It is suspected that mammals generally adopt a respiratory pattern that meets their metabolic needs with the least metabolic energy cost. Because of the phenotypic diversity in healthy dogs and cats, how this is achieved varies dramatically between patients - what works best for a Borzoi may not work at all for a Pug. In addition to these species and breed differences, respiratory diseases cause characteristic changes in respiratory frequency, tidal volume, airflow velocity, and patterns of muscle activation that can provide clues for the clinician to help localize the cause of abnormal respiratory function. This lecture will present the basic elements of how respiratory patterns change with disease states and emphasize important physical examination findings.

Dyspnea vs increased respiratory effort

The term dyspnea refers to the experience of distress secondary to respiratory disease. In man it is most often associated with increased resistance to airflow but may also be caused by anything leading to clinically significant hypoxemia or hypercapnia (especially if acute). Like pain, its presence is inferred from facial expressions and behavioral signs of distress. Dyspnea is a true medical emergency; our goal is to relieve it within minutes of presentation. In contrast, the terms labored breathing or increased respiratory effort refer to the physical manifestations of increased work of breathing. This may be objectively measured, but most of the time is inferred from subjective appraisal of physical signs. Whereas an animal with dyspnea is having a crisis, an animal with labored breathing may be very well adapted to its condition and be free from distress. Where a given patient lies on this spectrum must be inferred from their behavior. In general, as an animal approaches dyspnea it must focus more and more conscious effort on the act of breathing. Animals that are interactive and engaged with their environment usually still have significant physiological reserves; those that appear withdrawn and focused on the act of breathing may be close to the edge of respiratory arrest. A notable exception to this generalization is the puppy. Juvenile dogs will often remain active, alert, and hungry even with advanced respiratory failure and may go straight to the brink of death before showing compelling behavioral signs of distress.

Neoplasia, constrictions, compression from abscesses or trauma: A 'fixed' obstruction refers to one that does not change appreciably with the phase of respiration. As the airway lumen narrows with advancing disease, the animal will adopt a prolonged inspiration and exhalation that is characteristic of fixed obstructions. Respiratory frequency falls as the inspiratory phase becomes noticeably prolonged. As the problem becomes more severe, more energy is expended on both inspiration and exhalation. The intercostal muscles 'suck in' from markedly negative pleural pressures created by vigorous diaphragm contraction, and the animal may actively exhale, first by contracting the intercostal muscles alone, later by a combination of intercostals and abdominal muscles. The term 'abdominal breathing', referring to active contraction of the abdominal wall, can ONLY assist with exhalation and is a response to air trapping that slows exhalation to unacceptable rates.

Neoplasia and constrictions from scarring are typically insidious in onset, initially causing clinical signs only with exercise and usually allowing plenty of time for adaptive strengthening of the respiratory muscles. If acute, as for example from cervical swelling from trauma, the animal may have the same physical features as aspiration of a foreign body, including dyspnea and cyanosis.

Foreign body: This disorder is characterized by an acute onset of respiratory difficulty, often accompanied by coughing/retching and behavioral signs of dyspnea. Marked cyanosis may be present as hypoxemia occurs before there is any adaptive shift in hemoglobin oxygen affinity and cardiovascular responses are vigorous, with good perfusion of skin and mucus membranes. In the author's experience, inspiration is more severely affected than expiration but this would be expected to vary with location of the obstruction. Most obstructions are at the level of the larynx or tracheal bifurcation.

Brachycephalic syndrome: This syndrome is classically characterized as restriction of the upper airway secondary to excessive or extreme phenotypic expression of the abnormalities selectively bred for in some chondrodysplastic breeds. Although some anatomic features of the syndrome produce a dynamic obstruction, most individuals have at least some component of fixed obstruction. Any combination of stenotic nares, stenotic nasal cavity, elongated soft palate, myopathy of the pharyngeal opening musculature, laryngeal dysplasia, everted laryngeal saccules, and trachea hypoplasia are possible.^1-3 Collapse of the cervical and/or intrathoracic portion of the trachea is sometimes seen. Some dogs have concurrent abnormalities of respiratory drive⁴, and some acquire rib fractures and/or hiatal hernia.⁵ The breathing pattern is characterized by prolonged inspiration, and (usually) comparatively easy exhalation.⁶ Affected dogs routinely develop stertorous sounds with the slightest provocation. In spite of evidently high resistance to air flow, affected animals are generally not focused on breathing and relatively free from distress, even if their arterial pO₂ is very low. This finding fits with the observation in many species that chronic hypercapnia and hypoxemia invoke adaptive responses that resets chemoreceptor response thresholds to relatively high values of pCO₂ and low values of pO₂.