Fluid on the lungs, known medically as pulmonary oedema, is most commonly caused by heart problems. In the majority of cases, the underlying issue is heart failure, where the left side of the heart can no longer pump blood efficiently. But fluid can also collect in the lungs from pneumonia, severe infections, kidney disease, chest injuries, certain medications, and even travelling to high altitudes.
How Heart Problems Cause Fluid Buildup
The most common route to fluid on the lungs starts with the heart. When the left lower chamber of the heart (the left ventricle) is weakened or diseased, it struggles to pump out all the blood it receives from the lungs. This creates a backlog. Pressure builds in the blood vessels running through the lungs, and eventually that pressure forces fluid through the thin walls of those blood vessels into the tiny air sacs where oxygen exchange happens. Once those air sacs fill with fluid, breathing becomes increasingly difficult.
Several heart conditions can trigger this chain of events:
- Coronary artery disease: fatty deposits gradually narrow the arteries supplying the heart muscle, weakening the left ventricle over time. A blood clot in one of these narrowed arteries can cause a sudden worsening.
- Cardiomyopathy: direct damage to the heart muscle forces the heart to work harder. It may cope at rest but fail to keep up during exercise, infection, or a spike in blood pressure, allowing fluid to back up into the lungs.
- Heart valve problems: a narrowed or leaking valve disrupts normal blood flow through the heart. A valve that suddenly starts leaking can cause severe pulmonary oedema very quickly.
Heart-related fluid on the lungs is by far the most common type. Doctors call it “cardiogenic” pulmonary oedema, and it often develops gradually alongside worsening heart failure, though it can also come on suddenly after a heart attack or a valve problem.
Causes Not Related to the Heart
When the heart is working normally but fluid still accumulates in the lungs, the problem lies elsewhere. In these cases, the thin membrane separating the air sacs from the surrounding blood vessels becomes damaged or leaky, allowing fluid to seep through.
The most significant non-cardiac cause is acute respiratory distress syndrome (ARDS), a severe lung reaction that can develop after major infections, sepsis, or trauma. Pneumonia is another common culprit, though it works differently. With pneumonia, an infection (bacterial, viral, or fungal) causes infected fluid to fill the air sacs. With pulmonary oedema, the fluid is typically thinner and watery, and no infection is driving it.
Other non-cardiac causes include:
- Kidney failure: when the kidneys can no longer remove enough fluid from the body, the excess fluid can accumulate in the lungs.
- Smoke inhalation: chemicals in fire smoke damage the membrane between the air sacs and blood vessels, allowing fluid to leak through.
- Drug reactions: opioid overdose and aspirin toxicity are among the medications and substances that can trigger fluid buildup.
- Severe viral infections: certain viruses, including hantavirus and dengue, can cause pulmonary oedema directly.
- Chest trauma: physical injury to the chest wall can lead to fluid accumulation.
- Blood transfusions: in rare cases, transfusions cause fluid overload or a reaction called transfusion-related acute lung injury.
Sometimes both a heart problem and a non-cardiac cause contribute at the same time, which can make diagnosis more complex.
High Altitude as a Trigger
High-altitude pulmonary oedema (HAPE) is a distinct form that affects otherwise healthy people. It occurs at elevations above about 2,500 metres (roughly 8,200 feet) when someone ascends too quickly for their body to adjust to lower oxygen levels. The biggest risk factor is the speed of ascent, regardless of fitness level. Cold temperatures and respiratory infections increase the risk further.
HAPE can also affect people who normally live at high altitude but spend time at lower elevations. When they return home without allowing time to re-acclimatise, the same fluid buildup can occur. Air travel counts too. Flying directly to a high-altitude destination means your body still needs time to adjust, even though you arrived by plane rather than on foot.
Symptoms That Signal Fluid on the Lungs
Pulmonary oedema can develop gradually or come on within hours. The hallmark symptoms are breathlessness and difficulty breathing, especially when lying flat. Many people find they need to sit upright or prop themselves up with pillows to breathe comfortably at night. A persistent cough, sometimes producing frothy or pink-tinged sputum, is another characteristic sign.
Rapid-onset pulmonary oedema is a medical emergency. Sudden, severe breathlessness, a feeling of suffocating or drowning, wheezing, rapid heartbeat, and cold, clammy skin all warrant calling 999 immediately. In the slower-developing form linked to chronic heart failure, you might notice increasing breathlessness during activity, swollen ankles, weight gain from fluid retention, and waking up at night gasping for air.
How It Is Diagnosed
In hospital, doctors use a combination of tools to confirm fluid on the lungs and identify the cause. A chest X-ray is often the first step, as it can show fluid in and around the air sacs. Blood oxygen levels are measured using a small clip on your finger (pulse oximetry) or a blood gas test. An ECG checks for heart rhythm problems or signs of a heart attack. Blood tests can help distinguish whether the cause is heart-related or something else, and an echocardiogram (an ultrasound of the heart) shows how well the heart is pumping and whether the valves are working properly.
How Fluid on the Lungs Is Treated
Treatment depends on the severity and the underlying cause, but the immediate priority is always the same: improving oxygen levels and removing excess fluid.
Oxygen is the first line of treatment. This might be delivered through a face mask or nasal tubes. If breathing remains very laboured, a machine called CPAP (continuous positive airway pressure) can help keep the airways open and push oxygen into the lungs more effectively. In severe cases, a ventilator may be needed.
Diuretics, commonly called water tablets, are given to help the kidneys remove excess fluid from the body. In an acute episode, these are given directly into a vein so they work faster. The goal is to reduce the volume of fluid putting pressure on the lungs. Medicines to widen blood vessels and reduce the workload on the heart are often used alongside diuretics. If heart failure is the root cause, medications to strengthen the heart’s pumping action may be added.
For high-altitude pulmonary oedema, the most effective treatment is descending to a lower altitude as quickly as possible. Supplemental oxygen helps while descent is being arranged.
Recovery and Long-Term Outlook
An acute episode of pulmonary oedema typically requires a hospital stay. How long depends on the cause and how quickly you respond to treatment. Someone admitted with heart failure-related fluid on the lungs might spend several days in hospital while doctors adjust medications and stabilise the underlying condition.
The long-term outlook varies significantly depending on what caused the fluid buildup. If the trigger was a one-off event, like altitude sickness or a treatable infection, full recovery is common. When heart failure is the underlying cause, treatment focuses on managing the condition to prevent further episodes. This usually involves ongoing medication, dietary changes (particularly reducing salt intake), and regular monitoring. Each episode of acute pulmonary oedema can signal that the heart condition has worsened, so identifying and addressing the root cause is essential for reducing the risk of recurrence.