Pulmonary oedema or fluid in the lungs – what exactly does it mean and what is the cause of it? Fluid in the lungs is life threatening and can lead to death if not treated immediately. We have to distinguish here between fluid in the lungs (pulmonary oedema) and in the the chest (pleural effusion). Both conditions cause dyspnoea but cause and treatment can vary greatly. In this post I want to focus on pulmonary oedema and only briefly discuss pleural effusion in the last paragraph.


Fluid in the lungs – Pulmonary oedema


Underlying conditions



Heart failure



Pleural effusion  

Possible underlying disease¹:

  • Heart failure
  • Poison (e.g. Smoke)
  • Medication
  • Iatrogen (Overinfusion)
  • Loss of protein
  • Infection
  • Electric shock, attacks, head trauma
  • Blocked airways (e.g. foreign body)
  • „Aspiration“ of liquid (e.g. during anesthesia, under water)
  • Lack of oxygen (altitude sickness)

  Heart failure is by far the most common cause for pulmonary oedema. Other causes are rare with liquid build up rather late and in most cases different presentation of the patient than in heart disease.  

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Symptoms when fluid fills the lungs:

  • Dyspnoe
  • Rapid breathing during rest
  • Cough
  • Belly breathing
  • Open mouth breathing (cats)
  • Blue mucous membranes
  • Uneasiness

   When fluid fills the lungs, space normally available for gas exchange gets smaller and therefore breathing gets faster. This will progress into high grade dyspnoea. In the beginning we might notice a raised resting respiratory rate (> 40 breath per minute), belly breathing and panting. Some dogs start to cough, cats normally don’t cough. The animal gets more and more unsettled due to lack of oxygen and dyspnoea. At some point mucous membranes might turn slightly blue (you might check the gums for that). 

With cats the condition very often progresses very fast, mostly because they are very good in hiding it for so long. Especially panting in cats is rarely normal und should be taken very seriously. Dogs might show signs a little bit earlier. Often they have decreased appetite, cough and are more tired. But they as well can have a very acute and fast course.

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Diagnosis of pulmonary oedema

Patients with pulmonary oedema usually present themselves with dyspnoea. Because of that, they should be stressed as little as possible and the correct therapy should be started as quickly as possible. To diagnose pulmonary oedema x-rays of the chest are needed, which should be done fast and with no unnecessary stress. Sometimes, especially in cats, even X-Rays are too stressful and oxygen is needed to stabilised the animal, if possible in an oxygen cage. Once the patient has calmed down a little, X-rays can be obtained and an appropriate therapy started. Heart disease is the most common cause for pulmonary oedema. Other causes are rare, which is why I am only discussing heart problems in the following paragraphs. 

A heart scan will quickly tell us if and which heart problem is present. Unfortunately we cannot  scan the lungs as ultrasound beams get reflected completely when directed at air. Heart scans are performed once the patient is stable enough. There is a biomarker- quick- test available for cats which can eliminate or confirm heart disease when dyspnoea is present. The test can’t replace a scan, but is still a valid tool for making quick decisions, especially if no cardiologist is present. 


What happens in heart failure?

Once a heart disease has become so severe that the heart or body is not able to compensate the lack of heart function any longer we call the condition heart failure. The disease is most likely present since a long time but without obvious symptoms.

What happens exactly:

In a healthy heart-lung-circulation blood is pumped from the right heart into the lungs, loaded with oxygen and then back to the left heart. There, the strong left heart muscle pumps blood into the aorta and the whole body. Gas is exchanged in alveoli, little bubbles in the lungs, which are surrounded by vessels and filled with air. In heart disease it is possible that not all blood is pumped into the body anymore. We call it left heart failure. Left heart failure happens in many heart diseases like e.g. Mitral valve disease, Hypertrophic cardiomyopathy, Dilated cardiomyopathy and also in some congenital diseases. The left atrium dilates as a consequence of not enough blood being pumped forward. The body thinks there is not enough blood anymore and reacts by increasing total blood volume. This is a vicious cycle and the situation in the heart even gets worse.  At this stage however, no symptoms are usually present and gas exchange in the lungs is not impaired but dogs may start to cough due to the larger heart pressing against the bronchi. At some point the left atrium cannot dilate any further and blood gets backed up into the vessels of the lungs. Like a traffic jam thats gets longer and longer. The vessels in the lungs can only withstand a certain amount of pressure until fluid is pressed through the vessel walls into the lung tissue and alveoli. The little bubbles fill more and more with fluid, cannot be filled with air anymore and are not available for gas exchange. Dyspnoea is the consequence.


pulmonary oedema


Treatment of pulmonary oedema

The main treatment goal is to get fluid out of the lungs to ensure the patient can breath easier and will not suffocate. Unfortunately we can’t just tap the fluid and remove it with a syringe, because the fluid is in the tissue and tiny little alveoli. We have to remove the liquid with diuretics like Furosemide or Torasemide. With these medications we intend to „dry out“the body so fluid is taken up by the vessels again and removed from the lungs. Diuretics are very effective, but initially very high doses might be necessary to get over the critical situation. The earlier therapy is started the less aggressive it needs to be. After stabilisation the dose of diuretics will be reduced to find the smallest possible amount. This will happen over days. Depending on the underlying heart disease, other medication will also become necessary or might have already prescribed previously.

Will my pet have to stay in the hospital

It depends on how bad the condition of the dog or cat is when treatment is started. In case of severe dyspnoea it is usually helpful to give drugs intravenously and also supply oxygen for a while. This might be a lot easier in a clinical setting and especially the first 24 hours success rate is higher if animals are admitted.

What comes next

Once the dog or cat has overcome the most critical initial phase, medication needs to be adjusted so good life quality is possible. Daily diuretics are normally necessary, but the goal is to reduce them as much as possible but still give enough to keep the lungs clear. In order to archive this, the owner needs to do his or her homework and count resting respiratory rate regularly. In cases where the highest recommended daily dose of furosemide or torasemide is reached, a second diuretic might become necessary. 

How to count resting respiratory rate:

We count breathes per minute during resting or sleeping periods of our pet. It’s the same as in humans. One lift and lowering of the chest is one breath. In cats and dogs normal rates are between 20-30 and should definitely not go above 40 per minute.  The first sign of fluid accumulating in the lungs is an elevated resting respiratory rate. When the owner is counting regularly, he or she gets a feeling for what is normal and realises a lot quicker if something is not right and can react accordingly. The faster the right medication is started or the dose is increased, the better the chances for success. 





Most animals can be stabilised after being presented with dyspnoea due to heart failure. Unfortunately some come to the hospital too late and the body is too weak to recover. The first 24-48 hours are critical and of course the faster therapy is started the better the chances of survival. After the acute phase is over, the right mix of medication needs to be found and adjusted whenever necessary. The most common heart disease in dogs – mitral valve disease –  has a mean survival time after the first time with pulmonary oedema of 268 days.  With other diseases it is also more or less a year in average, apart from a few diseases with shorter survival times. But every animal is different and some live a lot longer, some live not as long as the statistic tells us. The most important thing is life quality during this time, which is normally quite good and therefore it is definitely worth to start treatment. 


Fluid in the chest – pleural effusion

Possible causes :

  • Heart failure (Right heart failure, pericardial effusion, heart worms, left heart failure in cats)
  • Tumor
  • Liver disease
  • Infektion (e.g. FIP, Leishmaniosis)
  • Bleeding (e.g. accident)
  • Loss of protein

Difference between pleural effusion and pulmonary oedema:

There is a small gap between chest wall and lungs with only little fluid to make sliding against each other smooth. In this gab fluid can accumulate e.g. due to right heart failure. With ultrasound it is very easy to see this fluid because there is no air between ultrasound probe and fluid like there is in pulmonary oedema. You might compare it to a sponge in a plastic bag. We can place the plastic bag with the sponge in a glas with the glas representing the chest, the sponge the lungs. If we fill the glas with water, we have pleural effusion. The water can be seen. If we fill the plastic bag with water, the sponge will absorb it and we can’t see it anymore, because it is in the tissue just like in pulmonary oedema. 

Cats are special:

Cats can develop pleural effusion, pulmonary oedema or both in left heart failure. We do not exactly know what will develop beforehand but the presence of pleural effusion is considered to have a worse prognosis. In my experience however, those cats can also live for quite some time if treated properly.


Therapy depends on the cause. In most cases it’s useful to tap the fluid and release pressure in the chest to help the patient breath easier. This is a simple procedure without anesthesia. Unfortunately tapping only gives us time and does not treat the underlying disease. In case of heart failure, treatment is usually similar to pulmonary oedema and consists of diuretics and maybe some other drugs. 


If you have questions or comments regarding this topic, please contact me via Mail or my contact page. You can also leave a comment in the comment section. 


  1. Canine and Feline Cardiology: Principles and Clinical Practice, by Philip R. FoxDavid Sisson, et al., Kapitel 13, Pages 251-264
  2.  ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs.Keene BW et al. Journal of Veterinary Internal Medicine. (2019)
  3. Porciello F et al. (2016). Sleeping and resting respiratory rates in dogs and cats with medically-controlled left-sided congestive heart failure. Veterinary Journal 2016 Jan;207:164-168.
  4. Amelie Beaumier et al.,(2018). Clinical findings and survival time in dogs with advanced heart failure. Journal of Veterinary Internal Medicine, Volume 32, Issue 3, Pages 944-950.
  5. Rush et al. (2002). Population and survival characteristics of cats with hypertrophic cardiomyopathy: 260 cases (1990-1999). J Am Vet Med Assoc. 2002 Jan 15;220(2):202-7.
  6. M. Johns et al.(2012), Left Atrial Function in Cats with Left‐Sided Cardiac Disease and Pleural Effusion or Pulmonary Edema. Journal of Veterinary Internal Medicine,Volume 26, Issue 5