The ECG shown here is from a 63 year old female who came to Emergency Department because of shortness of breath of rather sudden onset. She was taking l-thyroxin for borderline hypothyroidism and mild obesity and was in the habit of adjusting her dose of thyroxin as she felt appropriate for weight control and her desired activity level. The physical examination revealed a radial pulse rate of 115 and bilateral pulmonary rales. The chest X-ray showed pulmonary edema. What do you think accounts for these findings?