Sampling techniques
The collection of body cavity effusions for fluid evaluation is not only a diagnostic but also a therapeutic measure. When available, ultrasound-guided aspiration is recommended but not strictly required.
Light sedation and/or local anesthesia are recommended for thoraco- and pericardiocentesis in noncooperative patients. Sedation is usually preferred as the application of local anesthetic is sometimes more irritating than the sampling procedure. The hair at the puncture site is clipped and the skin surgically scrubbed. Sterile conditions should be pursued to prevent contamination of the body cavity. Eighteen- to 20-gauge hypodermic needles or butterfly cannulas (preferred) are most useful (Alleman, 2003). For the removal of larger amounts of fluid from the pleural or pericardial space, over-the-needle catheters have also been recommended. When a needle is used, the bevel of the needle should always face the parietal side of the body cavity to prevent irritation of internal organs by suctioning them into the needle lumen. A minimum of at least two EDTA-containing test tubes, a sterile plain tube, and a heparin-coated tube should be ready at hand before the sampling procedure is started. It is important to have more than one tube ready to allow fractionated sampling in cases of traumatic puncture.Thoracocentesis
Thoracocentesis requires special equipment such as tubing and a three-way stopcock to allow the removal of large amounts of fluid and prevent the formation of pneumothorax. The animal is held either standing or in sternal recumbency. Lateral recumbency should be avoided because it can aggravate respiratory distress. As the mediastinum is fenestrated in most dogs and cats, pleural effusions are usually present on both sides unless fibrotic pouches were formed. Therefore, diagnostic imaging prior to or during sampling is useful to assess the amount and localization of the fluid accumulation.
The skin over the puncture site should be gently drawn forward before inserting the needle so that intact skin will seal the puncture site when the needle is removed. Usually the 7th or 8th intercostal space at the costochondral junction is punctured. Make sure to enter the pleural cavity always on the cranial aspect of the rib to avoid injuries to intercostal blood vessels and nerves, which are located caudally.Pericardiocentesis
Pericardiocentesis also requires special equipment to allow the removal of large amounts of fluid and prevent the formation of pneumothorax. The procedure is usually performed from the right side in the 4th to 5th intercostal space where cardiac palpitations can be felt through the chest wall. Electrocardiographic monitoring and/or ultrasound guidance is recommended during pericardiocentesis, as premature ventricular contractions might occur. Also, the pericardium has to be punctured by a closed system to avoid the formation of pneumothorax.
Abdominocentesis
Abdominocentesis can be performed with a needle alone; the fluid is allowed to drain spontaneously. The urinary bladder should be emptied before the procedure. The patient should be in a standing position or fixed in lateral recumbency. The puncture site is either at the point of maximal dependency or 2–3 cm caudal to the umbilicus just beside the midline. Penetration of the abdominal wall through the musculature is enough to seal the puncture site.
Diagnostic peritoneal lavage is sometimes recommended for the detection of peritonitis, but it is strictly contraindicated in suspected neoplasia. A fenestrated large bore (10–14 G) over-the-needle catheter is inserted and fixed to the abdominal wall. Next, 20 ml/kg body weight of prewarmed buffered saline or Ringer’s solution is instilled. The animal is then rolled from side to side and the fluid removed for evaluation. Usually, only a small amount of fluid can be drained.