Sampling from the Respiratory Tract for Bacterial Culture and Cytology
A variety of techniques are commonly used to obtain samples from the respiratory system. Although readily accessible, pharyngeal swabs obtained from the awake animal are usually of little value. Since the pharynx is normally populated with a variety of bacteria, which may not be representative of the bacterial population in the lungs or nasal cavity, most sampling techniques involve some method of by-passing the pharynx.
Transtracheal Aspirates (TTA)
TTA may be useful in the evaluation of any animal with respiratory tract disease. It is indicated in any patient with evidence of pulmonary parenchymal or bronchial abnormalities. Bacterial culture and sensitivity results from fluid obtained by TTA are vital to the management of bacterial pneumonia, and can be helpful in directing antibiotic therapy in dogs with chronic airway disease such as bronchitis or bronchiectasis. Cytologic analysis may be of diagnostic importance in the presence of inflammatory disease--for example detection of the presence of eosinophils, neutrophils or lymphocytes. The presence of excess mucus often indicates chronic inflammation.
Equipment Required for TTA
1. One 'through the needle' catheter, 8-12 inch, approximately 17 gauge (e.g., 'Intracath' Deseret Medical Inc., Becton Dickinson, Sandy, Utah), with needle guard
2. 1-2 mL of lidocaine 20 mg/ml
3. One hemostat
5. Scrub solution
6. Sterile gloves
7. 3-5 sterile 10 ml syringes filled with sterile non-bacteriostatic saline
TTA can be performed in many dogs without the use of sedation, especially if the animal is debilitated. If light sedation is required, short-acting or reversible drugs should be used, which will have little depressant effect on respiratory function (for example, oxymorphone 0.02-0.2 mg/kg IV or ketamine hydrochloride 11-15 mg/kg IV, both administered with diazepam 0.22-0.55 mg/kg IV). The dog should be positioned in a sitting position or in sternal recumbency, and the head elevated. The choice of site for TTA is variable. In small dogs and in dogs with thick neck conformation, it is easier to enter the airway through the cricothyroid ligament. The cricothyroid ligament is a triangular depression on the midline between the prominent thyroid cartilage and the ridge of the cricoid cartilage of the larynx. In medium-sized to large dogs, however, it is usually best to penetrate the trachea between two tracheal rings, on the midline, about halfway down the ventral neck. Once the site is chosen, it should be clipped and surgically scrubbed. The area should be infiltrated with lidocaine for local anesthesia.
With the dog restrained by the assistant, the clinician should now carefully locate the site for TTA. The needle is inserted bevel-down in order to minimize the risk of tearing the catheter with the sharp bevel when the catheter is inserted. The needle is first advanced through the skin on the midline. The clinician then stabilizes the trachea with the thumb and forefinger of one hand, and slowly advances the needle in a horizontal direction towards the trachea with the other. It is important to direct the needle straight towards the midline of the trachea, since if it is approached at a tangent it will be difficult to penetrate the lumen. When the needle contacts the trachea, it may be necessary to 'walk' the needle a short distance up or down in order to find and penetrate the ligament between two tracheal rings.
When the needle penetrates the trachea, a distinct 'pop' can usually be felt. The animal will often cough or swallow, particularly when the needle is within the tracheal lumen. After penetration of the trachea, the needle is carefully held in position within the lumen, to prevent it from prematurely backing out of the airway. If the needle is advanced too far within the lumen, the dorsal wall of the trachea will prevent the catheter from passing through the needle. In some instances, the dorsal wall of the trachea may even be inadvertently penetrated. Once the needle is correctly positioned within the lumen of the trachea, it may be slowly raised from a horizontal position to a 45 degree angle, pointing bevel-down towards the carina. The catheter is then advanced through the needle as far as it will go. Most dogs will cough as the catheter irritates cough receptors of the tracheal mucosa. Once the catheter is in position, the needle can then be withdrawn a short distance from the skin, and the needle-guard secured in place. This prevents any further damage or laceration of the airway.
The TTA is then performed with the 10ml syringes of sterile saline, using a hemostat to stabilize the end of the catheter. A syringe is placed on the catheter, and 8-9 mL of saline are injected into the airway, leaving about one ml in the syringe. The clinician then attempts to aspirate as much of the fluid as possible back into the syringe. It is helpful at this time if the assistant can coupage the chest wall in order to encourage the dog to cough while the clinician is aspirating. Usually only 0.5-1 mL of fluid can be recovered from each wash. This process is repeated until flecks of mucoid or purulent material are recovered in the wash. Often the second or third attempts are more productive than the first. It is quite safe to instill 30-50 mL of saline into the lungs of all but the smallest dogs. The catheter may then be withdrawn and pressure placed over the site for a few minutes.
TTA can be difficult to perform in very small and toy breeds of dogs, and in cats, due to the small size of the airway. In such small patients, it is preferable to anesthetize and intubate, and to perform the wash through a sterile endotracheal tube. Similarly, TTA can be hazardous in dogs with tracheal collapse, and can precipitate severe coughing and respiratory distress.
TTA is a somewhat stressful procedure which should not be performed in animals in significant respiratory distress, even though these are the animals most likely to benefit from it. It can cause desaturation of hemoglobin and subsequent collapse in unstable patients. Desaturation and collapse can be accompanied by cardiac arrhythmias and hypotension. Clinicians and assistants should carefully observe the patient for cyanosis or excessive distress during the procedure, and should be prepared to discontinue and to administer oxygen by face-mask if necessary.
Fine Needle Aspirates
The anatomic landmarks for fine needle aspiration are variable, depending on the radiographic location of lesions. If diffuse disease is present, the right dorsal lung lobe is selected for aspiration. Once a mass or consolidated area is demonstrated radiographically, the approximate location is noted by counting rib spaces. Fine needle aspiration is accomplished using a regular 10 ml syringe and a 22-25 gauge needle. If possible, the animal is restrained in lateral recumbency and the area clipped and scrubbed. The needle is inserted gently, directly towards the tentative location of the lesion. To avoid severe laceration of lung, it is important that the animal should not move or struggle while the needle is within lung tissue. Once the needle is inserted, moderate suction is applied to the syringe two to three times, with negative pressure allowing the hub of the syringe to spring back between attempts at aspiration. The needle can be re-directed once or twice if necessary.
Surprisingly, fine needle aspirates of the lung are often well tolerated, and associated with relatively little morbidity. This is particularly true if they are reserved for sampling in dogs and cats with mass lesions adjacent to the thoracic wall. For obvious reasons, fine needle lung aspirates should be avoided in patients with coagulopathies. The most common complication of fine needle aspirates is the development of pneumothorax, and owners should be warned of this risk before the procedure is undertaken. Despite the obvious risk, however, clinically significant pneumothorax seems to occur infrequently. Patients should be monitored carefully for progression of hypoxemia and respiratory distress caused by pneumothorax following fine needle lung aspirates. Some may require thoracocentesis or even chest tube placement if severe air leakage occurs.
Thoracocentesis should be performed in any animal that has evidence of pleural effusion on physical examination or a thoracic radiograph. Since all fluids within the pleural cavity appear as the same soft-tissue density radiographically, it is vital to obtain samples in order to reach a diagnosis. Large amounts of pleural fluid or air are likely to significantly impair respiration, so their removal can often be a life-saving procedure.
In an emergency situation when an animal presents in respiratory distress, obtaining radiographs may be time-consuming and stressful. In this instance, diagnostic and therapeutic thoracocentesis may be performed immediately to rule out the presence of pneumothorax or pleural effusion, since this procedure is easy and may be life-saving. This is of particular importance if auscultation of the chest reveals dull or absent lung or heart sounds.
Equipment Required for Thoracocentesis
1. A needle of sufficient length to penetrate through the intercostal muscles into the pleural cavity--this will vary in length from 3/4 of an inch in cats to 2 inches in the most obese dogs
2. A piece of extension tubing 8-12 inches in length (a butterfly catheter (Abbott, Chicago, IL) may be used here to provide both the needle and the extension tubing)
3. A 3-way stopcock
4. A 20-60 ml syringe
6. Scrub solution
7. A bowl to collect the fluid when the syringe is full
8. Sample tubes and aerobic and anaerobic culturettes
Sedation is not usually required in dogs, but some cats may require short-acting or reversible sedatives (for example ketamine hydrochloride 11-15 mg/kg with diazepam 0.22-0.55 mg/kg IV) in order to carry out thoracocentesis. The dog or cat is restrained by one or two assistants in a sternal position if possible. The usual site for thoracocentesis is the 7th or 8th intercostal space, which can be located by counting backwards from the 13th rib. If fluid is present in the pleural space, the needle is inserted low down, near the costochondral junction, to maximize the chance of obtaining a sample. If a pneumothorax is thought to be present, the needle should be inserted more dorsally in the intercostal space. The clinician should attempt to insert the needle cranial to the rib, since the intercostal blood vessels run caudal to the rib.
Once the site has been located, it should be clipped and scrubbed. The needle and extension tubing are attached to the 3-way stopcock and the syringe. The needle is then inserted perpendicular to the skin and advanced slowly through the intercostal muscles. Once the needle is in the intercostal muscles, an assistant can begin to create some negative pressure using the syringe. As soon as the needle passes through the pleura into the intercostal space, fluid will be seen within the tubing, and the needle should then be kept stationary as fluid is aspirated into the syringe. If a pneumothorax is present, when the needle penetrates the intercostal muscles the assistant will find the syringe filling with air. The needle should be kept stationary as long as fluid or air continues to be obtained. If the clinician feels a scraping or bumping sensation of the needle against internal viscera, the needle should be withdrawn slightly. Eventually negative pressure will be reached and the needle can be removed. The same procedure may then be repeated on the other side of the chest.
The incidence of complications with thoracocentesis is generally low, and primarily relates to damage by the needle within the thoracic cavity. Lacerations of lungs leading to pneumothorax, or coronary vessels leading to hemorrhage, may occur. This can be minimized by careful control of the needle tip, and by reducing struggling by the animal. Coagulopathies are a relative contraindication of thoracocentesis--but sometimes the procedure cannot be avoided, especially in the case of life-threatening pleural hemorrhage.
References are available upon request.