Background

When someone is admitted to intensive care with serious breathing difficulties, doctors must act quickly to find the cause and begin treatment that could save their life. Two conditions that often come into question are Acute Lung Injury (ALI) and Acute Respiratory Distress Syndrome (ARDS). In the past, these were described as separate illnesses, but today they are viewed as part of the same disease spectrum. For doctors, nurses, and students working in emergency, intensive care, or respiratory medicine, understanding how these conditions are related — and how thinking about them has changed over time — is essential.

Understanding terminology

For many years, the terms ALI and ARDS caused confusion, as doctors were unsure how severe a patient’s illness was or which treatment was most suitable. Since 2012, when the Berlin Definition of ARDS was introduced, the criteria have become clearer and more standardised. Even so, it is still important for healthcare professionals and students to understand how the definitions have evolved. This helps ensure accurate diagnosis, more effective treatment decisions, and clearer communication between teams.¹

In 1994, the American-European Consensus Conference (AECC) introduced the term Acute Lung Injury (ALI) to describe patients who had sudden breathing failure due to low oxygen levels, but not caused by heart failure or excess fluid. Within this group, Acute Respiratory Distress Syndrome (ARDS) was considered the more severe form. At the time, the severity was measured by the PaO₂/FiO₂ ratio (a way of assessing oxygen levels in the blood compared to the oxygen being given). A ratio of 300 mmHg or below meant ALI, while 200 mmHg or below meant ARDS. In both cases, chest X-rays showed changes on both sides of the lungs, without signs of heart-related fluid overload. This system helped doctors identify and grade the extent of lung damage.² 

ARDS classification

Over time, doctors realised that the AECC definition had several problems. Different ways of setting up ventilators and interpreting scans often led to inconsistent diagnoses. To improve this, the Berlin Definition was introduced in 2012. This new system removed the separate category of ALI and instead grouped ARDS into three levels of severity based on oxygen levels in the blood:³

• Mild: PaO₂/FiO₂ between 200–300
• Moderate: PaO₂/FiO₂ between 100–200
• Severe: PaO₂/FiO₂ below 100

This clearer framework made it easier to diagnose ARDS consistently, compare patient outcomes, and guide treatment. While ALI is no longer an official medical category, it is still sometimes used to describe the milder end of ARDS.

Pathophysiology

Similarities 

Both ALI and ARDS develop when the delicate barrier between the tiny air sacs (alveoli) and blood vessels in the lungs becomes damaged. This damage causes fluid, rich in protein, to leak into the air sacs — a problem known as pulmonary oedema. Because this isn’t caused by heart failure, it is called non-cardiogenic pulmonary oedema. Common triggers include pneumonia, severe infections (sepsis), major injuries, or inhaling stomach contents (aspiration).In response, the body’s immune system floods the lungs with inflammatory cells, which damage the natural lung coating (surfactant) that normally keeps the air sacs open. Without this, the lungs become stiff, oxygen levels fall, and patients struggle to breathe. This explains why both ALI and ARDS cause similar symptoms: breathlessness, low oxygen, and widespread lung changes seen on scans.⁴

The underlying process in both ALI and ARDS is called diffuse alveolar injury — in other words, widespread damage to the tiny air sacs of the lungs. A trigger such as infection, trauma, or aspiration sets off a chain reaction in the immune system. Special white blood cells (neutrophils) release powerful chemicals, known as cytokines, that increase inflammation. This inflammation makes the lung’s blood vessels leaky, allowing protein-rich fluid to enter the air sacs. As a result, the lungs fill with fluid rather than air. At the same time, the natural surfactant that normally keeps the alveoli open is destroyed, leading to collapse of these air spaces (atelectasis). This combination of fluid build-up and collapsed alveoli makes gas exchange extremely inefficient, leaving patients severely short of oxygen.⁵ 

Differences

The key difference between ALI and ARDS lies in severity. ALI was once used to describe the milder form, where oxygen levels were low but not as critical. ARDS referred to the more severe form, with dangerously low oxygen. With the Berlin Definition, ALI is no longer a separate diagnosis, and ARDS is instead divided into mild, moderate, and severe. As the condition worsens, the lungs become stiffer, less able to expand, and harder to ventilate with machines. This “loss of compliance” is most noticeable in moderate and severe ARDS. In practice, the spectrum runs from relatively mild injury (what used to be called ALI) to life-threatening ARDS, all caused by the same underlying process but differing in intensity.⁷ 

Diagnostic criteria

Doctors usually suspect ARDS when a patient develops sudden breathing problems within about a week of a trigger, such as infection, injury, or aspiration. One warning sign is that even when patients are given extra oxygen, their oxygen levels in the blood remain very low. Imaging tests like chest X-rays or CT scans often show hazy “white-out” patches across both lungs, caused by fluid build-up rather than infection alone. A key part of the diagnosis is ruling out heart failure, since heart problems can also cause fluid in the lungs. To do this, doctors may check heart function using blood tests (BNP levels), ultrasound scans of the heart (echocardiography), or careful monitoring of fluid levels. If the underlying cause is not treated quickly, the condition can worsen, moving from a milder stage (previously called ALI) to severe ARDS.⁸ 

Treatment challenges

Recognising the severity of ARDS early makes a real difference because it guides treatment decisions. The mainstay of care is mechanical ventilation — a breathing machine — but doctors now use gentler settings to protect the lungs from further injury. This usually means giving smaller breaths of air (low tidal volume ventilation) and limiting the pressure inside the lungs. Managing fluids is also important; too much fluid can worsen lung swelling, so doctors carefully balance hydration with the need to avoid extra strain on the lungs. At present, there is no single medicine that cures ARDS or consistently improves survival. That said, steroids have shown benefits in some patients, particularly those with ARDS caused by COVID-19, and research is ongoing.¹⁰

The Berlin Definition officially removed ALI as a separate diagnosis, but the concept is still useful. It highlights that ARDS isn’t a fixed condition — it can improve or worsen depending on the patient’s overall health, the treatment given, and how early it is started. This means doctors must watch closely for the first signs of lung injury, since early action can change the course of illness.

There are also historical and research reasons why the term “ALI” is still mentioned. Before 2012, many studies and clinical trials used both ALI and ARDS, which makes older results harder to compare with modern research. Some textbooks and databases also continue to use the older terminology. For students and younger doctors, being able to recognise both terms helps them understand the medical literature and connect past knowledge with current practice.

Risk factors and etiology

Both ARDS and what used to be called ALI share similar risk factors. The most common trigger is sepsis, but other causes include major physical trauma, blood transfusion-related injury (TRALI), severe pancreatitis, and inhalation of harmful substances such as smoke or chemicals. The underlying cause may be the same, but what sets cases apart is not the trigger itself — it’s how much oxygen the patient can maintain and what their lung scans reveal.¹¹ 

Prognosis

The outlook for ARDS depends on how severe it is. Moderate to severe cases carry a higher risk of death, with rates ranging from 35–45%. These patients also tend to spend longer in intensive care and may struggle with breathing difficulties even after leaving hospital. Survivors often face reduced exercise capacity, ongoing lung changes, and sometimes mental health challenges such as anxiety or depression. What was once called ALI tended to have better outcomes but is now considered the milder end of the ARDS spectrum. Simplifying terminology has helped improve accuracy in diagnosis and communication between healthcare teams. However, the term "ALI" is still sometimes used in teaching or informal discussions to describe early or less severe lung injury. This makes it important for doctors and educators to explain clearly what low oxygen levels (hypoxaemia) mean, how severe they are, and how imaging and clinical findings fit into the overall picture.¹²

FAQs

What is the difference between ALI and ARDS?

ALI (Acute Lung Injury) used to describe milder lung injury, while ARDS (Acute Respiratory Distress Syndrome) described more severe cases. Today, doctors usually use ARDS as an umbrella term and classify it as mild, moderate, or severe based on oxygen levels and lung function.

How do doctors know if someone has ARDS?

Doctors look at how well the patient is getting oxygen, perform chest scans to check for fluid or inflammation in the lungs, and rule out heart problems as the cause. The Berlin Definition provides a standard way to classify severity, which helps guide treatment.

What causes ARDS?

The most common cause is sepsis, but it can also happen after major trauma, blood transfusions (TRALI), severe pancreatitis, or inhaling harmful substances. The severity depends on how badly the lungs are affected rather than the trigger itself.

Can ARDS be treated?

Yes. Treatment focuses on supporting breathing, usually with careful use of ventilators and oxygen therapy, and managing fluid levels in the body. There is no single drug that cures ARDS, but evidence-based strategies can help reduce complications and improve recovery.

What is the outlook for someone with ARDS?

The prognosis depends on severity. Mild cases often recover with supportive care, but moderate to severe ARDS has higher risks and may require longer intensive care. Survivors may experience ongoing breathing difficulties or reduced exercise tolerance, so follow-up and rehabilitation are important.

Summary

In summary, ALI and ARDS are not separate diseases but represent different levels of severity in acute lung injury. The Berlin Definition helps doctors classify patients more clearly, ensuring that diagnosis reflects the actual seriousness of the condition. Even though the term ALI is no longer formally used in guidelines, it is still helpful in practice and teaching to describe early or milder cases.

Early recognition and accurate categorisation are key to improving outcomes. Using evidence-based treatments and monitoring patients closely can make a real difference. Understanding the history of these terms also helps healthcare teams communicate more clearly, avoid confusion, and provide better care for people with breathing difficulties.