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.
