Introduction

ARDS is an acute, diffuse, inflammatory form of lung injury and a life-threatening condition in seriously ill patients, characterised by poor oxygenation, pulmonary infiltrates, and an acute onset.  It can be triggered by various factors such as trauma, infection, sepsis, or inhalation injuries.¹ ARDS disrupts the alveolar-capillary membrane, leading to pulmonary oedema and compromised oxygenation. The condition affects approximately 190,000 patients annually in the United States, and mortality can exceed 50% at times.² In this case, damage occurs to the alveolar-capillary membrane, which serves as the interface between air and blood in the lungs. This damage can be triggered by various insults, including direct lung injury (e.g., pneumonia, aspiration) or indirect systemic processes (e.g., sepsis, trauma).³

Pulmonary surfactant plays a vital role in normal lung function by reducing surface tension at the air-liquid interface of alveoli.³ This reduction in surface tension is essential for maintaining alveolar stability, preventing collapse during expiration, and facilitating gas exchange. In ARDS, the quantity and quality of surfactant are altered, contributing to alveolar instability and respiratory failure.⁴

Surfactant, a complex mixture of phospholipids and proteins, plays a crucial role in maintaining alveolar stability and preventing collapse.⁵ Pulmonary surfactant is significantly lost in ARDS for a number of reasons, such as:

• Damage to the alveolar epithelium affects the production and secretion of surfactants
• Increased phospholipase and protease catabolism of surfactants
• Alveolar inflammation-induced functional inhibition

Although the injection of surfactants can enhance oxygenation in patients suffering from acute respiratory distress syndrome (ARDS), there has been no consistent evidence of a survival advantage in adult populations. The complexity of ARDS, where surfactant failure is only one aspect of a complex clinical process, could be the cause of this disparity.⁶ 

Mechanism of surfactant therapy

Since surfactant deficiency plays a pivotal role in the pathophysiology of ARDS, the injection of exogenous surfactants has been suggested as a possible therapeutic option. The goal of this strategy is to halt the cycle of inflammation and lung damage, restore the depleted surfactant pool, and enhance alveolar stability.⁷ Surfactant therapy works in ARDS by lowering alveolar surface tension, which keeps alveolar collapse from occurring, boosting alveolar recruitment and stability, modulating local inflammation and immunological responses, thus improving lung compliance and gas exchange.^8,9  

Initial research and development

The discovery of the function of pulmonary surfactant in lung function in the 1950s marked the beginning of the surfactant therapy path. This resulted in pioneering studies on newborn respiratory distress syndrome, a disorder in preterm infants marked by a surfactant shortage.¹⁰ Early respiratory distress syndrome clinical trials were remarkably successful in their endeavours. When preterm infants with respiratory distress syndrome were given modified bovine surfactant, there were notable improvements in oxygenation and chest radiographs, according to a groundbreaking study published in 1980 by Fujiwara et al.¹⁰ This resulted in a number of randomised controlled trials in the 1980s, which validated the effectiveness of surfactant therapy in lowering preterm infant mortality and pulmonary air leakage.

Despite the promising theoretical basis, translating the success of surfactant therapy from neonatal respiratory distress syndrome to adult ARDS has proven challenging. A number of significant studies have been carried out to assess its efficacy:

Adult ARDS patients' mortality and oxygenation were not significantly improved by surfactant administration, according to a meta-analysis of randomised controlled trials. This result stands in stark contrast to the obvious advantages of neonatal respiratory distress syndrome.¹¹

Research on ARDS in children and newborns has shown more encouraging findings. Three of the eight trials indicated a decrease in mortality, while seven of the eight trials showed improvements in oxygenation in children.⁸ In the same way, 7 out of 10 trials in newborns reported better oxygenation, but only 1 out of 10 trials showed benefit in terms of mortality.

The effectiveness of surfactant therapy in treating acute respiratory distress syndrome is still being studied despite these obstacles.⁸ Present initiatives centre on creating surfactant formulations that are more resistant, improving administration strategies, and pinpointing particular ARDS subgroups that may benefit most from this treatment.

The discrepancy between neonatal and adult outcomes may be attributed to several factors:

• Timing of administration: In neonatal respiratory distress syndrome, surfactant is often given prophylactically or very early in the disease course, which may be more effective than later administration in established ARDS¹² 
• Disease heterogeneity: ARDS in adults is often secondary to various underlying conditions, making it more complex than primary surfactant deficiency in premature infants¹¹
• Dosage and delivery: The optimal dose and delivery method for adult ARDS patients may differ from those established for neonates¹²

Clinical evidence on effectiveness

Exogenous surfactant treatment dramatically increased oxygenation in ARDS patients, according to a randomised controlled study by Gregory et al. (1997).¹³ Still, other large-scale trials, such as the ARDS Network trial, did not demonstrate a statistically significant decrease in mortality with surfactant therapy when compared to placebo groups. There has been conflicting overall influence on survival rates despite surfactant therapy's ability to increase lung compliance and oxygenation in certain studies.¹³ There seems to be variation in the efficacy of surfactant therapy among ARDS subgroups. Studies suggest that individuals with direct lung injury (e.g., pneumonia, aspiration) may benefit from surfactant therapy more than those with indirect causes (e.g., sepsis, trauma). Furthermore, early surfactant therapy administration appears to have better results than delayed treatment when it is given during the first 48 hours of ARDS onset.¹⁴

There has been a comparison between surfactant therapy and common ARDS therapies, including prone positioning and mechanical ventilation. The primary therapy for ARDS oxygenation maintenance is mechanical breathing; however, this approach ignores the underlying surfactant shortage.¹⁵ It has been demonstrated that prone positioning, which 

enhances ventilation-perfusion matching, lowers mortality in severe ARDS.¹⁶ Additionally, it has been investigated to combine surfactant therapy with other cutting-edge procedures such as extracorporeal membrane oxygenation (ECMO). In cases of severe ARDS, ECMO is utilised to oxygenate blood externally and provide lung rest.¹⁷

Challenges and limitations

Clinical trials investigating surfactant therapy in ARDS have yielded inconsistent results, largely due to challenges in patient selection and timing of intervention. This lack of consistent positive outcomes may be attributed to the heterogeneity of ARDS populations and the complexity of the syndrome's pathophysiology.¹³ There seems to be a greater benefit to early intervention in preventing future lung injury when surfactant administration is timed appropriately.¹⁸ Because ARDS is frequently diagnosed late in its course, determining the best time to begin treatment is still difficult. Its efficacy is also greatly influenced by the surfactant delivery mechanism. Poor alveolar deposition has hampered the effectiveness of aerosolised surfactant delivery; estimates indicate that only approximately 5% of the medication reaches the target locations.¹³ On the other hand, studies have shown that instilling surfactant by bronchoscopic delivery yields better results; nevertheless, this approach has drawbacks as well, such as the possibility of transient hypoxemia after administration.¹⁹

While surfactant therapy shows promise in improving oxygenation, it is not without potential side effects. One major concern is the risk of infection associated with the administration procedure, particularly when using bronchoscopic instillation. Additionally, the distribution of surfactant in injured lungs can be problematic, potentially leading to uneven treatment effects and exacerbation of ventilation-perfusion mismatches.⁸ The cost-effectiveness of surfactant therapy in ARDS remains a significant concern. Natural surfactants are expensive to produce, and the required doses for adult ARDS patients are substantially larger than those used in neonatal care.⁹ Given the lack of consistent mortality benefits, justifying the high costs of treatment is challenging.

Logistically, implementing surfactant therapy in adult ICU settings presents several hurdles. The need for specialised equipment and trained personnel for bronchoscopic administration can limit its widespread adoption.¹⁹ Moreover, the potential for temporary clinical deterioration during administration necessitates close monitoring and may increase the overall burden on ICU resources.¹⁹

Future directions

Advancements in surfactant formulations are a key area of focus. While natural surfactants have been the mainstay of therapy, synthetic surfactants are gaining attention due to their potential for more consistent composition and reduced risk of immunogenicity. Researchers are working on developing synthetic surfactants that closely mimic the properties of natural surfactants, including the incorporation of surfactant proteins or their analogues.⁵

Innovations in delivery methods are also being explored to improve the effectiveness of surfactant therapy. Traditional instillation methods have limitations, particularly in achieving uniform distribution throughout the lungs. Aerosolisation of surfactant is a promising approach that could potentially overcome these challenges.⁵ Studies have shown that aerosolised surfactant can improve oxygenation in ARDS patients, although more research is needed to optimise delivery techniques.⁵ 

Furthermore, adjusting the surfactant formulation, dosage, and delivery mechanism to the unique needs of each patient may improve the efficacy of treatment. This strategy might take into account elements like the patient's genetic profile, the degree of lung damage, and the underlying aetiology of ARDS[6]. The future of surfactant therapy in ARDS may lie in personalised treatment protocols. Research suggests that the effectiveness of surfactant therapy may vary depending on the cause of ARDS (direct vs. indirect lung injury) and the stage of the disease.⁹

Conclusion

Surfactant therapy has shown promise in improving oxygenation in ARDS patients, particularly in the early stages of the disease. However, despite these physiological improvements, clinical trials have consistently failed to demonstrate a significant mortality benefit in adult ARDS patients. The effectiveness of surfactant therapy appears to differ among ARDS subgroups. Additionally, the timing of administration, dosage, and delivery method have been identified as critical factors influencing the therapy's efficacy.²⁰ Interestingly, surfactant therapy has shown more promising results in pediatric and neonatal ARDS. Several trials in children have demonstrated improvements in oxygenation and, in some cases, reduced mortality. 

The ongoing search for optimal ARDS treatments remains a critical area of research in critical care medicine. While surfactant therapy has not yet fulfilled its initial promise as a game-changing intervention for adult ARDS, it continues to be a valuable area of exploration. 

The positive results seen in pediatric populations and the physiological improvements observed in adults suggest that there may still be untapped potential in this approach.⁸

As our understanding of ARDS pathophysiology continues to evolve, surfactant therapy may find its place as part of a multimodal treatment strategy. Combining surfactant administration with other evidence-based interventions, such as lung-protective ventilation strategies and prone positioning, could potentially yield synergistic benefits. The ongoing refinement of this therapy, coupled with advances in our understanding of ARDS subtypes and personalised treatment approaches, may yet reveal its full potential in improving outcomes for ARDS patients.