Canine and Feline Respiratory Medicine. Lynelle R. Johnson
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tubing can puncture the airway if extended with force.
Thoracocentesis
Thoracocentesis is performed as a diagnostic and therapeutic technique. It can be done before radiographs are performed when physical examination suggests a pleural disorder and the animal is in distress, after thoracic ultrasound demonstrates pleural disease, or after radiographs when pleural space disease is confirmed. The region of the seventh to ninth intercostal space is clipped and scrubbed in the ventral one‐third of the chest for fluid and in the dorsal one‐third for air. A 20 or 22 gauge butterfly needle is adequate for use in small dogs and cats when small pleural effusion or mild pneumothorax is present. A fenestrated 14–18 gauge catheter with extension set works well for larger dogs and can allow relatively rapid removal of large pleural effusions. Prior to entering the chest, an extension set, three‐way stopcock, and syringe should be assembled and ready for use to limit introduction of air into the pleural space after penetration with the needle or catheter. A large bowl, ethylenediaminetetraacetic acid (EDTA) and red‐top tubes, and a culturette swab should also be readily available to allow efficient specimen collection. In some animals, sedation is needed to perform a chest tap safely. Whenever possible, it is useful to have three people available to perform a chest tap.
A sterile preparation of the lateral thorax is completed and a site on the chest wall is chosen. The needle or catheter is advanced through tented skin and then walked off the cranial border of the rib to penetrate the pleural space at a perpendicular angle. This will avoid the vessels and nerves lying along the caudal rib margin. As soon as the pleura has been penetrated, the needle or catheter is directed downward to avoid injury to the lung parenchyma (Figure 2.23). The needle stylet is held stationary while catheter is advanced over the needle into the pleural space. When the catheter is fully in the chest, the needle is withdrawn, and the extension set is rapidly attached.
Figure 2.23 (a) To initiate thoracocentesis, the skin is tented and the catheter with needle is inserted perpendicularly between the rib spaces. (b) After the pleural space has been entered, the needle is directed ventrally and the catheter is advanced fully into the chest while the needle is held stationary.
Fine‐Needle Lung Aspiration and Biopsy
FNA of the lung is a suitable technique for evaluating diffuse interstitial or alveolar disorders or peripheral mass lesions within the thorax. It can be performed as a blind technique or with ultrasound guidance. However, because it can be associated with pneumothorax or hemothorax, careful patient selection is advised to limit complications. In animals that are not severely tachypneic or hyperpneic, percutaneous FNA of the lung can be performed safely and with minimal sedation. A small region of the chest is shaved and minor surgical preparation is completed. A 23–27 gauge 0.75–1.5 in. needle attached to a 3 ml syringe filled with air is passed perpendicularly through the skin and subcutaneous tissue cranial to the border of the rib into the lung parenchyma. It is inserted back and forth gently and quickly; then the needle and syringe are removed together for preparation of cytologic specimens. The syringe is detached from the needle and filled with air, and contents of the needle hub are gently sprayed onto a slide or cover slip for cytologic examination and Gram staining, if possible. If aspiration with an empty syringe fails to dispel material onto the slide, the syringe can be filled with 0.5–1 ml of sterile saline, and the aspirated material is suspended in the saline for a cytospin preparation. Even when the sample appears to be of low cellularity, cytopathology is recommended to detect cellular atypia or inflammation.
Figure 2.24 Ultrasound guidance is used to obtain a lung biopsy using a Temno biopsy needle.
Percutaneous lung biopsy can be obtained with use of an ultrasound‐guided biopsy needle in the anesthetized patient. This is performed more commonly in dogs than in cats. A surgical preparation is performed, and a 16–18 gauge Temno™ biopsy needle (Merit Medical Systems, South Jordan, UT) is guided into the lesion to obtain a 2 cm core tissue sample for histopathology (Figure 2.24). After either aspiration or biopsy, the animal should be placed in lateral recumbency with the side of the aspiration facing downward for 15–30 minutes to promote the development of a clot or seal at the aspiration site. Typically, an ultrasound is performed after the procedure to screen for hemorrhage or pneumothorax. Visualization of the normal “glide” sign as the lung slides across the pleura rules out one of these complications. An increase in respiratory rate or effort or detection of absent lung sounds near the site of aspiration would indicate hemorrhage or pneumothorax and the need for intervention.
Sample Analysis
Nasal Cytology
Nasal cytology can be performed by pressing a microscope slide directly onto the nose of an animal or by obtaining material with a cotton swab and spreading it onto a slide. It is non‐invasive and inexpensive; however, there are substantial limitations to the amount of information obtained from this test. Its primary usefulness is in the diagnosis of nasal fungal infection with cryptococcosis (see Chapter 4). If eosinophils are noted in a dog with cough and nasal discharge, the possibility of eosinophilic lung disease should be considered (see Chapter 6). Accuracy in the diagnosis of nasal aspergillosis or neoplasia is poor unless an endoscopically obtained sample from a fungal plaque (De Lorenzi et al. 2006) or from a mass lesion is examined cytologically.
Nasal Culture
Bacterial culture of nasal discharge is not recommended. In fact, since most infections in the nose are secondary rather than primary, the value of culturing a deep nasal flush or culturette sample can also be questioned because of the difficulty in distinguishing normal flora from pathogens or opportunistic invaders. Interpretation of bacterial cultures is more difficult in animals that have been on multiple courses of antibiotic trials because this tends to favor the development of resistant species. Despite this, bacterial culture from a deep nasal swab or flush can provide some direction for antibiotic therapy in cats with chronic rhinosinusitis. The easiest way to obtain a sample from the nose of a cat is to direct a sterile cytology brush (Cytosoft™, Medical Packaging Co., Panorama City, CA) into the ventral portion of the nasal cavity and gently twirl the brush to collect cells and secretions. A deep nasal flush can be performed by inserting an 8 French red rubber catheter halfway into the ventral portion of the rostral nasal cavity, occluding the soft palate digitally, and instilling and aspirating 1–2 ml of sterile non‐bacteriostatic saline. Culture for aerobic bacteria and Mycoplasma spp. is recommended.
A study on the utility of fungal cultures in the diagnosis of canine nasal aspergillosis reported moderate sensitivity (77%) but high specificity (100%; Pomrantz et al. 2007). In that study, the material submitted for culture was from a visualized fungal plaque and therefore a low number of false‐positive values would be expected. In cases of cryptococcosis, culture and speciation can be helpful in determining the epidemiology of the infection, although subclinical colonization of the nasal cavity has been reported in 7–14% of dogs and cats (Malik et al. 1997).
Nasal Histology
Histopathology provides the diagnosis of aspergillosis in cases in which a plaque lesion has been sampled by direct visualization; however, a random sampling of the nasal mucosa or a blindly collected sample may not contain fungal elements, leading to