Fluid on the heart
What is pericardial effusion?
The heart sits inside a thin sac called the pericardium, which normally contains only a small amount of fluid. Pericardial effusion is an abnormal build-up of fluid within this sac.
As pressure increases, the heart cannot fill normally between beats. This reduces the amount of blood pumped around the body and can cause obstructive shock, known as cardiac tamponade. Fluid may accumulate rapidly or gradually, so signs can appear suddenly even when the problem has been developing for some time.
What causes it?
Pericardial effusion is much more common in dogs than cats. In dogs, the fluid is often blood and the most common broad causes are cancer and idiopathic pericardial effusion, where no underlying cause is identified.
- Heart tumours, including haemangiosarcoma and heart-base tumours
- Idiopathic inflammation or bleeding, with no mass identified
- Anticoagulant rodenticide toxicity or another clotting disorder
- Less commonly, infection, trauma, congenital disease or rupture of a heart chamber
- In cats, severe heart disease, cancer, feline infectious peritonitis and inflammation are recognised causes
What signs might I see?
- Sudden weakness, wobbliness, collapse or inability to stand
- Pale gums, cold limbs or profound lethargy
- Rapid breathing, breathlessness or reduced exercise tolerance
- A fast heart rate or faint, muffled heart sounds
- A swollen or pot-bellied abdomen caused by fluid accumulation
- Reduced appetite, weight loss or gradually slowing down
- Intermittent episodes of weakness that appear to improve and then recur
How is it diagnosed?
A rapid point-of-care ultrasound is the fastest and most reliable way to identify fluid around the heart and assess whether it is restricting heart filling. Initial stabilisation may need to begin before a complete investigation is possible.
Further tests may include:
- Physical examination, blood pressure, electrocardiogram and blood oxygen monitoring
- Blood tests to assess anaemia, organ function and other consequences of shock
- Clotting tests when anticoagulant rodenticide exposure or another bleeding disorder is possible
- Chest radiographs and abdominal ultrasound to look for other disease or evidence of cancer
- A detailed cardiac ultrasound (echocardiogram), often performed by a specialist or a CT scan
- Analysis of the drained fluid
How is it treated in an emergency?
When the fluid is causing cardiac tamponade, the priority is to relieve pressure around the heart. Oxygen, low-stress handling, intravenous access and carefully selected circulatory support may be used while the procedure is prepared.
- Pericardiocentesis: a needle or catheter is guided into the pericardial sac, usually with ultrasound, to drain the fluid
- Continuous heart-rhythm and blood-pressure monitoring during and after drainage
- Oxygen and supportive treatment for shock
- Vitamin K1 and/or blood products if a clotting disorder or significant blood loss is present
- Treatment directed at the underlying cause once the patient is stable
Improvement after pericardiocentesis is often rapid because the heart can fill and pump more effectively. The procedure treats the immediate crisis but does not necessarily prevent the fluid from returning.
What happens after stabilisation?
Your pet is usually monitored in hospital for recurrence, abnormal heart rhythms and ongoing shock while additional diagnostics are performed after stabilisation. The next steps depend on the suspected cause and may include specialist echocardiography, repeat imaging, oncology consultation or surgery. Once your pet is stable enough to travel, referral for a comprehensive echocardiogram may be recommended.
At QVS, some internal medicine clinicians can perform echocardiography; however, referral to a dedicated veterinary cardiology clinic may be advised for specialist assessment and ongoing management.
What is the outlook?
The prognosis depends almost entirely on the underlying cause. Dogs with idiopathic pericardial effusion can have a good long-term outcome, particularly when the fluid does not recur or is successfully managed surgically.
The prognosis is guarded to poor when the effusion is caused by haemangiosarcoma, while some heart-base tumours progress more slowly and may be managed for longer periods. In cats, outlook varies according to the underlying heart disease, infection or cancer.
A mass not being visible on the first ultrasound is reassuring but does not completely exclude cancer. Your veterinary team may recommend repeat imaging and will discuss the most likely cause, treatment options and expected quality of life for your pet.
Clinically reviewed and approved by Dr Giulia Agostini, Pet Emergency Senior Veterinarian and Clinical Manager, DVM MANZCVS (ECC) August 2026