An 82 lb (37kg) 8 year old neutered male German Shepherd mixed-breed dog is being evaluated because of a two-month history of decreased exercise tolerance. The client reports that the dog had two episodes of profound weakness on walks during the past week. The dog seems comfortable at home, but his heart is racing all the time. Two years ago, another veterinarian diagnosed the dog with hypothyroidism and prescribed oral levothyroxine 0.8mg twice daily. Dosage was reevaluated one month after initiation of treatment and was determined appropriate. Appetite, urination, and bowel movements are normal. BCS is 6/9. Temp is 101.2*F and heart rate is 180 bpm. The dog is panting. PE detects a pulse deficit that occurs three to five times each minute. No other abnormalities are noted. Results of complete blood count, serum biochemistry profile, and urinalysis are within normal limits. Which of the following is the most likely explanation for the episodic weakness in this dog?
a. Decreased cardiac output
b. Electrolyte imbalance
c. Neurogenic weakness
d. Osteoarthritis
e. Ventilation-perfusion mismatch