Introduction
An increased quantity of fluid in the abdominal or thoracic cavity is an indication of a pathological process and analysis of the fluid is a quick, easy, inexpensive and relatively safe way to obtain useful information for diagnosis, prognosis and treatment of disease.
Abdominocentesis
There are several techniques described. The most common site of needle insertion is 1-2 cm caudal to the umbilicus, avoiding the falciform fat. Ultrasound guidance may be required if the fluid accumulation is localised or only a small volume is present.
- First empty the urinary bladder to avoid accidental cystocentesis. Clip and scrub the site of needle insertion (1-2 cm caudal to the umbilicus) as for any aseptic surgical procedure. Usually neither local nor general anaesthesia is required although adequate restraint is needed
- The patient is restrained in lateral recumbency or standing. Insert a 21-23g, 1-1½” needle or an over-the-needle 20 g, 1¼” plastic catheter (dog) or a 23g ¾” needle (cat) in the ventral mid-line. When in lateral recumbency the needle is positioned 1cm below mid-line. Note that if there is a previous surgical incision the needle should be inserted at least 1.5 cm away to avoid adhesions between the abdominal viscera and the scar. A syringe is not usually attached because open-needle abdominocentesis has been found to be more sensitive than aspiration via a syringe
- If a syringe is attached only mild negative pressure is required because aspiration of abdominal viscera or omentum against the needle can inhibit fluid collection. When an over-the-needle catheter has been used and the stylet removed, gentle abdominal compression may make it easier to collect fluid
- A four quadrant tap technique may be used where localised fluid accumulation is suspected and ultrasound guidance not available. The abdomen is visually divided into four quadrants and centesis performed in the centre of each
- Other methods which can be used include inserting a closed end, multiply fenestrated tom cat catheter or peritoneal dialysis catheter
Abdominal lavage
This method is used when no fluid can be aspirated following ultrasound guidance or a four quadrant tap.
- A peritoneal dialysis catheter is most sensitive for collecting a small amount of fluid. Clip and prepare the ventral mid-line 1-2 cm caudal to the umbilicus, as for any surgical procedure. Place the patient in lateral recumbency and infiltrate the insertion site with local anaesthetic
- Make a small skin incision, divide the subcutaneous fat. Try to control any bleeding before entering the abdomen to prevent blood contaminating the sample
- Incise the linea alba and insert the catheter without any stylet, to avoid damaging the viscera. If fluid cannot be aspirated infuse warm saline (20ml/kg body weight) while gently massaging the abdomen. Collect the fluid (typically <2ml) by gravity drainage if possible. Please mention ‘abdominal lavage sample’ in the history because the saline will affect fluid analysis
Thoracocentesis
Usually pleural effusions are abundant and bilateral but they can be mild, unilateral or compartmentalised. Radiographs can help to show the extent and location of the fluid, and can be used to help guide thoracocentesis if the fluid is compartmentalised. If the fluid is not compartmentalized, thoracocentesis is done ventrally at the 6th, 7th or 8th intercostal space at the level of the costochondral junction.
- The animal is restrained in sternal recumbency or standing. Sedation and/or local anaesthesia are generally not necessary for collecting a small sample for analysis, but may be needed if a large volume of fluid is to be drained
- Clip and prepare the site of needle insertion as for any aseptic surgical procedure. A needle attached to a 5ml syringe can be used to sample the fluid. If a large volume of fluid is to be drained a similar gauge over-the-needle catheter or other catheter unit is preferred to reduce the chance of injury to intrathoracic organs (see below). Insert the needle next to the cranial surface of the rib in the 7th or 8th intercostal space to avoid the intercostal vessels which are located just caudal to each rib. Dogs: 19-23g, ¾-1” butterfly catheter or 18-20g needle. Cats and small dogs: 20-22g over-the-needle catheter
- If only a syringe full of fluid is to be withdrawn, the syringe can be attached directly to the catheter and the catheter is withdrawn when the syringe is full. To repeatedly withdraw fluid a 3-way stopcock should be attached to the catheter. A chest tube may be necessary to drain large quantities of fluid
Samples
- Collect a sample of the fluid in an EDTA tube for total nucleated cell count, red cell count, total protein and albumin determinations. If appropriate, this can also be used for cholesterol and triglyceride
- Collect a further sample in a PLAIN tube to be used for microbiology if necessary or other biochemical tests as required. If FIP is suspected, fluid may be used for detection of feline Coronavirus using PCR. Clot activator (‘doughnut’) tubes are not suitable
- It is also useful to fix an additional aliquot of plain fluid (for cytological examination) by adding 2 drops of formalin (from a histology pot) to 1ml of sample in a PLAIN tube
- If the fluid is very cloudy and/or red, two direct air dried smears should be made in the same way as blood smears. These should be rapidly air dried for the best cell morphology. For clear fluids direct smears may not be appropriate due to the low number of cells and smears made from centrifuged sediment is recommended, particularly if a delay in analysis is expected
Analysis
Physical examination. Appearance of the fluid, appearance of the supernatant, total nucleated cell count, red cell count, total protein and albumin concentrations.
Cytology report
A differential cell count, comments on cell preservation, the presence of microorganisms and assessment of cell morphology, including criteria of malignancy, if considered appropriate. Classification of the type of fluid (protein rich/poor transudate, exudate) and comments on possible causes where appropriate.
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