Several methods for obtaining TAs have been developed, each having advantages and disadvantages. The most important consideration when choosing a technique is whether microbiologic culture of the tracheobronchial secretions is indicated. In general, aspirates obtained endoscopically are unsuitable for this use because they invariably become contaminated by upper airway flora. However, a guarded catheter passed through the endoscope may be used to obtain samples suitable for microbiologic culture. Alternatively, the transtracheal (percutaneous) aspiration technique may be employed.
The choice of technique can affect significantly the numbers and types of cells obtained. Thus standardization of procedures with regard to type of technique, time of sampling, volume and type of fluid instilled, sample handling, and processing is recommended for meaningful interpretation and comparison of results.
The rationale for use of transtracheal aspiration is based on the assumption that the bacterial population derived from the upper airway of normal horses is negligible beyond the proximal trachea. Therefore organisms cultivated from a tracheal aspirates represent bacteria found in the distal trachea and lower airways. These bacteria may be present transiently, or they may be part of a pathologic process. The distinction between these phenomena is important.
Samples obtained by transtracheal aspiration are suitable for cytologic and Gram’s stain evaluation and bacteriologic or fungal cultivation. However, this technique is invasive, and possible complications have tended to preclude its widespread application. These include subcutaneous abscessation at the puncture site, tracheal laceration and hemorrhage, chondritis, and pneumomediastinum. In addition, the catheter may break off in the tracheal lumen, although in most cases it is coughed up rapidly and swallowed. Good technique prevents most untoward complications.
A variety of needle-catheter combinations may be used, but maintaining asepsis is critical. The components may be purchased either individually or prepackaged and include an introducer catheter-over-needle, flushing catheter, and a stylet (Catheter TW 1228 and 1628, Mila International, Phoenix, Ariz.). A convenient combination of catheters comprises a 12-gauge needle, 3-inch over-the-needle cannula, and number 5 French canine urinary catheter with the tip cut off obliquely.
Sedation generally is indicated when performing a tracheal aspiration, with xylazine (Rompun) used commonly. An area measuring approximately 6 by 6 cm over the middle third of the cervical trachea should be clipped and prepared for aseptic surgery. A bleb of local anaesthetic (approximately 1 ml) is injected subcutaneously over the midline and a stab incision is made through the skin and subcutaneous tissue with a number 15-scalpel blade. The trachea is stabilized with one hand and the cannula is introduced into the tracheal lumen between two cartilage rings. The stylet is removed, and the urinary catheter is passed down into the tracheal lumen to the level of the thoracic inlet, where the washing and aspiration is performed. In most cases 10 to 15 ml of sterile isotonic saline is adequate to obtain a satisfactory sample. However, repeated infusions may be required. Once an adequate sample has been collected, the catheter should be withdrawn, maintaining the cannula in situ during retraction to minimize contamination of peritracheal tissues.
Tracheal Aspirates: Endoscopic Technique Using Unguarded Catheters
An increasingly popular and well-tolerated alternative for collection of TAs is via a fiberoptic endoscope. However, samples collected using unguarded catheters are contaminated with nasopharyngeal bacteria and are unsuitable for microbial cultivation. Endoscopy allows visualization of the respiratory tract at the time of sampling, where evaluation of the mucosa of the trachea (degree of hyperemia) and its luminal contents (quantity and quality of mucus, mucopurulent secretions, and blood) may assist in interpretation of cytologic results. Furthermore, if the length of the endoscope permits, the large bronchi may be examined, and purulent debris draining from a specific bronchus suggestive of pulmonary abscess occasionally may be recognized.
A small polyethylene catheter is passed through the biopsy channel of the endoscope and 10 to 15 ml of sterile, isotonic saline instilled. Most horses have a ventrally depressed area in the trachea, anterior to the carina and level with the thoracic inlet. Fluid accumulates at this site and forms a puddle from where it can be aspirated. The principal use of samples collected using this technique is for cytologic examination.
Tracheal Aspirates: Endoscopic Technique Using Guarded Catheters
Recently, guarded systems have been evaluated for collection of uncontaminated samples from the lower airways via endoscopy. In adult horses, the advantages of collection using guarded catheters include noninvasiveness, speed with which samples can be obtained, visual inspection of the airways, and guidance of the catheter. These advantages generally outweigh those of the transtracheal method, which include reduced chance of bacterial or cellular contamination from the upper respiratory tract.
Several multilumen, telescoping, plugged catheters have been assessed. One is the endoscopic microbiology aspiration catheter (Catheter EMAC800, Mila International, Phoenix, Ariz.). This catheter contains a glycol plug in the outer catheter, to maintain sterility as the catheter is being advanced through the endoscope and trachea, and an inner catheter for retrieval of the sterile specimen. Another system involves a 5 French inner catheter within, an 8 French guiding catheter (Catheter V-EBAL-8.0-190, Cook Veterinary Products, Bloomington, Ind.). Before each sample collection the endoscope and its biopsy channel must be disinfected. Glutaraldehyde (Cidex) is an appropriate disinfectant.
Some controversy remains regarding the adequacy of samples collected through guarded catheters for microbiologic cultivation. Technical prowess definitely influences the quality of sample obtained. Factors that help prevent contamination include rapid collection of the sample, limited volume of infusate (10-15 ml), and advancement of only the inner catheter into the tracheal puddle rather than the catheter as a whole. In addition, if the horse has coughed frequently during the procedure, an increased risk of contamination is likely, and these samples are rarely appropriate for bacteriologic cultivation.