Dyspnea refers to labored or difficult breathing. Dyspnea is often accompanied by a variable tachypnea (increased respiratory rate), which may be missed by owners. Orthopnea is a more severe form of dyspnea for which the animal requires a specific positional orientation in order to breathe. This is often a position of sternal recumbency, standing, or sitting with the thoracic limbs abducted and neck and head extended. Orthopnea is most often associated with marked pericardial and/or pleural effusion, marked pneumothorax or pneumomediastium, diaphragmatic hernia, or severe pulmonary edema.

Cyanosis, the bluish tint to normally pink mucous membranes, is secondary to an increase in reduced hemoglobin (>5 g/dL). It is a sign of severe hypoxemia and is indicative of noncompensated respiratory dysfunction that results in an inability of carbon dioxide to be exchanged for oxygen in the lung.

The purpose of breathing is to ventilate alveolar structures of the lungs for the exchange of oxygen with carbon dioxide. Breathing rate and depth is controlled centrally by respiratory centers in the brain stem (on the ventral surface of medulla) and peripherally by chemoreceptors in the carotid and aortic bodies. Central chemoreceptors sense the pH of the cerebrospinal fluid. Ordinarily, the major determinant of cerebrospinal fluid pH is the concentration of carbon dioxide. However, any substance that produces alterations in cerebrospinal fluid pH will influence breathing. Peripheral chemoreceptors are most sensitive to arterial pH and carbon dioxide and only moderately sensitive to arterial oxygen. However, if arterial carbon dioxide and pH are maintained at normal levels, a decrease in arterial oxygen that results in serious hypoxemia (PaO₂ 60 mm Hg) will increase the sensitivity of these chemoreceptors to carbon dioxide concentration and pH, thereby altering ventilation. Another often overlooked and important control of breathing is core body temperature. Evaporative heat dissipation by ventilation is the major means of temperature regulation in companion animals. Receptors in the thermoregulatory centers of the brain stem, and others, provide continuous information to the medullary respiratory centers that coordinate the muscles of ventilation (diaphragm and intercostals).