Diagnostic approach to LRT disease in horses

What presenting signs indicate LRT disease?

• Cough

• Bilateral nasal discharge

• Tachypnoea/Dyspnoea

What are the causes of a cough in LRT?

Stimulation of irritant receptors

Forced expiration against closed glottis

high velocity expiration

Possible causes:

• Physical stimulus (Foreign material, turbulent air, mucus)

• Chemical stimulus (Osmolarity, irritant)

Increase response to stimulus in inflammation

How does airway inflammation cause bilateral nasal discharge?

• Increased mucus production

• Altered mucus composition

(result in bilateral mucopurulent discharge)

What is the significance of bilateral/unilateral discharge?

Helps determine location of origin

bilateral = behind nasal septum

unilateral = either side of nasal septum

if guttural pouch can begin unilateral + become bilateral

How does LRT disease cause tachypnoea/dyspnoea?

• Hypoventilation

• V/Q mismatch

• Impaired gas diffusion in alveolus

What is VO₂ max? and the significance of this in horses?

Maximal oxygen consumption

A fit thoroughbred only uses 4% VO₂ at rest so signs of resp disease not always apparent at rest

What history and signalment factors can determine likelihood of resp disease presentation?

• Disease time course and features

• Herd or individual problem

• Age and use of horse

• Management and environment

• Coexisting problems

What should be observed from a distance before physical exam?

• Posture (extended head and neck = severe respiratory distress, tail lifted)

• Abdominal effort

• Respiratory Rate

• Respiratory Depth –

• Pattern – biphasic?

• Hypertrophy of Ext. ab. oblique

  • ‘Heave line’

Why is collapse more likely in URT during inspiration?

• Relatively negative pressure in trachea cf to atmorsphere whereas relatively positive in lungs compared to body so collapse more likely in trachea

• Inverse true in expiration

Where is collapse most likely to occur on inspiration and expiration?

Inspiration- URT

Expiration - LRT

What would you look at on clinical examination of suspected LRT disease?

• All systems

• Temp / Heart rate

• Ventral oedema? (more likely cardiogenic cause)

• Guttural Pouches & Lymph nodes (submandibular)

  • enlargement, discharges

• Nares and Nasal Passages

  • airflow obstruction

  • discharges

Where should you auscultate the resp tract in horses?

• Base of the trachea

• Then the green circle- where normal lung sound is going to be loudest, bifurcation of trachea

• And then whole lung

What do normal breath sounds sound like?

turbulent air in large (>2mm) airways

Soft blowing sound

Inspiration > expiration

Faster air = louder

Low frequency sounds travel best through normal lung

What are the main adventitiouos (abnormal) sounds?

1. Wheezes

2. Crackles

3. Pleural rubs

4. Cough

5. Expiratory Grunts/groans

What does a wheeze sound like?

'Musical note, whistling sound'

What causing wheezing?

• Airway narrowing and vibration which can be caused by:

  • Thickened wall

  • Intraluminal obstruction (mucus/foreign body)

  • Bronchospasm

  • Extra luminal compression

What is being shown here

Mucus accumulation

What are the types of wheezes?

• Polyphonic wheeze

• Monophonic wheeze- single sound (in image)

What is the bernoulli effect?

• High velocity air causing lower pressure

• Further narrowing of airway

• Can cause wheeze

When are the lower airways smallest and most likely to collapse (Insp/exp, early/end)?

• Inspiration

• Expiration, particularly at end

When does wheezing occur most commonly in the LRT and URT?

• LRT – most common end expiratory

• URT – most common inspiratory

(often insp. + exp.)

What are the two types of crackles?

• Coarse crackles

  • Bubbling mucus

  • Insp or expiration

  • Radiate widely

• Fine crackles

  • Popping open of collapsed small airways

  • Most common: early insp

What are pleural friction rubs?

• Inflamed parietal and visceral pleural membrane (ex- if they have fibrin on)

• Variable- fine crackles to sandpaper rubbing together

• Usually insp and exp at same point in resp cycle

How can you do equine thoracic auscultation (using rebreathing)?

• Increases PaCO2

• Increases Respiratory Rate and Tidal volume + resp effort

• Increases normal and abnormal resp. sounds

• Cough = abnormal (sign of inflam)

• To exacerbate adventitious sounds

  

How can you use thoracic percussion?

• If you have airfilled lungs will get resonant percussion

• Dull sound with fluid (ex- pleural effusion)

What further diagnostic tests can you do for LRT disease?

• Laboratory and Clinical Pathology

• Nasopharyngeal swab

• Endoscopy and transendoscopic tracheal aspirate

• Percutaneous tracheal aspirate

• Bronchoalveolar lavage

• Thoracocentesis

• Imaging

  • Radiography

  • Ultrasonography

• Lung biopsy

What blood samples can you take for LRT disease?

• Inflammatory profile:

  • WBC / proteins / Fibrinogen /Serum Amyloid A —> all increase in inflam

  • fibrinogen = acute but remain longer than SAA

  • serum amyloid A (SAA) = acute + decrease over 24hrs

• Lactate

  • Tissue hypoxia (if marked resp. disease)

• Blood gas profile

  • Arterial Blood gas (will indicate cause of hypoxia)

  • Hypoxaemia

  • Hypercapnia

What tests can you do for specific viruses/bacteria in the LRT?

• Polymerase Chain Reaction (PCR) to identify RNA/DNA of specific viruses / bacteria

• Paired serology

• Virus isolation from:

  • buffy coat

  • nasopharyngeal swabs (insert to level of lateral canthus)

• Bacterial Culture / identification

What can an endoscope be used for?

• Imaging

• Sample

(URT, trachea, carina —> 1st point bifarcation)

What LRT samples can be taken?

1. Tracheal aspirate (TA) Transendscopic or transtracheal

2. Bronchoalveolar Lavage (BAL)

3. Thoracocentesis

How is a transendoscopic tracheal aspirate performed?

add saline

collect fluid from tracheal sump

What are the advantages and disadvantages of transendoscopic tracheal aspirate?

+ve

• easy

• non-invasive

• sample representative of whole lung

-ve

• sample contaminated by nasopharyngeal flora and equipment (not best for bacteriology)

• specialist equipment required

How is a transtracheal aspirate performed?

puncture through skin (between cartilage rings)

What are the advantages and disadvantages of transtracheal aspirate

+ve

• no pharyngeal contamination

• no specialised equipment

• useful in young foals when endoscopes too large

-ve

• Horse may cough catheter into pharynx and contaminate sample (can cause invert)

• invasive

  • cellulitis

  • subcutaneous emphysema

How can you analyse a tracheal aspirate sample?

• Differential cell counts

  • macrophages should be predominant cell type in normal

  • Abnormal = >20% neutrophils

  • Abnormal = Presence of mast cells, eosinophils (inflam)

• Presence of mucus, amount, Curschmann’s spirals

• Gram stain

  • Especially for intracellular organisms

• Bacterial culture / sensitivity

What does this show?

non septic

LRT inflam

neutrophils (not degenerate)

How is a bronchoalveolar larvage carried out? What is it suitable/unsuitable for?

• Specific BAL tube or >2m endocope

• Small area of distal airway lavaged with saline

  • Best for diffuse lung disease

• Good for cytology

• Unsuitable for bacteriology

What should a BAL sample look like?

• Surfactant so must get foam on your sample

(surfactant = line bronchioles/alveoli + increase compliance of lungs)

What are the advantages and disadvantages of BAL?

+ve

• sample obtained from DISTAL airways = most commonly affected

• Best correlation with pulmonary function and histopathology

• equipment cheap and accessible (unless endoscopically obtained)

-ve

• Site may not be appropriate in animals with

  • localised pulmonary abscesses or pneumonias (cranioventral lobes)

• Pharyngeal contamination

  • Culture not useful

• Invasive

What characteristics of LRT disease might be seen when performing BAL?

Mostly severe / chronic / treatment failures

• e.g. neutrophilic vs mast cell inflammation

• mature vs degenerate neutrophils

• identify pathogens

When should BAL used vs TA?

BAL: better correlation with:

• Airway obstruction (pulmonary function testing)

• Exercise induced hypoxaemia

• Lung histopathology

TA is most useful for

• Bacteriology

• Focal lung lesions e.g. Abscess/neoplasia

• Tracheal inflammation

What can be seen on this radiograph? Is it normal?

normal

What can be seen on this radiograph? Is it normal?

show pleural effusion

can see pleural surface + lung anatomy due to fluid

can see diaphragm

When is thoracocentesis indicated?

Whenever there is a pleural effusion

What are the clinical signs of pleural effusion?

• Increased resp. rate

• Dull thoracic percussion ventrally

• Pleurodynia

• Ultrasound