Pathophysiology of airway obstruction in horses: a review.

Obstruction of the upper and lower airways is common in horses. In the upper airway, paresis of abductor muscles of the nares and larynx allows inspiratory collapse of soft tissues, which is accentuated by factors increasing upper airway resistance and by high inspiratory flow rates. Intrapulmonary airway obstruction occurs due to accumulation of secretions, release of chemical mediators in response to a variety of stimuli, and parasympathetic stimulation of airways. Obstruction of large airways increases the work of breathing, whereas obstruction of small airways may cause no measurable increase in resistance but does cause uneven distribution of ventilation and hypoxemia, especially during exercise. Poor collateral ventilation accentuates gas exchange problems in the horse with airway obstruction and may be a factor causing intrapulmonary hemorrhage during racing.