ISSN 2398-2977      

Respiratory: dyspnea

pequis

Synonym(s): Difficult/labored breathing


Introduction

  • Respiratory distress or dyspnea is characterized by an inappropriate degree of effort to breathe, based on assessment of respiratory rate, rhythm and character.
  • Cause: dyspnea can be divided into expiratory, inspiratory, both inspiratory and expiratory (mixed), and non-respiratory. Expiratory dyspnea is due to intra-thoracic airway obstruction, eg equine asthma. Inspiratory dyspnea is either caused by extra-thoracic airway obstruction, eg laryngeal hemiplegia, or intra-thoracic restrictive diseases, eg pleural effusion. Mixed dyspnea is caused by fixed obstructions in extra-thoracic airways, eg neoplasia. Non-pulmonary respiratory distress originates from outside of the respiratory tract.
  • Signs: there is an inappropriate degree of effort in breathing with changes in rate, rhythm and character of respiration. Expiratory dyspnea cases have a prolonged and labored expiratory phase with an exaggerated abdominal effort at the end of expiration ('heave'). Inspiratory dyspnea cases  have a prolonged and labored inspiratory phase with increased inspiratory effort and exaggerated diaphragmatic and external intercostal muscle activity. Other signs common to both forms and which may variably occur include exercise intolerance, inactivity, abnormal respiratory noise, nostril flare, abducted elbows, anxious expression and cyanosis.
  • Diagnosis: complete physical examination, thoracic auscultation and percussion, thoracic radiography and ultrasonography, respiratory tract endoscopy, collection of samples from respiratory tract, arterial blood gas analysis.
  • Treatment: depends on the specific cause.
  • Prognosis: good to hopeless, depending on the specific cause.

Print off the Owner factsheet on Respiratory problems in your horse - not a good wheeze! to give to your client.

Pathogenesis

Etiology

  • Difficult breathing is defined as having respiratory dyspnea, although in animals the term respiratory distress is probably more correct.
  • There is an increase in effort during the respiratory cycle and in some cases evidence of distress.
  • The dyspnea may occur throughout the respiratory cycle or mainly during inspiration or expiration.
  • Expiratory dyspnea is caused by the obstruction of the airwas within the thorax as seen in equine asthma Equine asthma:
    • Because the resistance of the small airways in the lungs is normally low (<20% of total airway resistance is attributable to them), advanced and extensive disease has to be present within the lungs before respiratory dyspnea is detected.
    • Normally small airways are pulled open by negative intra-thoracic pressure during inspiration and resistance to airflow is low.
    • During expiration intra-thoracic pressure increases and the diameter of the small airways decreases thereby increasing the resistance to airflow.
    • The inflamed airways are narrowed (bronchoconstriction, luminal inflammatory exudates and edema) causing them to collapse even more during expiration (dynamic airway collapse) and severely limiting airflow. This leads the horse to breathe at higher lung volumes and recruit abdominal muscles to increase the intrathoracic pressure during expiration.
  • Inspiratory dyspnea is most often caused by a non-fixed obstruction of the extra-thoracic airways such as the larynx or trachea, eg bilateral laryngeal paralysis   Larynx: hemiplegia   and soft palate disorders   Soft palate: dorsal displacement  :
    • The subatmospheric pressure that develops in the extra-thoracic airways during inspiration causes any structure that is not stable, eg paralysed larynx, to collapse into that airway causing an obstruction, breathing difficulties and often an abnormal respiratory noise. These cases are often worse at exercise due to the higher negative pressures generated at this time.
    • Less common disease processes that reduce the compliance of the chest wall or lungs, or act as space-occupying lesions within the thorax, can also lead to inspiratory dyspnea due to compromises in lung inflation. Examples of these restrictive diseases may be classified as intrapulmonary (pulmonary fibrosis   Lung: multinodular fibrosis  , silicosis, interstitial pneumonia   Lung: pneumonia - interstitial (foals)  ) and extrapulmonary (pleural effusion, pneumothorax   Pneumothorax  , mediastinal mass, botulism   Botulism  ).
    • Those horses with restrictive disease are often hypoxemic due to ventilation-perfusion mismatch   Lung: ventilation-perfusion mismatching  and diffusion impairment of gases (intrapulmonary) or hypoventilation (extrapulmonary).
  • Mixed inspiratory and expiratory dyspnea (an even degree of dyspnea throughout the breath cycle) is caused by fixed extrathoracic obstructions within the airway which do not change during the cycle, ie solid space-occupying  intra- or extraluminal mass affecting the nasal cavity (septum deviations, severe bilateral inflammation, ethmoid hematoma   Ethmoid: hematoma  ), pharynx (pharyngeal cysts   Epiglottis: cyst - subepiglottic  , enlarged lymph nodes, guttural pouch enlargement), larynx (epiglottiditis   Epiglottis: epiglottitis - acute  , arytenoid chondritis   Larynx: arytenoid chondritis  ) or trachea (deformity, masses, foreign body). There is often an accompanying upper respiratory tract noise.
  • Non-pulmonary respiratory distress originates from outside of the respiratory system and can be related to inadequate oxygen-carrying capacity of the blood (anemia   Anemia: overview  or dysfunction of the red blood cells), metabolic acidosis compensation, pain (laminitis   Foot: laminitis  , rhabdomyolysis   Muscle: myopathy - exertional rhabdomyolysis  , colic   Abdomen: pain - adult  ), and hyperthermia (fever   Pyrexia: overview  , exercise, environmental).

Predisposing factors

General

  • Dependent on underlying cause.

Specific

  • Dependent on underlying cause.

Pathophysiology

  • Dyspnea results from airway obstruction which can either be intra- or extrathoracic, fixed or non-fixed, ie mobile, or restrictive diseases which limit lung expansion.

Timecourse

  • Acute onset.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Mazan M R (2015) Update on noninfectious inflammatory diseases of the lower airwayVet Clin North Am Equine Pract 31 (1), 159-185 PubMed.
  • Schwarz B et al (2013) Equine multinodular pulmonary fibrosis (EMPF): Five case reportsActa Vet Hung 61 (3), 319-332 PubMed.
  • Marqués F J et al (2012) Respiratory distress due to retropharyngeal and neck swelling in a horse with mediastinal lymphosarcomaComp Cont Educ Pract Vet 34 (5), E5 PubMed.
  • Hughes K J, McGorum B C, Love S & Dixon P M (2009) Bilateral laryngeal paralysis associated with hepatic dysfunction and hepatic encephalopathy in six ponies and four horsesVet Rec 164 (5), 142-147 PubMed.

Other sources of information

  • McGorum B C & Dixon P M (2007) Clinical Examination of the Respiratory Tract. In: Equine Respiratory Medicine and Surgery. Eds: McGorum B C et al. Saunders Elsevier, USA. pp 103-117.

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