3 Equine Respiratory Tract Examination

• Equine respiratory tract examination involves identifying the origin of nasal discharge and sources of airway impairment

• It also includes distinguishing between intra- and extra-pulmonary diseases

• Study of the pathophysiology of upper and lower airway obstruction, along with lower airway restrictive disease is key

Observation at Rest

• Oxygen exchange impairment can manifest as exercise intolerance, respiratory difficulty at rest, or cyanosis

• Animals in respiratory distress at rest often exhibit anxiety, head extension, bulging eyes, and anorexia

• Horses are obligate nasal breathers

• Mild to moderate respiratory insufficiency can be investigated through observation during and after exercise

• Rate and character of respiration should be assessed while the horse is at rest

• Normal respiratory rate is 8-12 bpm, also known as eupnea

• Biphasic respiration is considered normal

• Abdominal pressing with expiratory difficulty indicates lower airway obstruction

• Extended head and neck position accompanied by inspiratory noise suggests upper airway obstruction

• Location and obstructive/restrictive nature of airway obstruction affects airflow during inspiration, expiration, or both and is associated with respiratory noise

• Dyspnea is the subjective feeling of shortness of breath or air hunger

• Orthopnea involves shortness of breath when lying flat

• Hyperpnea refers to an increased depth of breathing

• Tachypnea is rapid breathing, and hyperventilation at rest

• Hyperpnea is an increase in breathing proportional to metabolic rate

• Hyperventilation involves increased alveoli ventilation relative to carbon dioxide production

• Hypoventilation, or respiratory depression, causes increased carbon dioxide concentration, or hypercapnia, and respiratory acidosis

• Apnea is the suspension of external breathing with no respiratory muscle movement

Impact of Respiratory Physiology

• Airway caliber is the greatest determinant of airway resistance, according to Puiseuille’s Law
• Resistance = 8(viscosity)(length) / (driving pressure x π)(radius^4)

Upper Airway Obstruction

• 80% of resistance to airflow originates in the upper airway
• A 50% decrease in airway radius increases resistance 16-fold
• This means even small changes in upper airway diameter can dramatically increase resistance to airflow
• Extrathoracic airway pressures are sub-atmospheric during inspiration
• Poorly supported soft tissues in the upper airway collapse during inspiration, called dynamic collapse
• Laryngeal hemiplegia, extrathoracic tracheal collapse, retropharyngeal abscess, and arytenoid chondritis can cause equine respiratory disorders that cause inspiratory difficulty
• Stridor, stertor, and roaring are audible sounds that are signs of respiratory tract issues
• Tracheotomy provides immediate relief of upper airway obstruction.

Lower Airway Obstruction

• 20% of total airway resistance comes from the lower airways
• Bronchiole radius is small, their overall contribution to pulmonary resistance is low because of their surface area
• Bronchiole resistance is low meaning there needs to be advanced disease present before abnormalities can be detected or respiratory distress occurs
• Intrathoracic pressures are sub-atmospheric during pulmonary inflation
• Small airways are pulled open and stretched at high lung volumes, which lowers airflow resistance during inspiration
• Intrathoracic pressure is positive during exhalation
• Diameter of small airways is decreased in the process which means they can be closed at low lung volumes
• Airflow resistance is highest during the expiratory phase of respiration
• Narrowed bronchioles collapse (dynamic airway collapse), trapping air behind the closed airways
• Animals with this collapse breathe at higher lung volumes and maintain a higher functional residual capacity
• Wheezing is loudest at the end of expiration because that is when dynamic airway narrowing and collapse take place
• Intrathoracic tracheal collapse means there will be expiratory difficulty
• Positive intrathoracic pressure exacerbates collapse during the expiratory phase of respiration

Restrictive Disease

• Restrictive pulmonary disease inhibits expansion and leads to inspiratory respiratory distress

• Lung vital capacity and compliance, or elasticity, are reduced

• A rapid and shallow breathing pattern at low lung volumes is characteristic of restrictive pulmonary disease

• This high compliance at low lung volumes takes advantage of their low-stress breathing

• Increased ventilation is a large disadvantage with this respiratory pattern

• Intrapulmonary restrictive diseases: pulmonary fibrosis, interstitial pneumonia, infiltrative metastatic pulmonary neoplasia

• Extrapulmonary restrictive disease is caused by mechanical impedance: pleural effusion, pneumothorax, mediastinal mass, abdominal distension, trauma to the thoracic wall, diaphragmatic hernia or neuromuscular dysfunction

Respiratory Examination

• Examination starts at the nares

• Check the nostrils for airflow symmetry, flare, discharge, plaques, and odor

• Epistaxis means there is blood at the nares

• Hemoptysis indicates blood originating from the lower respiratory tract

• Pulmonary edema is indicated by the presence of pink frothy foam

• Malodorous nasal discharge or facial deformity can indicate disease of the sinuses

• Dull sounds during percussion of sinuses filled with fluid

• Submandibular lymph node lymphadenopathy should be evaluated for pain and nature of swelling

• Cellulitis with discharge means strangles

• Discrete swelling means viral or neoplastic

• Prominent muscular process of the arytenoid cartilage is laryngeal hemiplegia

• Coughing after tracheal compression shows lower airway inflammation

• Tracheal rattle means there is exudate in the trachea

• Plaque of edema means pleural effusion.

• Horses with pleurodynia will stand with abducted elbows and will resent auscultation and percussion

• Pulmonary Borders:

  • 17th ICS at tuber coxae
  • 16th ICS at tuber ischii
  • 13th ICS at mid-thorax
  • 11th ICS at shoulder
  • 5th ICS at elbow

• Expanded pulmonary fields indicates obstructive pulmonary disease

• Diminished lung borders indicates fluid line, pulmonary consolidation, diaphragmatic hernia, or neoplasia

• Rebreathing procedure increases the rate and depth of respiration, but is contraindicated in horses dyspneic at rest

• Monitor respiratory impairment and recovery time and a cough indicates lower airway inflammation

• Crackles are short, explosive sounds from bubbling or opening of airway

• Wheezes are long, musical sounds due to airway narrowing

• Pleural fluid or pulmonary consolidation means there are absent/radiating cardiac sounds

• In horses, pleural friction rub indicates inflamed visceral and parietal pleura, and borborygmi over the thoracic cavity are common

• Coughing is an indication of disease in the lower respiratory tract

• Determine if the cough is productive/nonproductive, diurnal, or paroxysmal

• Pneumonia means there will be productive coughing

• Nonproductive coughs will occur if diseases are irritating and noninfectious

• Coughing up white, pink, or blood-tinged foam means terminal cardiac disease

• Exudate appearance cannot determine productive from nonproductive cough because horses will swallow the coughed-up material