Diagnostic techniques for the respiratory tract (Proceedings)

Transtracheal aspirates

Transtracheal aspiration (TTA) allows samples to be obtained from the airways for cytologic and microbiologic evaluation. Although other techniques, such as bronchoalveolar lavage, may provide more information about disease processes deep within the pulmonary parenchyma, these techniques require anesthesia and the use of a bronchoscope. Transtracheal aspiration, on the other hand, is readily available and requires minimal specialized equipment or facilities. In many cases, TTA can be performed in the awake animal, thus minimizing the morbidity associated with anesthesia. Samples obtained by TTA, although only from the upper airways and bronchi, can often provide valuable information about disease processes deep within the lung.

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. Neoplasms of the lung such as bronchogenic carcinoma may also occasionally be detected cytologically.

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, butorphanol 0.2-0.4 mg/kg IV with diazepam 0.2-0.5 mg/kg IV). At least one, and possibly two assistants will be required for restraint. 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 depending on the individual animal. In small dogs and in dogs with thick neck conformation, it is easiest to enter the airway through the cricothyroid ligament. The cricothyroid ligament can be felt as 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 of the catheter should first be 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 the hemostat to stabilize the end of the catheter. A syringe is placed on the catheter, and 8-9mls 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 mls 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 mls 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.

Other complications associated with TTA are usually few and minor. Occasionally subcutaneous emphysema can occur which is usually mild and self-limiting. Thoracic radiographs obtained after TTA may reveal self-limiting and asymptomatic pneumomediastinum. Hemoptysis may occur which is typically brief, although could be severe in animals with coagulopathy. Rarely seen but potentially serious complications include tracheal laceration, or catheter breakage with subsequent loss into the airway.

Fine needle aspirates

Percutaneous fine needle aspiration of the lung can be a useful diagnostic technique that requires minimal equipment. Fine needle aspiration is particularly useful in patients with mass or localized consolidated lesions of the lung, particularly if such lesions are located adjacent to the thoracic wall. Fine needle aspiration is also occasionally performed in patients with diffuse lung disease. The technique can be performed blind or with guidance by ultrasound or radiography. Since it is a relatively stress-free procedure, fine needle aspiration is a particularly useful technique if such patients are too unstable to undergo sedation for bronchoscopy or restraint for TTA.

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 cc 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. The needle can be re-directed once or twice if necessary.

In general, unless the lesion is fluid-filled, the fine needle aspirate will yield only a small amount of material in the hub of the syringe. If suction is applied while the needle is withdrawn, the sample can be lost by being sucked into the barrel of the syringe by the sudden rush of air as the needle exits the thoracic cavity. Negative pressure should be released, therefore, before withdrawal of the needle from the thorax. After aspiration, the needle (with sample material in the hub), should be detached from the syringe, and the syringe filled with air. When the syringe is then re-attached to the needle, the sample material can be rapidly expelled onto culturettes for aerobic and anaerobic culture, or onto glass slides for cytology.

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

Thoracocentesis is a readily available and practical technique. When indicated, it is often of immense diagnostic as well as potentially life-saving therapeutic value, and in most cases no other technique or therapy can be substituted for its use. Thoracocentesis should be performed in any animal that has evidence of pleural effusion on a thoracic radiograph. Since all fluids within the pleural cavity appear 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 the case of an animal with pyothorax, drainage of even moderate amounts of purulent fluid may also be life-saving by minimizing the risk of sepsis.

In an emergency 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 a 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.

Sedation is not usually required in dogs, but some cats may require short-acting or reversible sedatives in order to carry out thoracocentesis. The dog or cat is restrained by one or two assistants in a sternal position if possible. It can be more difficult to perform a successful thoracocentesis if the animal is in lateral recumbency, since fluid will tend to drain downwards towards the dependent side. 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 costrochondral 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 continue to be obtained. If a scraping or bumping sensation of the needle is felt, 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.

In some patients, especially if fluid is loculated within the pleural cavity by fibrin tags and fibrous adhesions, it can be difficult to clear all of the fluid by aspirating from one site. These animals can be very frustrating to treat since multiple sites must then be used, either selected at random or using thoracic radiographs for guidance. In these cases, ultrasonographic guidance can be very helpful, by helping to locate fluid pockets within the chest.

Thoracocentesis is not a particularly stressful or painful procedure, but some animals in respiratory distress (especially cats) may object to the restraint that is required. If excessive struggling occurs in the dyspneic animal, this can be life-threatening. Sedation or anesthesia may be vital in this situation in order to allow thoracocentesis. If the clinician elects to use chemical restraint, respiration must be observed carefully, and ideally the animal should be intubated and ventilated with oxygen if it is anesthetized.

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. Occasionally air may escape into the pleural cavity through the needle, but this usually leads to only minor pneumothorax.

Complications such as re-expansion pulmonary edema and air embolism are complications which may occur occasionally, but are difficult to document clinically. Pleural shock is thought be an exaggerated vagal response occuring mainly in feline patients when large volumes of fluid are removed. It responds well to atropine and volume expansion.