of TU to toxic cyanamide, we pretreated rats with .OH scavengers, dimethylsulfoxide (DMSO), ethanol, and mannitol, prior to treatment with TU (3 mg/kg), preventing 91, 63, and 53%, respectively, of increases in lung weight to body weight ratios and 93, 67, and 46% of increases in lung lavage albumin concentrations. Furthermore, treatment of rats with cyanamide (CYN) (100 mg/kg) also caused increases in lung weight to body weight ratios (CYN: 7.39 +/- 0.57 X 10(-3) vs. controls: 5.46 +/- 0.26). N,N'-dimethylation of TU (DMTU) prevented TU toxicity, because treatment with DMTU did not significantly increase lung weight to body weight ratios (DMTU: 5.12 +/- 0.16 X 10(-3) vs. controls: 5.46 +/- 0.26) or lung lavage albumin (DMTU: 14 +/- 1 mg/100 ml vs. controls: 11 +/- 1). DMTU remained a very effective in vivo .OH scavenger, increasing survival of lethally irradiated mice treated with 600 mg/kg DMTU to 79% compared with 8% in untreated controls.

Mepacrine but not methylprednisolone decreases acute edematous lung injury after injection of phorbol myristate acetate in rabbits.

A good model of the adult respiratory distress syndrome (ARDS) is intravenously injected phorbol myristate acetate (PMA), which causes pulmonary sequestration of neutrophils and a neutrophil-dependent acute edematous lung injury in rabbits. In the present study, pretreatment of rabbits with the antimalarial agent, mepacrine, prevented lung edema after injection of PMA without altering initial accumulations of neutrophils in the lung. Mepacrine also decreased oxygen radical production and degranulation by neutrophils stimulated by PMA in vitro. In contrast, pretreatment with methylprednisolone did not decrease edematous lung injury in rabbits given PMA nor did it inhibit O2 radical production or degranulation by neutrophils treated with PMA in vitro. Our results suggest that agents that modify neutrophil function may be useful in decreasing lung injury after PMA treatment and, perhaps, in treating patients with ARDS.

Oxygen-radical-mediated permeability edema and vasoconstriction in isolated perfused rabbit lungs.

Oxygen radicals have been implicated in the pathogenesis of permeability pulmonary edema. To determine directly if O2 radicals can cause increased alveolar-capillary membrane (ACM) permeability and low-pressure permeability edema, we chemically produced O2 radicals in the sale perfusates of isolated rabbit lungs. The O2 radicals generated by xanthine oxidase caused protein-rich edema and increases in lung perfusion pressures that were inhibitable by catalase (hydrogen peroxide scavenger) or dimethylthiourea (hydroxyl radical scavenger) but not by superoxide dismutase. To determine the effect of O2 radicals on ACM permeability without interference from increased perfusion pressures, we used papaverine to maintain baseline perfusion pressures during O2 radical exposure and then assessed ACM integrity by evaluating the response of isolated lungs to elevated outflow pressures (10 mmHg for 10 min). Under these conditions, increased ACM permeability manifested by weight gains and lavage albumin accumulations occurred in lungs treated with xanthine oxidase but not in control lungs. We conclude that O2 radicals can cause increased ACM permeability and vasoconstriction in isolated lungs.

Granulocytes mediate acute edematous lung injury in rabbits and in isolated rabbit lungs perfused with phorbol myristate acetate: role of oxygen radicals.

Acute edematous lung injury is associated with a marked increase in the number of granulocytes in the alveoli and microvasculature of the lung. Phorbol myristate acetate (PMA) causes granulocytes to adhere, aggregate, and release oxygen radicals and granular enzymes. We found that intravenously injected PMA caused a protein-rich edema in lungs of control rabbits but not in granulocytopenic rabbits pretreated with nitrogen mustard. Specifically, control rabbits treated with PMA had higher lung weight to body weight ratios (6.4 +/- 1.0 X 10(-3)) and lung lavage albumin concentrations (190 +/- 44 mg/dl) than granulocytopenic rabbits pretreated with nitrogen mustard and then given PMA (4.74 +/- 0.23 X 10(-3) and 9.9 +/- 3.8 mg/dl, respectively). To further clarify the role of granulocytes in the production of edema, additional experiments were conducted in an isolated perfused rabbit lung. Addition of purified granulocytes and PMA to the balanced salt perfusate caused lung edema, whereas neither granulocytes nor PMA alone caused edema. Specifically, increases in lung weights (42 +/- 9.2 g) and albumin concentrations (1,182 +/- mg/dl) in lung lavages from isolated lungs exposed to granulocytes and PMA were greater than increases in lung weights or albumin concentrations in lung lavages from isolated lungs exposed to granulocytes alone (2.0 +/- 0.4 g and 15 +/- 0.6 mg/dl), or to PMA alone (6.0 +/- 0.6 g and 81 +/- 34 mg/dl). To determine the contribution of oxygen radicals to the pathogenesis of the edema, chronic granulomatous disease granulocytes, which are deficient in oxygen radical production, were added to the isolated lung perfusate. Chronic granulomatous disease granulocytes and PMA did not cause edema in isolated lungs (delta lung weight 1.0 +/- 0.2 g and lavage albumin 12 +/- 5.0 mg/dl) whereas granulocytes from normal human subjects and PMA did (delta lung weight 43 +/- 5.2 g and lavage albumin 1,120 +/- 54 mg/dl). These data suggest that oxygen radicals released from stimulated granulocytes contribute to the pathogenesis of acute edematous lung injury.

Angiotensin converting enzyme concentrations in the lung lavage of normal rabbits and rabbits treated with nitrogen mustard exposed to hyperoxia.

Increased concentrations of angiotensin converting enzyme (ACE) were found in lung lavages from rabbits exposed to hyperoxia for 72 h and the concentrations of ACE were correlated with ratios of extravascular lung water to body weight (r = 0.69, p less than 0.05) and albumin concentrations in lung lavages (r = 0.89, p less than 0.01). In parallel studies, rabbits treated with nitrogen mustard in which granulocytopenia was maintained throughout the 72-h hyperoxic exposure period had less evidence of edematous lung injury and lower concentrations of ACE in their lung lavages than similarly treated rabbits in which granulocytopenia was not maintained. The results suggested that granulocytes contribute to acute edematous lung injury from hyperoxia and that ACE concentrations in lung lavages reflect this process.