A 69 years old man presents with worsening substernal chest pressure after gardening. He describes the feeling as a squeezing pain that is radiating to his jaw and left shoulder. He appears anxious and is sweating profusely. His past medical history is significant for hypertension and diabetes. On examination, his blood pressure is 159/93 mmHg and he has a heart rate of 112 bpm. Three doses of sublingual nitroglycerin provide little relief. What is the most likely diagnosis?