What's causing this cat's open-mouth breathing and frantic facial expression?
Sabrina, a 13-year-old spayed Siamese cat, was initially presented because of restlessness and hyperactivity, with occasional wheezing and rapid breathing at rest. Her appetite had been normal, with no vomiting or diarrhea. Moderate weight loss had occurred, but she had been fed a reducing diet for a few months. No polyuria or polydipsia had been noted. On physical examination, Sabrina was bright and alert with a body condition score of 4/9, with mild to moderate muscle wasting over her back. Her body temperature was slightly elevated at 39.8 C (103.7 F). On auscultation, tachycardia with a 2/6 cardiac murmur was detected. An electrocardiogram revealed a heart rate of 260 beats/min. with an occasional ventricular premature contraction. Because of her temperament, she did not allow cervical palpation. During examination, Sabrina developed severe respiratory distress, with open-mouth breathing and a frantic facial expression (see below).
What is the most likely cause of Sabrina's respiratory distress, cardiac problems (tachycardia, murmur, arrthymias), fever, and weight loss?