Trachael Aspirate and Bronchiolar Lavage

Indications:

These methods are used to collect cells from the trachea and lower airways in order to study a pathological cellular response. The procedures are particularly useful in the investigation of coughing dogs and cats, and those in which airway disease is suspected. The material collected may also be useful for microbiological studies.

It is important to note that interstitial disease (e.g., neoplasia) may not be reflected in changes of the lower airways and therefore could be missed if this procedure is not used in conjunction with radiography.

Tracheal aspirates of varying volume may be obtained by the two methods described. The preparation of smears immediately after collection allows good cellular preservation and therefore gives the best chance of a meaningful interpretation. Direct squash preparations of mucus and smears of concentrated material are most useful. Material to be submitted for culture should not be used for smear preparation.

The most common problem in collection of tracheal washes is contamination with cells and bacteria from the oropharynx. This may affect both the cytological and culture findings. The use of transtracheal wash technique greatly reduces the chances of contamination and is performed under sedation rather than general anaesthesia. This makes the transtracheal technique very useful in the dog, where sedation generally provides sufficient restraint. In the cat and in fractious animals, general anaesthesia and collection of a sample by the endotracheal method may be required.

Transtracheal Wash

Equipment:

• 18 gauge jugular catheter; Benkat, Granby House, Belfield Street, Ilkeston, Derbyshire, DE7 8DU. Tel: 0115 930 9716
• Sterile isotonic saline

Procedure:

In the dog, the skin of the midline larynx and surrounding area is prepared as for surgery. The animal is restrained in the sitting position/sternal recumbency and a small amount of local anaesthetic is infiltrated into the subcutaneous tissue of the midline larynx. Before inserting the needle, the catheter should be measured for length. It should be inserted to a level just proximal to the carina, which approximates to the 4-5th intercostal space in the dog.

The cricothyroid ligament may be palpated as a depression cranial to the cricoid cartilage. The needle is inserted through the cricothyroid ligament into the lumen of the larynx. The catheter is advanced through the needle to the level of the carina. This process often induces coughing.

Once the catheter is in position, the needle is withdrawn from the tracheal lumen, and sterile saline is injected into the tracheal/bronchial lumen (1-2 ml/5 kg). When the animal starts to cough, or at the end of the injection, aspiration of the saline should begin. A very small quantity of saline is generally retrieved. When the catheter is removed, digital pressure should be applied to the wound to prevent the development of subcutaneous emphysema.

Samples should be collected into EDTA. If possible, slides of mucus or a centrifuged deposit should be made immediately. Rapid air-drying is essential to preserve the morphology, especially for samples with a high mucus content. The slides and fluid should be submitted to the laboratory. Where it is not possible to make smears in the practice, a second sample may be collected into EDTA. To the second sample, add a single drop of 10% formal saline. In some cases, this may help to preserve the morphology of the cells.

Problems:

Oropharyngeal contamination must be avoided when collecting tracheal aspirates. Using the transtracheal method, contamination may occur at two points:

a) When the needle is inserted, care must be taken to direct it such that the catheter travels towards the thoracic inlet. Occasionally the catheter may bend, resulting in contact with the pharyngeal region.

b) Coughing during collection can result in transfer of material to the oropharynx followed by re-aspiration.

Figure 1: Transtracheal Tube Method

Endotracheal Tube Method

Procedure:

After the induction of anaesthesia, the endotracheal tube should be passed with minimal oropharyngeal contact. With the animal in lateral recumbency, a catheter is placed through the lumen of the endotracheal tube. Saline (1-2 mg/5 kg) is infused and immediately retrieved. If there is difficulty retrieving fluid, it may help to roll the animal onto the opposite side. Sample collection is noted as above.

Problems:

Oropharyngeal contamination.

If a bronchial wash needs to be repeated, this should be done immediately or after a delay of 48 hours, since the procedure results in a neutrophilic response.

Smear-Making

Squash Preparations:

This method is ideal for tracheal washes. A small amount of the sample is placed on one slide and a second slide placed on top. The second slide may be perpendicular or parallel to the first. The material will spread between the two slides (Figure 2a). If necessary, very gentle pressure may be used to facilitate spreading (Figure 2b) and the top slide is gently pulled across the bottom until the two slides are separated (Figures 2c and 2d). The slides should slide apart, and should not be lifted away from each other.

Rapid drying of the slides gives the best preservation of cellular morphology. This may be achieved by directing a hairdryer (cool/warm settings, not hot) onto the back of the slide from a distance of approximately 6-8 inches.

Figure 2.