Most other causes of dyspnea are associated with mixed respiratory patterns. Small airway disease, such as feline asthma, classically presents with a mixed inspiratory and expiratory dyspnea but with a longer expiratory phase and increased abdominal effort. An inspiratory dyspnea without stridor in a cat can occasionally occur with severe, chronic pleural effusion. Dynamic upper airway obstruction is usually associated with prolonged inspiration with inspiratory stridor or stertor, followed by a short expiration. In a dyspneic animal, the respiratory pattern can sometimes help localize the level of the respiratory tract affected. The respiratory system can be divided into five divisions: the upper airway, small airways, pulmonary parenchyma, pleural space and the chest wall, and diaphragm. In addition to paradoxical abdominal movement, the intercostal muscles were actually being sucked in on inspiration. The radiograph to the left is from a cat with diffuse interstitial metastasis and extremely non-compliant lungs. This can occur due to decreased lung compliance, upper airway obstruction, diaphragmatic rupture or paralysis, and occasionally, in cats with severe pleural effusion. Paradoxical abdominal movement occurs when increased intercostal contraction draws the diaphragm and abdominal viscera cranially on inspiration and the abdominal wall moves in (i.e., the chest and abdomen move in opposite directions). Remember that deciding to actively take control of the airway (which often only requires very small doses of sedative in severely dyspneic cats) is vastly superior to tubing them following a respiratory arrest. Another flag to pull out the endotracheal tubes is the marked mydriasis that cats will develop immediately prior to respiratory arrest. Lateral recumbency due to dyspnea is a serious sign in a dog however, it often means impending respiratory arrest in a cat. Constantly changing body position in cats implies a much worse degree of dyspnea than it does in dogs. Dogs prefer to stand with abducted elbows, while cats tend to sit in sternal recumbency. Straightening of the neck and open mouth breathing occur in both dogs and cats, however, some other postural manifestations of more severe dyspnea vary between species.
One should look for the postural manifestations of dyspnea such as an extended neck, abducted elbows, open mouth breathing, an anxious facial expression, increased abdominal movement, and paradoxical abdominal movement. Much information can be gleaned from simply observing the breathing pattern of the patient while in 100% oxygen.
This should not be confused with paradoxical abdominal movement, which is a manifestation of severe dyspnea. It should therefore be intuitive that contraction of the abdominal muscles (abdominal effort) can only assist with expiration.
During normal inspiration, diaphragmatic contraction displaces abdominal viscera caudally and the abdominal wall moves out passively (i.e., the chest and abdomen move out together). A normal animal should have a respiratory rate of 15–30 breaths per minute and, because the majority of a resting inspiration is due to diaphragmatic contraction, there should be very little apparent chest movement. Initial evaluation of the respiratory system comprises respiratory rate, effort and respiratory auscultation. Once an animal has suffered a respiratory arrest, the odds are hugely stacked against you-prevention is inordinately better than cure! Apart from the most severe upper airway obstructions, most animals will benefit from a period in 100% oxygen in an oxygen cage prior to a complete major body system evaluation. Dyspneic animals are often as their most fragile immediately following presentation and gentle restraint can prove life threatening. The stress of life-threatening disease coupled with transport and the unfamiliar surroundings of a noisy emergency clinic should never be underestimated. Consequently, the risks of any manipulation must be carefully weighed against the potential benefits. In a critically dyspneic animal, even a brief major body system evaluation can prove fatal, especially in cats. Successful emergency management of the animal with difficulty breathing demands that the clinician remain acutely aware of the fragility of the dyspneic patient.