What are the typical electrocardiographic changes and their timeline after an acute myocardial infarction?

ECG Changes After Myocardial Infarction

The ECG evolves through a predictable sequence after acute MI, beginning with hyperacute T waves within minutes, progressing to ST-segment elevation within hours, followed by T-wave inversion over days to weeks, and culminating in pathological Q waves that may persist indefinitely. 1

Immediate Changes (Minutes to Hours)

Hyperacute Phase

• Hyperacute T waves are often the earliest ECG sign, appearing within minutes of coronary occlusion and preceding ST-segment elevation 1, 2
• Increased R-wave amplitude and width may develop in leads showing ST elevation, reflecting conduction delay in ischemic myocardium 1
• These changes can occur rapidly after coronary artery occlusion, sometimes before the patient reaches medical attention 2

ST-Segment Elevation Phase

• ST-segment elevation at the J-point in two or more contiguous leads develops within hours and represents the hallmark of STEMI 1
• Diagnostic thresholds are: ≥0.1 mV in all leads except V2-V3, where men ≥40 years require ≥0.2 mV, men <40 years require ≥0.25 mV, and women require ≥0.15 mV 1
• Reciprocal ST-segment depression (≥1 mm) in electrically opposite leads indicates larger area of myocardium at risk and correlates with higher myocardial salvage potential (61% vs 17%, p<0.001) when promptly revascularized 1

Early Evolution (Hours to Days)

T-Wave Changes

• T-wave inversion typically begins within 24-48 hours after ST elevation and represents a critical prognostic marker 3, 4
• Accelerated T-wave inversion (maximum negativity ≥3 mm within 48 hours) indicates successful reperfusion in 90% of cases 3
• Early T-wave inversion after reperfusion therapy is a marker of myocardial reperfusion and carries good prognosis 5

Critical caveat: Persistently positive T waves ≥48 hours after infarction or premature reversal of inverted T waves to positive deflections suggests regional pericarditis (sensitivity 100%, specificity 77%) 3

Q-Wave Development

• Pathological Q waves can appear early (within hours) or late (days) in the MI process 2
• Diagnostic criteria include Q waves ≥0.03 seconds duration and ≥0.1 mV deep in at least two contiguous leads 1
• Q waves develop in many but not all patients with MI 1

Intermediate Changes (Days to Weeks)

ST-Segment Resolution

• Complete ST-segment resolution (≥70%) within 60-180 minutes after reperfusion therapy predicts successful reperfusion and better outcomes 1, 5
• Partial resolution (30-70%) or absent resolution (<30%) correlates with worse outcomes and impaired left ventricular function 5
• ST segments gradually return toward baseline over days to weeks 2

T-Wave Evolution

• T-wave inversion may persist for weeks to months after the acute event 1
• Normalization of negative T waves during follow-up predicts recovery of regional dysfunction better than QRS changes 4
• Lack of T-wave normalization or late appearance of new negative T waves predicts unfavorable remodeling with progressive ventricular dysfunction 4

Late Changes (Weeks to Months)

Chronic Phase

• Q waves may persist indefinitely as markers of prior MI 1
• T waves may eventually normalize or remain inverted depending on extent of myocardial scarring 4
• ST segments typically return to baseline unless ventricular aneurysm develops (persistent ST elevation) 2

Serial ECG Monitoring Protocol

Acute Phase Monitoring

• Obtain initial ECG within 10 minutes of first medical contact 1
• Perform serial ECGs at 15-30 minute intervals in symptomatic patients with initially non-diagnostic ECG 1
• Serial 12-lead ECG monitoring detects injury in an additional 16.2% of AMI patients, representing a 34% relative increase in patients eligible for emergency reperfusion 1

Post-Diagnosis Monitoring

• Continue continuous ECG monitoring for 48-72 hours after MI to detect arrhythmias and ongoing ischemia 1
• Extend monitoring beyond 72 hours if hemodynamic instability, persistent ischemia, or ongoing arrhythmias present 1
• Obtain repeat ECG at 60-180 minutes after fibrinolytic therapy to assess reperfusion success 1

Special Considerations and Pitfalls

Atypical Presentations

• 10-30% of STEMI patients present with atypical symptoms and may lack classic ECG findings 1
• The ECG may be completely normal in up to 55% of NSTEMI cases initially 1
• Posterior MI shows ST depression in V1-V3 with positive terminal T waves; confirm with posterior leads V7-V9 showing ST elevation ≥0.05 mV 1

Confounding Conditions

• In left bundle branch block, concordant ST elevation (same direction as QRS) strongly suggests acute MI 1
• New or presumed new LBBB with clinical suspicion warrants immediate reperfusion therapy 6
• Ventricular pacing obscures ST-T changes; consider reprogramming pacemaker in non-dependent patients or proceed directly to angiography 6, 7

Prognostic Implications

• Patients with diagnostic changes on serial ECG have 2.5 times greater risk of ACS, 9.6 times greater risk of life-threatening complications, and 12.3 times greater risk of death 1
• More profound ST-segment shift involving multiple leads correlates with greater myocardial ischemia and worse prognosis 6
• ST-segment depression on presenting ECG carries the highest risk of death at 6 months 1

Essential action point: Do not wait for cardiac biomarker results to initiate reperfusion therapy in STEMI—the ECG diagnosis alone is sufficient 1