Thursday, August 22, 2013

Three things I learned: acute and chronic, upper and lower respiratory tract signs in dogs and cats

This Burmese cat presented with dyspnoea secondary to a marked pleural effusion which is being suctioned via a syringe with a three way stopcock. Mild intravenous sedation was administered.
Keen SAT followers will note my increasingly liberal use of the number "three". But here goes...this week in the CVE's TimeOnline respiratory physiology course I've been reading about respiratory patterns and diagnosing common causes of dyspnoea. And if you keep reading there is a handy table just for you.

These can be tricky to classify in practice, as our tutors pointed out, because animals don't read the textbooks before they present with dyspnoea (ie difficulty breathing, shortness of breath or "air hunger" depending on which definition you prefer). This is especially the case when multiple areas of the respiratory tract are affected simaltaneously, for example laryngeal paralysis (upper respirtatory tract or URT) with secondary aspiration pneumonia (LRT) or trauma with pneumothorax and concurrent pulmonary contusions.

The work of breathing = the amount of energy and oxygen consumed in the process, which might be 2-3% of our total energy and oxygen consumption. But if moderate to severe respiratory tract pathology is present, that can shoot up to 30% which is exhausting and not sustainable long-term, especially if onset is acute and there is little time for the body to compensate/adapt.

So what does it feel like? Well, according to our instructors
  • an obstructive respiratory pattern feels like you are breathing through a straw or size 4 ET tube on either inspiration or expiration for five minutes. Both the effort and time it takes to breath increase when the airways are narrowed.
  • a restrictive pattern is more like having your chest wrapped in a strong rubber band or being mummified with vetwrap. Instead of taking deep slow breaths you would be forced to take faster, shallower breaths to minimise the effort expended in stretching that big rubber band.

They provided key differentials for chronic and acute causes of dyspnoea, as well as via location in the respiratory tract which I put into table form.

Differential diagnoses for upper respiratory tract disease (characterised by an obstructive inspiratory pattern, and audible stridor which is loudest on inspiration)
Chronic causes
Acute causes
Elongated soft palate
Laryngeal paralysis
Everted laryngeal saccules
Tracheal collapse
Brachycephalic airway syndrome
Chronic URT infection
Tracheal trauma
Submucosal tracheal haemorrhage (eg secondary to anti-coagulant rodenticide)
Nasopharyngeal polyps (cats)
Smoke inhalation
Inhaled foreign bodies
Laryngeal spasm
Infections
Anaphylaxis in cats
Acute or chronic presentation
Granuloma (eg fungal granuloma)
Infection
Neoplasia (eg nasal lymphoma)
Epistaxis
Nasal pathology causing nasal cavity obstruction
Pharyngeal/laryngeal oedema may present acutely but may be secondary to a chronic disease process

Differential diagnoses for lower respiratory tract disease (characterised by an obstructive or restrictive pattern depending on the site, with wheezes that are loudest on expiration)
Bronchi
Pulmonary parenchyma
Pleural space disease
Paralysis
Feline asthma
Chronic bronchitis
Bronchiectasia
Parasitic airway disease
Left sided CHF
Pneumonia (bacterial, fungal, viral, protozoal)
Pneumonitis
Dirofilariasis
Neoplasia
Vitamin K antagonist rodenticide toxicity
Non-cardiogenic pulmonary oedema
ARDS
Pulmonary contusions
Alveolar haemorrhage (coagulopathy, neoplasia, aspiration)
PTE
Smoke inhalation
Pneumothorax (trauma, idiopathic bulla rupture, oesophageal or tracheal perforation)
Haemothorax (trauma, neoplasia, vitamin K antagonist rodenticide)
Pyothorax
Chylothorax
Pleural exudate (FIP, neoplasia)
Pleural transudate (eg LS CHF in cats, pericardial effusion, hypoalbuminaemia)
Diaphragmatic hernia
Lung lobe torsion
Toxin – snake envenomation, ixodes (tick paralysis), tetrodotoxin, botulism

Immune – myasthenia gravis, polyradiculoneuritis (Coonhound paralysis)

Cervical spinal disease

Hypokalaemia

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