What are the expected ECG changes in an adult patient with a probable old inferior myocardial infarct (MI) and a history of cardiovascular disease?

ECG Changes in Probable Old Inferior Myocardial Infarction

In a patient with a probable old inferior MI, the ECG will typically show pathological Q waves (≥0.03 seconds duration and ≥0.1 mV deep) in at least two contiguous inferior leads (II, III, aVF), often accompanied by T-wave inversions or normalized T-waves in the same distribution. 1

Diagnostic Q-Wave Criteria for Old Inferior MI

The hallmark finding is the presence of pathological Q waves meeting specific criteria: 1

• Q waves ≥0.03 seconds in duration AND ≥0.1 mV (1 mm) deep in leads II, III, or aVF
• Must be present in at least two contiguous leads of the inferior lead grouping (II, III, aVF) 1
• Alternatively, a QS complex (entirely negative deflection) in these leads indicates prior infarction 1

Important Caveats About Q Waves

Not all Q waves in inferior leads represent pathology: 1

• Isolated Q waves in lead III alone may be normal, especially without repolarization abnormalities in leads II or aVF 1
• Q waves <0.03 seconds and <25% of R-wave amplitude in lead III are normal if the frontal QRS axis is between 30° and 0° 1
• Always compare with prior ECGs when available to confirm these are established (old) rather than evolving changes 1

Associated Repolarization Changes

Beyond Q waves, old inferior MIs demonstrate characteristic T-wave and ST-segment patterns: 1

• T-wave inversions in leads II, III, and aVF are common and increase diagnostic likelihood when accompanying Q waves 1
• Normalized or upright T waves may be present if the infarct is remote (months to years old) 1
• Absence of ST-segment elevation distinguishes old from acute infarction—ST elevation suggests acute or recent MI, not old infarction 1
• Minor Q waves (0.02-0.03 seconds) become more suggestive of prior MI when accompanied by inverted T waves in the same lead group 1

Temporal Evolution and Q-Wave Regression

A critical pitfall is that Q waves can regress or disappear over time, particularly in successfully reperfused infarctions: 2

• The incidence of non-diagnostic ECGs increases from 23% immediately post-infarction to 44% at 5-year follow-up 2
• Smaller infarct sizes (particularly <6.2% of left ventricular mass) are more likely to show Q-wave regression 2
• Non-anterior (inferior/lateral) infarcts show higher rates of Q-wave regression compared to anterior infarcts 2
• Therefore, absence of Q waves does not exclude prior inferior MI, especially if the infarct occurred years ago 2

Localization Accuracy and Limitations

While Q waves in inferior leads indicate inferior MI, the ECG has limitations in precise localization: 2

• In 7 of 19 nonanterior Q-wave infarcts, the location or extent was misjudged by ECG alone 2
• The ECG cannot reliably distinguish between inferolateral, inferoposterior, or isolated inferior involvement without additional leads 1
• Right ventricular involvement should be assessed with right precordial leads (V3R, V4R) showing ST elevation ≥0.05 mV in acute settings, though this is less relevant for old infarcts 1

Differential Diagnosis and Confounders

Several conditions can produce Q waves mimicking old inferior MI: 1

• Pre-excitation (Wolff-Parkinson-White syndrome) with posterior accessory pathways creates pseudo-infarct patterns with inferior Q waves 1, 3
• Left ventricular hypertrophy, left anterior hemiblock, or LBBB may generate Q waves without infarction 1
• Cardiomyopathies (dilated, hypertrophic, stress-induced, or infiltrative like amyloidosis) produce Q waves from myocardial fibrosis 1
• Acute cor pulmonale or hyperkalemia can transiently create Q waves 1

Key Distinguishing Features

To differentiate true old MI from mimics: 1

• Regional wall motion abnormalities on echocardiography or fixed perfusion defects on nuclear imaging confirm true infarction 3, 2
• Comparison with old ECGs is invaluable—new Q waves suggest recent infarction or alternative diagnosis 1
• Clinical context matters: presence of coronary artery disease risk factors, prior symptoms, or known CAD increases likelihood of true infarction 3, 4

Prognostic Implications

The presence of established Q waves from old inferior MI carries prognostic significance: 1

• Q waves ≥0.04 seconds indicate high likelihood of significant coronary artery disease 1
• They suggest prior myocardial necrosis and reduced left ventricular function in that territory 1
• However, inferior MI with Q waves has better prognosis than anterior MI with Q waves 4
• Abnormal Q waves on admission ECG in inferior MI are not independently associated with adverse outcomes, unlike anterior MI 4

Practical Diagnostic Approach

When evaluating an ECG showing probable old inferior infarct: 1

1. Measure Q-wave duration and depth in leads II, III, and aVF—must meet ≥0.03 sec and ≥0.1 mV criteria in ≥2 contiguous leads 1
2. Assess for T-wave abnormalities in the same distribution to increase diagnostic confidence 1
3. Verify absence of ST elevation to confirm this is not acute/recent infarction 1
4. Compare with prior ECGs if available to establish chronicity 1
5. Consider imaging confirmation (echocardiography or nuclear perfusion) if diagnosis impacts management, especially given Q-wave regression over time 2
6. Evaluate for confounders like pre-excitation, bundle branch blocks, or cardiomyopathy that could produce pseudo-infarct patterns 1, 3