Acute myocardial infarction is divided into three stages: the hyperacute phase, the acute phase, and the recovery phase. The hyperacute phase generally occurs within half an hour of the cardiac vessel blockage. During this time, the myocardium has not completely necrosed, but there can be high, pointed T-waves on the electrocardiogram (ECG), and the ST segment remains unchanged. This stage is prone to misdiagnosis. In the acute phase, which lasts for more than 30 minutes, the ST segment elevation begins to appear on the ECGs of most people. Most of these elevations are horizontal or saddleback, although slanting elevations are less common. During the recovery phase, the ST segment on the ECG gradually returns to normal, and later, T-wave changes occur. Some individuals may develop ventricular aneurysms, which might prevent the ST segment from returning completely to baseline.