Medical management of upper airway obstruction in dogs (Proceedings)
Clinical signs of upper airway obstruction
Dogs and cats with partial or complete upper airway obstruction can present with varying degrees of dyspnea. Animals with complete airway obstruction may present in an agonal state and are generally quite easy to recognize! In these cases, absence of air movement is accompanied by frantic or gasping attempts to breathe with an open or gaping mouth.
Patients with varying degrees of partial airway obstruction usually have a fairly typical presentation. The hallmark clinical sign of upper airway obstruction is the presence of an audible noise during breathing. Forced movement of air through narrowed airways results in generation of loud sounds that can easily be heard without a stethoscope. Animals with laryngeal disease often make a rasping noise that is worse during inspiration, which is termed stridor. In contrast, animals with nasal, pharyngeal or soft palate disease tend to make of a snoring noise that can occur during either inspiration or exhalation, which is termed stertor. Animals with laryngeal disease often have a history of a change in sound quality of their bark.The pattern of respiration can also provide some useful information about the location of the respiratory disease. Animals with dynamic upper airway obstruction (for example laryngeal paralysis) tend to have prolonged inspiratory efforts as they try to move air into the lungs through the narrowed part of the airway. The respiratory rate may not be greatly increased above normal, but there is increased respiratory effort, sometimes associated with rhythmical opening of the mouth towards the end of each inspiration. In these animals, the expiratory phase of respiration is often completely normal as the pressure changes in the airway tend to "blow open" the larynx during exhalation. In contrast, animals with fixed upper airway obstructions, for example brachycephalic dogs or those with airway masses, tend to have trouble during both inhalation and exhalation.
The condition of animals with upper airway obstruction is often considerably worsened by stress or exercise. Even slight increases in oxygen demand can result in serious problems for some of these patients. Increased respiratory drive stimulates an increase in respiratory effort, which results in generation of greater negative pressure within the airway. Negative pressure within the airway tends to exacerbate collapse of the upper airway at the site of the obstruction, much as would happen if attempts are made to suck on a straw that is blocked at the end. The more the animal tries to inhale against the obstruction, the worse the airway obstruction becomes. A vicious cycle of progressive obstruction and dyspnea therefore ensues, that can spiral out of control within minutes.
Dyspnea can also be profoundly worsened by the presence of hyperthermia. Increased activity of the respiratory muscles, combined with stress associated with high ambient temperatures and excitement, often results in significant elevation of the body temperature of these patients. Normal thermoregulation in the dog involves evaporative cooling as water evaporates from the surface of the tongue during panting. In animals with upper airway obstruction, there may be minimal to no movement of air over the surface of the tongue, considerably restricting their ability to thermoregulate. The higher the body temperature becomes, the more they attempt to pant, resulting in generation of even greater swings in airway pressure, and further worsening of the airway obstruction. Extreme hyperthermia can have serious consequences including disseminated intravascular coagulation and shock.
Animals with upper airway obstruction are prone to the development of secondary pulmonary complications that can also exacerbate respiratory distress. The most common complication is aspiration pneumonia, particularly if the laryngeal function is abnormal and the airway is therefore unprotected. Regurgitation and vomiting are common consequences in these patients as a result of aerophagia, further exacerbating the risk of aspiration. The other common pulmonary complication is the development of non-cardiogenic pulmonary edema due to profound negative pressure within the thorax and poorly understood changes in vascular permeability.
Thus, animals with partial airway obstructions frequently present with a chronic history suggesting that airway disease has been present for a prolonged period of time. The acute presentation is often associated with a sudden and rapid decompensation to respiratory distress that is often triggered by stress, exercise, excitement, hyperthermia, or the development of pulmonary complications.