ECG Diagnosis: Inferior Wall Myocardial Infarction
Based on the ECG findings described, the most likely diagnosis is A) Inferior wall myocardial infarction. This diagnosis is supported by characteristic ST-segment elevations in the inferior leads (II, III, aVF) that define this condition.
Diagnostic ECG Criteria for Inferior Wall MI
The electrocardiographic diagnosis of inferior wall myocardial infarction relies on specific findings in the inferior leads 1:
- ST-segment elevation ≥1 mm in at least two contiguous inferior leads (II, III, aVF) is the hallmark finding that confirms acute inferior MI and warrants immediate reperfusion therapy 1
- ST-segment elevation in lead III greater than lead II specifically suggests right coronary artery (RCA) occlusion with 86% sensitivity and 94% specificity 2
- Reciprocal ST-segment depression in lateral leads (I, aVL) further supports the diagnosis, with an S/R wave ratio >0.33 plus ST depression >1 mm in aVL having 92% sensitivity and 94% specificity for RCA involvement 2
Why Other Options Are Incorrect
Acute pericarditis (Option B) presents with diffuse ST-segment elevation across multiple leads with characteristic PR-segment depression, not isolated inferior lead changes 1. The ST elevations in pericarditis are typically concave upward and widespread, contrasting with the convex upward ST elevations localized to inferior leads in inferior MI 1.
Pericardial tamponade (Option C) manifests with low voltage QRS complexes across all leads and electrical alternans, not ST-segment elevations 1. This is a hemodynamic diagnosis requiring echocardiographic confirmation, not an ECG diagnosis 1.
Hypothyroidism (Option D) produces sinus bradycardia, low voltage QRS complexes, and flattened T waves—none of which include acute ST-segment elevation patterns 1. This represents a chronic metabolic condition with gradual ECG changes, not acute ischemic findings 1.
Critical Clinical Considerations
- Obtain the ECG within 10 minutes of presentation when acute coronary syndrome is suspected, as this is the critical time window for diagnosis 1
- Serial ECGs at 15-30 minute intervals are essential if the initial tracing is non-diagnostic but clinical suspicion remains high 1, 3
- Compare with prior ECGs when available, as pre-existing Q waves from old inferior MI can complicate interpretation 1
- Consider right ventricular infarction by obtaining right-sided leads (V3R-V4R) in all inferior MI patients, as RV involvement occurs in up to 50% of cases and dramatically alters hemodynamic management 4
- Assess for posterior wall extension using posterior leads (V7-V9), as circumflex artery occlusions causing inferoposterior MI are frequently missed on standard 12-lead ECG 1, 3
Common Diagnostic Pitfalls
The presence of left anterior hemiblock can mask inferior MI by producing initial r waves in inferior leads, but a Q wave or Q equivalent in lead II remains diagnostic 5. Deep symmetrical T-wave inversions in inferior leads without ST elevation may represent a prior inferior MI rather than acute ischemia 1. A completely normal ECG does not exclude acute coronary syndrome, as approximately 5% of patients with normal ECGs ultimately have MI or unstable angina 1.