'Open Artery' After Acute Myocardial Infarction and Myocardial Viability Detected by Dobutamine Stress Echocardiography Before Hospital Discharge - A Possible Relationship? [Arteria 'Aberta' Apos Enfarte Agudo Do Miocardio. Relacao Com A Deteccao De Miocardio 'Viavel' Por Ecocardiografia De Sobrecarga Com Dobutamina Realizada Pre-Alta?]