Better Out Than In: Thoracocentesis for Pleural Effusion

Josh Cruz, DVM, explains the emergency presentation of veterinary patients with pleural effusion, and walks the reader step by step through performing life-saving thoracocentesis on these patients before continuing with diagnostics.

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The presence of pleural effusion and subsequent presentation of the dyspneic patient is a common emergency in both the dog and cat. Unfortunately, the onset of clinical symptoms for pleural effusion is often so severe and acute that they prevent a complete diagnostic evaluation prior to intervention. The thoracocentesis is the primary method of therapy for patients exhibiting dyspnea due to pleural effusion. Performing a thoracocentesis and finding evidence of effusion, however, may not be definitive for the underlying disease process. The thoracocentesis in the emergent patient is therefore aimed at providing comfort, time and a method of guiding clients towards making the best next decision for their pets. 

VD X-Ray of feline pleural effusion


Physical examination can often differentiate the cause for the patient’s dyspnea until more time is available to get more information. While dyspnea and tachypnea are noted by the owner, the clinician usually appreciates a restrictive breathing pattern, with dull, muffled or absent heart and lung sounds. This is noted primarily as exaggerated inspiration, with a shortened, fast expiration. Often, a fast abdominal component is recognized as well.  Jugular distention, lymphadenopathy, epistaxis/hemopytsis, decreased cranial thoracic compliance, uveitis, and superficial injuries/rib fractures may also be useful findings to guide the clinician.   

While even a cursory physical examination may already have you thinking of pleural effusion, we are often tentative about the next diagnostic step in such severely debilitated patients. Initial therapy, however, can provide substantial diagnostic benefits as well as much needed comfort for your patient. Oxygen therapy as flow-by, mask or oxygen cage can alleviate symptoms to some degree. Partial mu-agonist narcotics like butorphanol are often helpful for their safe, sedative effects. Conversely, full mu-agonists like hydromorphone can themselves lead to tachypnea, making evaluation of dyspnea that much more difficult. Placement of an intravenous catheter is usually possible in these patients and should be considered prior to thoracocentesis. However, even catheterization can be too stressful in some patients, so this decision should be made on a case-by-case basis. 

Our inherent and justified desire to “do no harm” often prevents us from providing the most effective and immediate therapeutic intervention for pleural effusion: the thoracocentesis. Complications associated with thoracocentesis include hemorrhage, pneumothorax, cardiac arrhythmias/trauma, pain and even death. While these complications are possibilities, they should not prevent you from performing this technique. Preparing patients with sedation, oxygen and catheterization, and taking precautions with cage-side ultrasonography if possible, will hopefully minimize these risks. 

ultrasound guided thoracocentesisBedside ultrasonography is quite possibly the easiest and most reliable technique for evaluation of pleural effusion. If this approach is available, use it!  Even the worst ultrasound machine can provide more information at less cost to the patient than some radiographs. If ultrasound is not available, and your suspicion is strong for pleural effusion, stick a needle in it anyway! Even a blind, negative thoracocentesis, if done safely, rarely causes significant complications and still provides valuable information. 


Supplies: clippers, aseptic prep, gloves, a needle of your choice (butterfly catheter/needles/catheter), an extension set, a three-way stop cock, an appropriate syringe, a measuring container, cytology tubes (EDTA, red top), and help (this is often a 2-3 person procedure)

Step 1 – Shave and prep area. Connect needle of choice, extension set, three-way stop cock and syringe. Have measuring container and tubes close by. 

Step 2 - Aim low between 7th and 9th cranial rib on right side. If using ultrasound, simply aim where there is the most amount of fluid. Injecting a bleb of lidocaine subcutaneously may be useful for therapeutic thoracocentesis, but is not always necessary. 

Step 3 – It’s a personal choice, but I usually start by inserting the needle bevel side up.  This helps remind you where the needle opening is and where to guide it to get as much fluid as possible. Aiming the needle sternally and along the body wall once you have penetrated into pleural space will maximize drainage and prevent trauma. 

Step 4 – Pull, save, quantify and qualify fluid. Slow and steady wins the race. Often, excessive negative pressure can prevent easy aspiration of fluid, and can risk more trauma. Small clots of cells or tissue often cause obstruction of the needle. Small, gentle pulsing of fluid back into the patient can dislodge obstructions, allowing for continued aspiration. 

Step 5 - Reassess the patient throughout for pain, progressive dyspnea, collapse. Allow negative pressure and positive flow of fluid to guide repositioning of needle. If you feel lung/heart/other tissue, discontinue. If your syringe suddenly aspirates blood, discontinue. If there is no fluid, consider repeating on the other side, or consider more dorsal thoracocentesis. (If no ultrasound is available, ask yourself if you have ruled out pneumothorax.) Once negative pressure is achieved in spite of repositioning and you feel confident you maximized fluid removal (or have enough fluid for sampling), discontinue. 


Hopefully, by this time, your patient is back in oxygen and breathing more comfortably, and you have a few tubes filled with fluid. While you decide what to do with these tubes, also remember to reassess your patient for improvement. Often this seems to be the better time to get a complete history, measure temperature, take blood pressure, and pulse oximetry (all the things the patient did not have time for earlier). 

As the hours go by, you will have the time to wean your patient off of oxygen, evaluate fluid in-house, prepare for pathologic review, sample blood, and perform radiographs and other diagnostics as indicated. In other words, now the hard part starts: finding the primary problem, communicating with the owner, and starting a therapeutic plan. While it’s true that these patients often present in the worst of shape, your ability to provide almost immediate relief improves as you become adept with thoracocentesis. Hence, I always find myself saying, “Better out than in.”


Fluid analysis and cytology

Effusions are usually separated into three categories: 

Pleural  Effusion

Protein concentration

Nucleated Cell Count

Pure transudate



Modified transudate






  • Pure transudates are most commonly indicative of hypoalbuminemia.
  • Modified transudates are most commonly indicative of heart failure (right-sided), diaphragmatic hernia, neoplasia and lung lobe torsions. 
  • Exudates are further classified as non-septic, septic, hemorrhagic or chylous effusions.       

While in-house evaluation of cells can often be diagnostic, a pathologic review often provides the more definitive answer, specifically when modified transudates or non-septic exudates are appreciated. 

Blood work

Blood work often plays second chair to fluid analysis and cytology in these cases, but it can often provide confirmation. CBC, chemistry, retroviral screening, heart worm testing, coagulation parameters, triglyceride levels, and other infectious disease screening should be considered. 


Commonly, radiography is the first diagnostic step that confirms pleural effusion and the need for the thoracocentesis in the first place. It can be argued, however, that radiography after the thoracocentesis is more valuable. When the effusion is gone, the cardiac silhouette, pulmonary structures, vasculature and patterns, and possible mass effects become visible. 

Thorough thoracic ultrasonography and echocardiography can also be very useful for ruling out cardiac disease and neoplasia.

Computed tomography (CT) can be helpful in ruling out penetrating injury, foreign material (grass awns, etc), lung lobe torsions and neoplasia. The downside is obvious: the need for anesthesia. In a dyspneic patient, this approach is rarely performed in emergency settings. 

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