Fluid around the lungs in dogs – pleural effusion

Overview

The fluid is around the lungs

Pleural effusion means abnormal fluid accumulation in the pleural space. This differs from pulmonary oedema, where fluid accumulates in lung tissue and air sacs. Fluid location guides investigation and treatment.

Chest fluid can be blood, clear fluid, lymphatic fluid or pus. Different fluid types can cause similar breathing difficulty. Signs alone therefore do not establish the cause.

A case group studying effusions in dogs included 103 pleural effusions. Cancer was the cause in 33 dogs and increased vascular permeability in 25. Cases were selected when the cause could be established or was strongly suspected. This does not describe disease risk for all dogs.

Causes

Effects of pressure and proteins

Increased pressure in blood vessels can push fluid into the pleural space. In the study above, right-sided heart failure explained 8/21, or 38%, of canine pleural effusions caused by increased pressure. Reduced blood proteins are another mechanism. Among pleural effusions associated with low oncotic pressure, protein-losing enteropathy was found in 10/13 dogs. Severe liver disease underlay 2/13 cases, or 15%, in the same group.

Inflammation, tumours and bleeding

Inflammation can increase blood vessel permeability. In pyothorax, pus-filled infected fluid accumulates in the pleural space. A penetrating injury, plant foreign body or lung abscess can underlie it.

Tumours are another important cause group. Among canine cancer-related pleural effusions, lymphoma was found in 10/33 dogs, or 30%. Lung adenocarcinoma was found in 7/33, or 21%, in the same study group. Blood can accumulate because of injury or a clotting disorder. Lung lobe torsion can also cause bloody effusion.

Impaired lymphatic circulation

In chylothorax, fat-rich lymphatic fluid leaks into the chest cavity. The thoracic duct normally carries lymph towards the bloodstream. If outflow is blocked, lymph can back up and leak into the chest. Dietary fats are broken into small particles that enter intestinal lymph vessels. A lymph-collecting reservoir lies towards the front of the abdominal cavity near the kidneys. It also receives fluid from elsewhere in the abdomen and the hindlimbs. Sometimes no explanatory disease is found and the condition is called idiopathic.

Risk factors

Effusion risk relates to the disease causing it. Chest injuries and clotting disorders predispose to bloody fluid. Fungal disease can also disrupt lymph flow in the chest. Heart disease can involve clear fluid accumulation. Diseases causing low blood proteins are another possible background.

Lymphatic fluid accumulation has been associated with conditions including tumours, heart disease and clots in major veins. Bites and other penetrating injuries are considered in suspected pyothorax.

Of the 103 dogs in the case study, 21 were in the increased vascular pressure group and 13 in the low oncotic pressure group. Such underlying diseases are investigated individually.

Typical symptoms

Fluid type cannot be identified from the breathing pattern. Possible signs at home include:

• Rapid, shallow breathing.
• Laboured inhalation.
• Weakness.
• A dry cough.
• Reduced appetite.
• Greater tiredness than usual.
• Possible weight loss with prolonged chylothorax.

A healthy dog's usual breathing rate is around 12–30 breaths per minute. At rest, breathing is generally slower and deeper. Pus-forming infection can also involve fever and pain.

Pleural disease can cause asynchronous chest and abdominal movement. The chest may expand while the abdomen moves inwards. Clearly worsening breathing is more urgent than an isolated breathing rate reading.

Complications and consequences

The lungs cannot expand normally

Fluid accumulation can prevent the lungs filling with air. During respiratory distress, oxygen supply is inadequate for the body's needs. Respiratory exhaustion may require mechanical ventilation.

Prolonged lymph loss can deprive the body of protein, white blood cells and vitamins. It can weaken immune defences and cause metabolic disturbances. Chronic lymph exposure can inflame the pleura and pericardium.

The outlook depends on the disease causing the fluid. Many diseases causing chest effusion have a guarded or poor outlook. In chylothorax, chronic pleural inflammation worsens the prospect of clinical recovery. Conservative care, meaning drainage and other non-surgical treatment, rarely resolves idiopathic chylothorax.

Diagnosis at the vet

First assessing breathing safety

Your vet assesses the breathing pattern and listens to the chest. Fluid can muffle heart and breath sounds. X-rays are taken only once the patient can tolerate them. A stressful imaging procedure can rapidly worsen a patient with breathing difficulty. Imaging must not delay essential treatment.

Ultrasound can identify pleural fluid. Some dogs tolerate it better than X-rays. Draining fluid can ease breathing and aid diagnosis.

Examining fluid contents and cells

Pleural fluid can undergo cytology. Assessment includes protein concentration, cell count and cell types. Bacteria or tumour cells may be identified microscopically. If infection is suspected, both aerobic and anaerobic cultures are performed.

Fluid low in protein and cells is called a transudate. An exudate, by contrast, is generally high in protein and cells. Intermediate fluid is called a modified transudate. Categories guide investigation, but their defining thresholds vary.

In one study, 11/14 canine pleural samples classified as transudates, or 79%, were associated with low oncotic pressure. Of exudates, 22/42, or 52%, were cancer-related. In the same group, 15/42, or 36%, involved increased vascular permeability. Of modified transudates, 8/24, or 33%, involved increased vascular pressure. Cancer was found in 7/24 samples, or 29%, in the same group. Underlying diagnoses should not be excluded on classification alone.

Inflammatory canine pleural effusions had a higher proportion of neutrophils, inflammatory cells, than other cause groups. Effusions from low oncotic pressure had lower protein concentrations and nucleated cell counts than other groups. Interpretation therefore combines these findings with your dog's other investigations.

Confirming lymphatic fluid and finding the cause

Milky fluid raises suspicion of chylothorax. Both fluid and blood samples can be submitted for confirmation. Comparing blood and fluid triglycerides helps establish fluid type.

X-rays show fluid but do not confirm chylothorax. Heart ultrasound looks for cardiac disease. Chest ultrasound or CT can help look for a tumour. A heart murmur is one finding affecting assessment of heart disease.

Sample quality affects laboratory cytology. A sample for automated cell counting must not be clotted or lumpy. Bacterial culture needs its own sample alongside the cell-count sample.

Treatment

Draining fluid eases breathing

Your vet removes pleural fluid impairing breathing by a chest tap. If repeated drainage is needed within minutes or hours, a drain may be placed. It allows continuous suction. A dog with a chest drain needs hospital care.

Oxygen may be needed to support breathing. Severe respiratory exhaustion can require intubation and mechanical breathing support. Drainage aims to let the lungs expand and relieve breathlessness.

Further treatment depends on the fluid's cause

Your vet plans further care according to the underlying diagnosis. Pyothorax treatment combines pus drainage and antimicrobials. Some pyothorax cases need surgical debridement. With bloody fluid, clotting disorders are assessed and treated.

Idiopathic chylothorax is often treated surgically. The most common procedure is thoracic duct ligation. This aims to redirect lymph through new routes into the bloodstream. A pericardial procedure can be combined with it. Another surgical option is ablation of the abdominal lymph reservoir. The aim is to establish alternative lymphatic routes into the bloodstream. This can relieve thoracic duct pressure. In some cases, surgery can be performed with endoscopic assistance.

Diet and monitoring after surgery

Low-fat food may be used for chylothorax. However, diet alone generally does not reduce lymph flow sufficiently. The effectiveness of the supplement rutin in treating chylothorax has not been demonstrated.

After chylothorax surgery, your dog may need intensive care, pain relief and drainage through a chest drain. Discharge is possible when fluid production stops or falls enough for drain removal. In successful cases, chyle leakage stops over days or weeks.

Success rates of 80–100% have been reported in dogs for combined thoracic duct ligation and a pericardial procedure. This figure concerns surgical chylothorax treatment. Metabolic disturbances in chylothorax patients can increase anaesthetic risk. Nerves controlling diaphragm function can also be injured during surgery. Possible surgical problems include bleeding, infection and continued effusion.

Prevention

Prevention depends on the underlying cause. Bloody effusion associated with injury and clotting disorders needs different assessment from chyle leakage. Managing identified heart disease and diseases causing low blood proteins forms part of care.

In idiopathic chylothorax, an identifiable source is often not found. No single home measure can therefore be named to prevent every pleural effusion. New breathing difficulty after a previous effusion is assessed promptly. If fluid recurs, its cause is investigated again rather than automatically assuming continuation of the previous diagnosis.

Important for dog owners

Watch breathing effort and establish the follow-up plan

Keep a dog with breathing difficulty calm during transport. A cool car reduces heat-induced panting. If your dog cannot stand, transport them lying on their breastbone with the chin supported and neck extended. Phone the emergency vet beforehand if possible.

At home, record resting breathing rate, breathing effort, cough, appetite and activity. Establish what is known about the fluid and which investigations remain pending. Fluid visible on an X-ray does not alone establish its precise type.

Before discharge, ask how any recurrent fluid will be managed. After chylothorax surgery, another chest tap may be performed if signs return. Keep test results and agreed follow-up dates. Rapidly worsening breathing or collapse means emergency care is needed again.

Frequently asked questions

Is fluid in the chest the same as pulmonary oedema?

The fluid is in a different location. In pleural effusion, it is in the pleural space. In pulmonary oedema, fluid accumulates between lung tissues and in air sacs. Regardless of fluid location, your dog may need prompt help with breathing difficulty. Your vet assesses location using the breathing pattern, chest sounds and imaging.

Does milky fluid definitely mean chylothorax?

Milky fluid raises strong suspicion of chylothorax. Blood can be tested alongside the fluid to confirm it. Underlying causes such as heart disease or a tumour are also sought. An X-ray cannot establish that the fluid is specifically lymphatic. Appearance is therefore a starting point for investigation.

Does fluid with many cells always mean bacterial infection?

No. In one study, 22/42 canine pleural exudates, or 52%, were cancer-related. Cell classification helps narrow the possibilities, but underlying diseases are not excluded on this basis. Fluid can be examined for both bacteria and tumour cells.

Can fluid need draining many times?

Yes. If drainage is needed every few minutes or hours, your vet can place a chest drain allowing continuous suction. A drain requires hospital care. Removing fluid aims to improve lung expansion and breathing. The underlying cause is investigated at the same time so further treatment can be targeted.

Does low-fat food help chylothorax?

Low-fat food can be used as part of treatment. However, it generally does not reduce thoracic duct lymph flow sufficiently on its own. Idiopathic chylothorax is often treated surgically. The supplement rutin has no demonstrated effectiveness for this disease. Diet is therefore selected as part of the wider treatment plan.