Discussion

Summary:
Investigation of the lower respiratory tract begins with consideration of the history and careful physical examination. Endoscopic examination of the tracheal and proximal small airways may occasionally reveal a specific lesion. However, airway cytology remains the single most important tool in identification of lower airway disease, particularly in inflammatory disorders. The selection of sample technique for airway cytology and bacterial culture depends on the particular clinical problem under consideration as each has strengths and weaknesses. Ultrasonography and radiology are often complimentary.

Clinical presentation:
Attention should be paid to age and type of horse, whether the problem affects one or a group and details of environment and careful physical examination is mandatory.

Samples for cytology and bacterial culture:
Selection of sampling technique is driven by the specific differential diagnoses under consideration:

Endoscopically guided tracheal aspirate:
Technique.
- Position endoscope a few centimetres proximal to the thoracic inlet
- Advance catheter and insert 30 ml sterile (buffered) saline and withdraw sample
- Care: lift the horse's head to a normal position if sedated.
- Ideally sample should be obtained following vigorous exercise

Advantages:
- Easy and well tolerated
- Sample is representative of the whole lung

Disadvantages:
- Contamination from pharyngeal flora or equipment
- Wide range in normal cell populations
- Cells poorly preserved.

Common indications:
Due primarily to ease of sampling, endoscopically guided tracheal aspirates are the most commonly performed technique in young racehorses with lower airway disease where bacterial infection is common and bacterial culture is a priority. It is also used in weanling and foal pneumonias. Because of potential contamination, the clinician must be prepared to ignore positive cultures of organisms that are not recognised aetiological agents of the diseases under consideration.

Transtracheal aspirate:
Technique.
- Surgically prepare site in lower third of the trachea
- Insert local anaesthetic
- Insert guide catheter (e.g. 10 gauge, 3 inch) or needle between tracheal rings
- Insert sample catheter (e.g. 16 - 14 gauge, 30 cm)
- Insert 25 - 30 ml sterile saline
- Aspirate sample
- Remove sample catheter
- Remove guide catheter last to avoid contamination of subcutaneous tissues

Advantages:
- No pharyngeal contamination if performed successfully
- No specialised equipment

Disadvantages:
- Catheter can be coughed into pharynx and contaminate sample
- Invasive
- Cellulitis
- Subcutaneous emphysema.

Common indications:
Indicated in pneumonias and pleuropneumonias where causative organisms are also potential pharyngeal commensals and accurate information from bacterial culture and antimicrobial sensitivity testing may influence treatment substantially.

Bronchoalveolar lavage (BAL)
Blind technique
- Insert guide catheter into trachea (if available)
- Advance BAL tube into bronchus until it will not advance further
- Inflate balloon (if available)
- Insert 120- 200 ml sterile saline
- Withdraw sample ensuring that it is frothy (i.e. contains surfactant)

Endoscopic technique:
- Advance endoscope to carina
- Insert 5 - 10 ml mepivicaine
- Proceed to insert until it will not advance further using more mepivicaine if necessary, may choose right or left lung or dorsal or ventral location
- Insert 120 - 200 ml sterile saline
- Withdraw frothy sample.

Advantages:
- Obtained from the area of tract that is most likely to be affected by e.g. exercise-induced pulmonary haemorrhage (EIPH) and recurrent airway obstruction (RAO)
- Narrow range of cell populations aids interpretation
- Equipment of blind technique is cheap and accessible.




Disadvantages:
- Site may not be appropriate in animals with localised lesions
(abscesses, pneumonia).

Common indications:
Inflammatory airway disease (IAD), RAO and EIPH (exercise- induced pulmonary haemorrhage).

Pleurocentesis (Thoracocentesis):
Technique:
- Performed with ultrasonographic site selection
- Select site, usually in seventh or eight intercostal space, above lateral thoracic vein
- Surgical preparation and local anaesthesia
- Stab incision
- Prepare to close drain rapidly if necessary, (artery forceps etc.)
- Blunt cannula or drain
- Withdraw sample
- Place purse string suture and tighten as removing cannula.

Advantages:
- Allows characterisation of pleural effusion
- Important part of therapy for equine pleurisy.

Disadvantages:
- Invasive with potential for trauma to intrathoracic structures and introduction of air.

Common indications:
- Where pleural effusion has been identified.

Lung biopsy:
Technique, advantages and disadvantages
Performed under ultrasonographic or pleuroscopic guidance usually to biopsy solid, nodular lesions
Identify lesion, surgical preparation, local anaesthesia, insert tru-cut device (14 - 18 g, 15 - 20 cm)
- Used for identification of specific forms of pathology
- Invasive: complications include uncontrollable haemorrhage and pneumothorax.

Common indications:
Restricted to cases where interstitial lesions such as granulomas, neoplasia or interstitial pneumonia are suspected. Primarily aimed at obtaining prognostic information.

Imaging modalities:

Thoracic radiography:
Equipment and technique:
- Rotating anode, stationary, capacity 500 mA; 180 kV with grid
- Usually 4 or 5 lateral images per side for adults
- Projections from both sides often helpful.

Interpretation:
Lung patterns are classified as interstitial, bronchiolar, alveolar and vascular and mixed, focal or diffuse
In the pleural cavity and mediastinum air, fluid and masses can be detected.

Thoracic ultrasonography:
Equipment and technique:
- Sector or microconvex transducers with a range of frequencies: 3.5 - 8+ MHz probe
- Obtain transverse images from 5th - 16th intercostal spaces on the left and 3rd, 5th - 16th intercostal spaces on right.

Interpretation:
- Sound does not penetrate normal aerated lung which has a smooth, freely gliding surface with minimal fluid in pleural cavity.

Selecting imaging modalities:
The most appropriate imaging modality depends on:
- Specific disease process suspected
- Availability of equipment
- Size of patient
- Safety issues e.g. radiation
- Ultrasonography is most useful for imaging lesions in the pleural space, subtle peripheral lesions and aiding in biopsy and aspiration
- Ultrasound is less useful in forms of pathology that do not extend to the surface of the lung
- Radiography gives the accurate assessment of the anatomical distribution of lesions which may help support a specific diagnosis or estimate severity
- Radiography is less useful for characterising effusion or the internal architecture of pulmonary lesions.

Pulmonary function tests:
- Useful to quantify the effects of disease and monitor response to therapy
- May identify functional abnormalities that are specific to specific forms of pathology e.g. bronchoconstriction in RAO
- On-board technologies are being developed.