A 52-year-old patient, who has been suffering from angina pectoris for 2 weeks, has more and more frequent pain attacks in the area behind his sternum and his need for nitroglycerine increased. Objectively: the condition is of moderate severity. Skin is pale. Heart sounds are weakened, rhythmic. Heart rate is 84 per minute. ECG shows no signs of focal myocardial damage. What is the most likely diagnosis?