Time course of ECG changes in myocardial infarction

What it is

The ECG in myocardial infarction does not stay still. It moves through an order of changes, and the same infarction looks different at ten minutes, at ten hours and at ten weeks. Reading it well means asking not only whether the ECG has changed but how far along that change it has gone.

The ECG also sets the first division within acute coronary syndrome, together with cardiac biomarkers. Unstable angina shows no ST elevation with negative biomarkers and symptoms at rest. NSTEMI shows no ST elevation with positive biomarkers. STEMI shows ST elevation with positive biomarkers. Clinical findings in combination with ECG and troponin are the mainstays of diagnosis. In cardiac arrest caused by coronary thrombosis, the 12 lead ECG shows ST segment changes, T-wave inversions, and/or Q waves.

The six stages in order

The changes run in this order:

• Normal ECG.
• Hyperacute T-wave, over minutes to hours.
• ST-elevation, over 0-12 hours.
• Q-wave developing over 1-12 hours.
• ST-elevation with T-wave inversion, over 2-5 days.
• T-wave recovery, over weeks to months.

So the change that follows the normal ECG is the hyperacute T wave, and it belongs to the first minutes to hours, before any ST elevation has had time to appear. ST elevation belongs to the early hours and is still there at two to five days alongside the T-wave inversion, and what the weeks-months stage shows is T-wave recovery.

The same order can be seen on one patient. An ECG of an inferior MI, read in the inferior leads II, III and aVF, shows ST elevation and Q-waves; the rhythm there is atrial fibrillation, irregularly irregular with a narrow QRS. The ECG of the same inferior MI later in time shows, in those same inferior leads, ST elevation, Q-waves and T-wave inversion. The T-wave inversion is what time has added, and that is the 2-5 day stage of the list above.