Myocardial Infarction Stages and Characteristics of Each Phase

Myocardial infarction, commonly known as a heart attack, is typically categorized into four distinct phases: the hyperacute phase, acute phase, subacute phase, and chronic (old) phase. Each phase is marked by specific electrocardiogram (ECG) changes and clinical features that reflect the progression of heart muscle damage over time.

Hyperacute Phase

The hyperacute phase occurs within the first few minutes after the onset of myocardial infarction. During this critical period, ECG readings may show tall, peaked T waves, which are often the earliest signs of myocardial injury. These changes typically appear before the development of more definitive signs of heart damage, such as ST-segment elevation. This phase quickly transitions into the acute phase.

Acute Phase

The acute phase generally spans from a few hours to several weeks following the heart attack. One of the most notable ECG characteristics during this stage is the elevation of the ST segment with a convex upward pattern, often described as a "bowtie" shape. Over time, this ST elevation gradually subsides. Patients in this phase may still experience chest pain and other symptoms related to myocardial injury, and they remain at high risk for complications such as arrhythmias or heart failure.

Subacute Phase

The subacute phase, sometimes referred to as the recent phase, occurs several weeks to months after the heart attack. By this time, the ST segment typically returns to the baseline level on the ECG. Additionally, the previously inverted T waves—indicative of ischemia—begin to normalize and become less pronounced. However, the Q waves, which signify areas of myocardial necrosis, continue to persist, serving as a lasting marker of prior heart damage.

Chronic or Old Phase

The chronic or old phase begins approximately 3 to 6 months after the acute myocardial infarction and may continue indefinitely. In this phase, the heart has completed the healing process, and the most prominent ECG finding is the presence of persistent pathological Q waves. These waves indicate scar tissue formation in the affected area of the heart muscle. While the acute symptoms have resolved, patients may experience long-term effects such as reduced cardiac function or ongoing angina.