Respiratory Arrest and Reversible Dilated Cardiomyopathy in a Hypothyroid Dog With Chronic Collapsing Episodes Progressing to Myxedema Crisis.

A 6-year-old male intact English Springer Spaniel was presented to the Cardiology Service at Charleston Veterinary Referral Center for evaluation of 2 months' duration of collapsing episodes that had progressed in frequency and severity, decreased appetite, exercise intolerance, and lethargy. On presentation, the dog was depressed and dull, had a "tragic facial expression," and a dry, thin, and brittle haircoat. A gallop sound and Grade II/VI left apical systolic murmur was ausculted with a regular rhythm but subjectively decreased pulse quality bilaterally. An echocardiogram revealed severe dilated cardiomyopathy, and a complete thyroid panel was submitted to corroborate a low total T4 that was detected on referral bloodwork from the day prior. A Holter monitor was placed to evaluate for occult ventricular arrhythmias that could explain the collapse episodes. Upon discharge from the hospital, the patient collapsed and experienced respiratory arrest. CPR was initiated and performed for approximately 2 min until spontaneous breathing occurred. Holter results (21 min in duration) at the time of arrest showed no arrhythmias and a sustained sinus rhythm. The dog was treated with heart failure medications and a loading dose of oral levothyroxine that was titrated on subsequent rechecks to achieve a total T4 within normal limits. Six months following diagnosis, his cardiac measurements had improved significantly, his cardiac medications had been reduced, and he had not experienced any additional respiratory arrest episodes since the day of his diagnosis.