There are two main types of 'fluid in the lung' - pulmonary oedema and a pleural effusion. Technically, a pleural effusion is fluid just outside the lung, in the pleural space (between the lung and the ribcage). Let's talk about pulmonary oedema as it literally means "lung fluid".

Pulmonary oedema is the medical term for fluid sitting in the lung's airways. Most commonly, if you have oedema in one lung, you'll have oedema in the other also. Pulmonary oedema is typically a whole-body, fluid management issue. Usually, there is a problem with another organ's functioning - and the excess fluid settles in the airways. 

The most common cause of pulmonary oedema is heart failure, specifically the left-hand side. The normal route for the re-oxygenation of blood is this - right atrium collects all the oxygen-depleted blood, moves into the right ventricle before it is shot across to the lungs. Here it collects the oxygen sitting in the alveoli, before returning to the heart's left atrium and then into the left ventricle to be shot around the body. 
- If the left ventricle isn't pumping effectively, you can have a back up of fluid (like bad traffic) into the left atrium, and subsequently into the lungs vessels. With all the extra fluid, the lungs' vessels can't hold it and some spills over into the alveoli (airway). Now, you have fluid in your lungs! 

Other causes for pulmonary oedema include kidney failure, liver failure, low blood protein (albumin) - basically, when the body's fluid management system is awry. 

Pulmonary oedema will render the patient short of breath, as technically they're partially 'drowning! We treat the patient by supporting their breathing with oxygen while correcting the underlying cause. Usually, this involves reducing the amount of fluid in the body through restriction of intake and diuresis (giving medication to make you pee more than a dog on a walk). 

A pleural effusion is a collection of fluid in the pleural space. The pleural space is the gap between the outside of the lung and the chest wall, bordered by the diaphragm below. Pleural effusions can be on both sides, but very often it will only affect one lung. The pleura is like a membrane that surrounds the pleural space, if it is irritated, fluid can be produced - usually a straw-yellow colour. 

If you have a pleural effusion on both sides, it's very likely that you also will have pulmonary oedema. It's indicative of a fluid management problem, and the excess fluid has found a home in both the lung and the pleural space. These effusions will usually go away when the underlying problem is treated. 

A pleural effusion on one side is most commonly caused by a lung infection (such as a pneumonia) that causes irritation of the pleura. If it is small, the doctor will treat the underlying infection and the body will naturally resorb the pleural fluid over time. 
If the cause of the effusion is unknown, or the effusion is large enough to compress the lung and cause shortness of breath, a sample will need to be taken for testing. If the fluid is causing symptoms, we will simply treat the effusion and send a portion of the fluid for testing. 

Treatment of a pleural effusion sounds rather medieval, but often the simple ways are the best! A chest drain (see-through tube) will be placed between the ribs (usually near the side of your chest or on your back) and lowered into the fluid. This is quite a simple procedure which uses local anaesthetic, and can therefore be done at the bedside by an experienced doctor. With one side of the tube in the fluid, the other side will be fed into a bucket with a small amount of water at the bottom. The water aids the pressure difference and encourages fluid to leave the chest without any going in! This is known as a water seal. A chest drain will often be fitted with a three-way tap, allowing us to "clamp" the chest drain when needed. Once the fluid is adequately removed and your breathing restored, the chest drain can be removed.

The other causes of a pleural effusion include autoimmune diseases, cancer, and a blood clot. The fluid is sent to the lab to be cultured, to see if we grow any bacteria. Cytology is also performed - checking the cells for evidence of cancer - as well as a plethora of biochemistry testing that aids the doctors in making a diagnosis.