Classical descriptions of ARDS, based on lung biopsy and postmortem specimens, have artificially divided the condition into three phases – exudative, proliferative and fibrotic 10 – although in practice these phases often overlap 1. The early phases are characterized by infiltration with neutrophils, macrophages and inflammatory cytokines, and disruption of the alveolar capillary barrier, leading to an influx of protein-rich oedema fluid into the alveolar spaces 11. Although controversy still exists regarding the role of polymorphonuclear neutrophils in all causes of ALI 12, it is likely that they play a central role in early stages 13. Analysis of bronchoalveolar lavage (BAL) fluid from patients with ARDS has revealed increased numbers of activated neutrophils in the early stages of ARDS 1314. The number of neutrophils in BAL fluid correlates with the severity of lung injury 15, and persistence of neutrophils in BAL fluid by day 7 is associated with increased mortality 14.

Pulmonary neutrophil sequestration occurs within minutes of exposure to an inflammatory insult 1617. The insult causes an increase in neutrophil stiffness and a reduction in deformability 18, leading to sequestration into the pulmonary capillaries followed by emigration into the alveolar space. The process of neutrophil emigration occurs by at least two different pathways. Neutrophil emigration is dependent on CD11/18 adhesion molecule interactions in response to Gram-negative organisms, IL-1α and phorbol 12-myristate 13-acetate. Gram-positive organisms, hyperoxia and the complement anaphylatoxins (C5a) appear to induce neutrophil emigration through a CD11/18 independent pathway 19.

Neutrophils are a potent source of reactive oxygen and nitrogen species, inflammatory cytokines, proteolytic enzymes and lipid mediators. A recent study examining ARDS BAL fluid 20 demonstrated a positive correlation between neutrophil myeloperoxidase and oxidatively modified amino acids, suggesting an association between pulmonary neutrophil activation and oxidative protein damage. Carden and coworkers 20 reported that damage to human surfactant protein A in BAL fluid from patients with ARDS resembled the damage caused when it is cleaved by neutrophil elastase in patients with ARDS. Therapeutic interventions with neutrophil elastase inhibitors in animal models of ARDS have shown that inhibition of neutrophil function can limit the degree of lung injury caused by ischaemia–reperfusion 21 and lipopolysaccharide (LPS) 22.

The importance of regulation of neutrophil apoptosis in ARDS was recently reviewed in detail 23. It is known that ARDS BAL fluid delays neutrophil apoptosis in vitro 24. At present the relationship between neutrophil apoptosis and survival from ARDS has not been clearly defined, although it has been suggested that increasing neutrophil apoptosis could be beneficial in aiding resolution of ARDS 23. Apoptotic neutrophils are cleared from the alveolar space by alveolar macrophages. Interestingly, this process changes the inflammatory cytokine profile produced by the macrophage from an inflammatory to anti-inflammatory phenotype 25. Furthermore, in a recent study conducted in mice 26, stimulating neutrophil apoptosis led to reduced lung injury and improved survival. This suggests that acceleration of neutrophil apoptosis could be beneficial in the treatment of ARDS. Modulation of neutrophil recruitment, activation and apoptosis are thus potential therapeutic targets for the treatment of patients with ARDS.

β-Adrenergic stimulation has been shown to reduce pulmonary neutrophil sequestration in several different models of lung injury. Using a murine model of direct lung injury (endotoxin inhalation), Dhingra and coworkers 27 showed that pretreatment with intravenous dobutamine reduced BAL fluid neutrophilia by 30% in parallel with reduced pulmonary IL-6, IL-10 and macrophage inflammatory protein-2 productions. Similarly, in a rodent model of indirect lung injury following endotoxic shock, pretreatment with intravenous terbutaline before exposure to endotoxin blocked pulmonary neutrophil accumulation, prevented circulatory failure and reduced mortality 28. In normal human volunteers, in a placebo-controlled trial, treatment with 300 μg inhaled salbutamol was able to prevent platelet-activating factor induced pulmonary sequestration of radio-labelled neutrophils 29.

The precise mechanisms of reduced pulmonary neutrophil sequestration have not fully been elucidated, although they may involve modulation of adhesion and emigration, accelerated apoptosis and reduced generation of inflammatory mediators.

A complex network of cytokines, proinflammatory and anti-inflammatory substances are involved in the inflammatory response in ARDS. Inflammatory cytokines such as IL-8, TNF-α and IL-1β have been found in high concentrations in the early phase of ARDS 4647. The balance between proinflammatory and anti-inflammatory cytokines is likely to be critical in the development and persistence ARDS 48. High initial titres and persistence of inflammatory cytokines have been shown to be predictors of poor outcome 49. IL-8, a cytokine that is seen early in the inflammatory response, is important in pulmonary neutrophil recruitment and activation 50. Treatment with anti-IL-8 monoclonal antibody in experimental animal models of ARDS has been shown to decrease the magnitude of ALI 505152], suggesting that modulation of cytokine production may have a role to play in ameliorating lung injury.

β-Adrenergic stimulation in vitro reduces inflammatory cytokine production (IL-1β 53, TNF-α 54555657], IL-6 58 and IL-8 5960) and enhances release of the anti-inflammatory cytokine IL-10 61 from whole blood, monocytes and macrophages. In an in vivo mouse model of LPS-induced septic shock, Wu and coworkers 28 demonstrated that treatment with terbutaline was able to reduce TNF-α production, enhance IL-10 production and improve survival. In an ex vivo model using human lung explants in culture, treatment with 1 ng/ml isoproterenol attenuated LPS-induced release of TNF-α and reduced lipid peroxidation, which was associated with an increase in intracellular cAMP levels 62. Van der Poll and coworkers 63 extended these findings in vivo in human volunteers using adrenaline before LPS exposure. That study confirmed that adrenaline reduced LPS-induced TNF-α release in vivo and in whole blood ex vivo. This occurred in parallel with an increase in the release of the anti-inflammatory cytokine IL-10. In addition β-adrenergic stimulation, in contrast to α-receptor stimulation, caused an increase in IL-10 similar to that with adrenaline. These data suggest that treatment with β₂-agonists may have a role to play in reducing the excessive proinflammatory effects of the cytokine network during the early phases of ARDS.

Extensive damage to the alveolar–capillary barrier and microvascular thrombosis are prominent features in the early stages of ARDS 64. This leads to alveolar flooding and the development of noncardiogenic pulmonary oedema, which impairs gas exchange and contributes to the refractory hypoxia that characterizes ARDS.

In vitro studies using pulmonary artery endothelial cells have shown that incubation with isoprotenerol reduces baseline monolayer permeability to albumin and can block the effects of thrombin-induced increase in permeability 6566. These findings have been confirmed in vivo in a sheep ARDS model using terbutaline 67 and a rat ARDS model using isoproten-erol 68. In a small nonrandomized study conducted in humans, administration of intravenous terbutaline to 10 patients with ARDS was associated with a significant reduction in lung vascular permeability (measured by radio-labelled transferrin) in six patients, which was associated with an increased probability of survival 69. The mechanism appears to be related to inhibition of endothelial cell contraction and increased force between endothelial cell tight junctions.

Alterations to the coagulation/fibrinolysis pathways may be important in the pathogenesis of ARDS 70. Two recent studies from Matthay and coworkers 7172 showed that plasma and oedema fluid levels of protein C and oedema fluid levels of thrombomodulin and plasminogen activator inhibitor-1 are associated with increased mortality in patients with ARDS. There is some preliminary evidence from studies in healthy volunteers that the intravenous administration of isoproterenol increases the release of tissue plasminogen activator and urokinase plasminogen activator, which may enhance fibrinolysis and vessel patency 7374. The effects of β-adrenergic stimulation on the coagulation–fibrinolysis cascade in ARDS, however, remains to be determined.

Clearance of fluid from the alveolar space is dependent on active sodium and chloride transport. The alveolar type II cell appears to be responsible for the majority of ion transport via the apical sodium and chloride conductive pathways and the basolateral Na/K-ATPase, although the alveolar type I cell and distal airway epithelium may also contribute 75. Experimental studies in animals, as well as in the ex vivo human lung, have demonstrated that β-adrenergic agonists accelerate the rate of alveolar fluid clearance 7677. The mechanism underlying increased alveolar fluid clearance is proposed to be due to an increase in intracellular cAMP, resulting in increased sodium transport across alveolar type II cells by upregulation of the apical sodium and chloride pathways and Na/K-ATPase and probably cystic fibrosis transmembrane conductance regulator 75. β₂-Adrenergic stimulation is more important than β₁-adrenergic stimulation in mediating alveolar epithelialsodium and fluid transport. Dopamine, at doses associated with only a β₁ effect, whether by intra-alveolar or intravenous route of administration, had no effect on alveolar fluid clearance in vivo in rats. Moreover, the increase in alveolar fluid clearance caused by dobutamine is blocked by selective β₂-adrenergic antagonists 78. Finally, β₁-adrenergic stimulation by high-dose terbutaline has been found to downregulate alveolar fluid clearance in the ex vivo rat lung 79.

Impaired ability of the alveolar epithelium to remove alveolar oedema fluid is associated with increased mortality in ARDS 8081. This has important implications for the potential use of β₂-agonists in the treatment of ALI/ARDS. If the alveolar epithelium is extensively injured, then pharmacological intervention aimed at improving epithelial function may be difficult because of the extent of injury. Alveolar epithelial fluid clearance mechanisms are intact after mild to moderate lung injury and can be upregulated by β-adrenergic agonists 8283. However, in some experimental models neutrophil-dependent oxidant injury to the alveolar epithelium is more resistant to β-adrenergic upregulation of alveolar fluid clearance 848586]. β-Agonists have also been shown to upregulate fluid transport in hydrostatic oedema 878889], hyperoxic lung injury 839091 and ventilator-associated lung injury 92. In addition, β₂-agonists can overcome the depressant effects of hypoxia on alveolar fluid clearance 9394. In a randomized, placebo-controlled clinical trial 95, inhaled salmeterol (a long-acting β₂-agonist) reduced the incidence of high-altitude pulmonary oedema in volunteers who were known to be at risk for this condition. The authors postulated that this may be due to an increase in alveolar fluid clearance, although beneficial effects of salmeterol on minute ventilation and pulmonary artery pressures could not be excluded. On the basis of these experimental data augmentation of alveolar epithelial fluid clearance with β₂-adrenergic agonists may accelerate resolution of pulmonary oedema and improve outcome in ALI/ARDS.

Surfactant, a mixture of dipalmitoyl-phosphatidylcholine and other lipids and proteins, is produced by type II alveolar epithelial cells. Surfactant is a lipid surface-tension-lowering agent and it helps to prevent pulmonary oedema. Surfactant plays an increasingly recognized role in immune defence. Surfactant protein (SP)-A is known to promote phagocytosis of bacteria by alveolar macrophages, and SP-D also has antimicrobial properties 9697. Deficiency in these specific proteins may well contribute to the increase risk for infection in ARDS patients.

Short-acting and long-acting β₂-agonists augment total surfactant secretion from alveolar type II cells through activation of β-adrenergic receptors and a cAMP-dependent protein kinase. Several β₂-agonists stimulate secretion of phophatidylcholine, the principal lipid component of surfactant 9899. In particular, terbutaline is a potent secretagogue 100. β₂-Agonists also stimulate secretion of SP-Band SP-C, the two hydrophobic proteins that are involved in the main biophysical functions of surfactant 101. Fenoterol has been shown to restore lung phospholipid metabolism, which was altered by sepsis, toward normal 99. These studies suggest a potential role for β₂-agonists as a treatment for surfactant abnormalities in ARDS.

Nosocomial pneumonia contributes to morbidity and mortality on the intensive care unit 102. Central to the development of these infections is colonization followed by invasion of the epithelial cell layer. Several studies have investigated the effect of salmeterol on Pseudomonas aeruginosa and Haemophilus influenzae induced epithelial damage 103104. In the Pseudomonas study, there was not only reduced pyocyanin-induced cytoplasmic blebbing and reduced mitochondrial damage but also a significant reduction in adherent bacteria. These data suggest that salmeterol has a cytoprotective effect on respiratory epithelial cells, most likely related to maintaining structural integrity of the epithelial cells rather than increasing antibacterial activity. Interestingly, salbutamol and isoproterenol have also been shown to increase monocyte adhesion to human airway epithelial cells in vitro, monocytes being integral to the bacterial immune response in the lung 105. It is possible, therefore, that β₂-agonists have a role to play in the prevention of ventilator associated pneumonia, which commonly complicates ALI/ARDS, by augmenting host epithelial resistance to infection.

In ARDS, histological studies have confirmed that there is a physical breach of both the alveolar endothelial and epithelial barriers. This physical damage results in pulmonary oedema that is central to the need for mechanical ventilation. Recovery of the barrier function is vital for effective alveolar epithelial repair. This process is regulated by keratinocyte growth factors (KGFs) and other related cytokines (e.g. IL-1β) that are capable of stimulating alveolar epithelial cell proliferation and migration. In a rat study, pretreatment with KGF before induction of lung injury reduced the severity of injury 106. The protective capability of KGF is probably due to upregulation of the number of type II alveolar epithelial cells, with a corresponding increase in net alveolar fluid transport 107. Salbutamol is a potent upregulator of human airway epithelial cells, probably via a protein kinase cascade, and isoproterenol directly increased the migration of bovine epithelial cells, speeding up the closure of mechanically and enzymatically induced wounds 108. Currently, it is not known whether stimulating epithelial regeneration in humans improves outcome in patients with ARDS.

The physiological consequences of extensive alveolar–epithelial injury include a reduction in pulmonary compliance 5 and increased airway resistance 109, which are associated with an increased work of breathing and requirement for mechanical ventilation. Several studies have shown that both intravenous and nebulized salbutamol reduce peak airway and plateau pressures 109110111] in patients with ARDS. The reduction in peak airway pressure reflects a reduction in airway resistance due to the bronchodilator effects of β₂-agonists. However, the reduction in plateau pressure suggests an improvement in respiratory compliance, through as yet undetermined mechanisms. These studies suggest that β-agonists may have a beneficial role to play in improving respiratory mechanics in patients with ARDS.