There are several ways to obtain samples from the airways and lower respiratory tract for cytologic evaluation and microbiologic
culture. In general, samples should be handled quickly since respiratory epithelial cells degrade with some rapidity after
retrieval, and samples should be handled gently to avoid damaging cell membranes. Ideally, samples are placed on ice if there
will be a delay in preparation for cytologic exam. Preferably, samples may be prepared by cytospin. Alternatively, lavage
samples can be sedimented in a manner similar to urine specimens and a gentle line smear prepared and allowed to air dry.
Samples can be evaluated cytologically for quantification of cell types, identification of neoplastic or reactive cells, and
identification of infectious organisms or foreign material.
Cultures should be performed quantitatively since normal airways are not necessarily sterile. It is also important to interpret
culture in light of cytologic findings, because bacterial growth in the absence of inflammation may not be meaningful but
may instead represent contamination or normal flora. Since up to 25% of bacterial pneumonia is caused by anaerobic pathogens,
anaerobic culture assumes an importance that may not be intuitive in lung disease. When samples are sent out, transport tubes
to support anaerobic growth should be used as well as routine transport media. Liquid samples can be placed in transport
media directly, or swabs of the liquid may be used for culture. Special cultures or PCR procedures are needed to identify
Mycoplasma sp. or fungal organisms.
Tracheal lavage is indicated for diagnosis of large airway disease, and is particularly useful to obtain microbiologic and
cytologic samples in animals with productive pneumonia. These samples can sometimes provide useful diagnostic material for
diffuse alveolar disease, but tracheal lavage is rarely useful in the diagnosis of interstitial lung disease. Tracheal lavage
is a simple, inexpensive technique that requires no special equipment and is relatively safe and minimally invasive. The
two techniques are endotracheal lavage and transtracheal lavage.
Endotracheal lavage is useful for cats and small dogs. It does require a brief general anesthesia but is extremely simple
to perform and complications are very rare. Supplies needed include laryngoscope, sterile endotracheal tube, sterile red
rubber catheter, several syringes of sterile saline, and sterile collection tubes. Syringes should be pre-filled with from
5-8 ml of sterile saline. The animal is administered injectable anesthetic to a depth that allows endotracheal intubation.
Intubation is facilitated by topical application of lidocaine to the laryngeal cartilages of cats, and by the use of a laryngoscope.
In either species, the tongue should be grasped with a gauze sponge and pulled forward to improve visualization of the larynx.
The animal is intubated as cleanly as possible, trying to avoid allowing the tip of the endotracheal tube to contact the oral
surfaces. From 2-10 breaths of 100% oxygen should be administered prior to completion of the wash procedure, usually by attaching
the endotracheal tube to an anesthetic machine. The endotracheal tube is disconnected from oxygen, and the sterile catheter
is advanced through the endotracheal tube until its tip is at approximately the level of the carina. The first fluid filled
syringe is attached and the fluid injected as a bolus. The syringe is then used to rapidly aspirate back in an attempt to
recover injected fluid. This procedure is repeated 2-3 times. Because the anesthetized animal does not actively cough, fluid
recovery may be poorer than for TTW and only a small volume of fluid is recovered (the rest is reabsorbed). The lumen size
of a red rubber catheter may mean that more suction needs to be applied to recover fluid all the way into the syringe; this
may be better accomplished with a larger syringe (20 cc) as opposed to repeatedly emptying the air from the smaller injection
syringe. As soon as adequate diagnostic samples (1-2 ml of turbid fluid) are recovered, anesthesia may be discontinued. Tipping
the animal with head down and abdomen elevated may allow drainage of excess fluid through the endotracheal tube, and this
fluid can be used for cytologic exam (but not for culture because of oral contamination). The animal should be administered
oxygen via the endotracheal tube until a laryngeal reflex (swallow) has returned, and then extubated. The animal should be
observed for cyanosis or dyspnea for 15-30 minutes after completion of the procedure.