What is the immediate management and ECG interpretation for a patient with a myocardial infarction (MI)?

Myocardial Infarction: Immediate Management and ECG Interpretation

Immediate Actions (First 10 Minutes)

Obtain a 12-lead ECG within 10 minutes of first medical contact and initiate continuous cardiac monitoring with defibrillator capacity immediately. 1, 2

• The ECG must be interpreted by a qualified physician immediately to distinguish STEMI from NSTE-ACS, as this determines whether immediate reperfusion therapy is indicated 1
• Place the patient on continuous ECG monitoring with defibrillator available to detect and treat life-threatening arrhythmias 1
• If the initial ECG is equivocal but clinical suspicion remains high, repeat ECGs immediately and compare with previous tracings 1

ECG Interpretation for STEMI

ST-segment elevation ≥1 mm in at least 2 contiguous leads (except V2-V3, which require ≥2.5 mm in men <40 years, ≥2 mm in men ≥40 years, or ≥1.5 mm in women) indicates STEMI requiring immediate reperfusion. 1

Standard STEMI Patterns:

• Measure ST elevation at the J-point using standard calibration (1 mm = 0.1 mV) 1
• For inferior MI (leads II, III, aVF), obtain right precordial leads V3R and V4R to identify right ventricular infarction 1
• Look for reciprocal ST depression in opposite leads, which increases diagnostic confidence 1, 3

STEMI-Equivalents Requiring Immediate Reperfusion:

Posterior MI: ST-segment depression ≥0.5 mm in leads V1-V3 with upright T-waves represents posterior wall injury and must be treated as STEMI 1, 2

• Confirm with posterior leads V7-V9 showing ST elevation ≥0.5 mm 1, 2
• This pattern is often missed—do not withhold reperfusion based solely on ST depression if posterior MI is suspected 2

Left circumflex occlusion: May present without ST elevation in standard leads—obtain V7-V9 leads if clinical suspicion is high 1

Left bundle branch block (LBBB): Apply modified Sgarbossa criteria to identify acute MI despite LBBB 3

ECG Interpretation for NSTE-ACS

ST-segment depression, transient ST elevation, or T-wave inversion in the setting of ischemic symptoms indicates NSTE-ACS. 1

• ST depression ≥0.5 mm in ≥2 contiguous leads carries higher risk than isolated T-wave changes 1
• The magnitude matters: ST depression ≥2 mm or involvement of ≥3 leads indicates 3-4 times higher risk 1
• A normal ECG does not exclude ACS—occurs in >33% of NSTE-ACS patients 1
• Transient ST changes during symptoms that resolve when asymptomatic strongly suggest severe CAD 1

Risk Stratification by ECG Pattern (Highest to Lowest Mortality):

1. Bundle branch block or paced rhythm with ischemic symptoms 1
2. ST-segment deviation (elevation or depression) 1
3. Isolated T-wave inversion 1
4. Normal ECG 1

Immediate Pharmacologic Management

For All MI Types:

Aspirin 160-325 mg orally immediately unless contraindicated. 1, 4, 5

Oxygen only if SaO2 <90%—routine oxygen is contraindicated when saturation is adequate as it may increase myocardial injury. 1, 2

Titrated IV morphine for chest pain relief, recognizing it delays oral antiplatelet absorption. 1, 2

Sublingual nitroglycerin for ongoing chest discomfort unless contraindicated by hypotension (SBP <90 mmHg), right ventricular involvement, or recent phosphodiesterase inhibitor use. 4, 5

For STEMI (ST Elevation or STEMI-Equivalent):

Primary PCI is the preferred reperfusion strategy and must be initiated immediately. 1, 2

• Administer a potent P2Y12 inhibitor (prasugrel or ticagrelor preferred over clopidogrel) before or during PCI 1
• If primary PCI cannot be performed within appropriate time windows, administer fibrinolytic therapy within 12 hours of symptom onset if no contraindications exist 1
• Patients should bypass the emergency department and go directly to the catheterization laboratory 1

For NSTE-ACS:

Measure high-sensitivity cardiac troponin immediately—elevation above the 99th percentile with dynamic changes confirms MI. 1

• High-sensitivity troponin assays detect MI within 1 hour of symptom onset 1
• The higher the troponin level, the greater the likelihood of MI and worse prognosis 1
• Repeat troponin measurement to demonstrate dynamic changes (rising or falling pattern) 1

Dual antiplatelet therapy with aspirin plus a P2Y12 inhibitor (ticagrelor or prasugrel preferred over clopidogrel) should be initiated. 1

Critical Pitfalls to Avoid

• Do not delay reperfusion for troponin results in STEMI—the ECG diagnosis is sufficient 1
• Do not give fibrinolytics to undifferentiated ST-depression patients—this increases mortality unless posterior MI is confirmed 2
• Do not miss posterior MI—ST depression in V1-V3 with upright T-waves requires posterior leads and STEMI treatment 2
• Do not assume a normal ECG excludes MI—repeat ECGs and measure troponin 1
• Do not perform routine PCI of an occluded artery >48 hours after STEMI onset in asymptomatic patients—this is contraindicated 1

Special Populations

Cardiac arrest with STEMI pattern: Primary PCI is indicated immediately in resuscitated patients 1, 2

• Initiate targeted temperature management (32-36°C for ≥24 hours) but do not delay PCI 1, 2
• Do not use rapid infusion of cold IV fluids for pre-hospital cooling 1

MINOCA (MI with No Obstructive Coronary Artery disease): Perform additional diagnostic tests including echocardiography, cardiac MRI, and consider provocative testing to identify the specific cause (coronary spasm, spontaneous dissection, thromboembolism, myocarditis, Takotsubo) 1, 6

Post-Reperfusion Management

Monitor patients for at least 24 hours in a telemetry bed regardless of MI type. 1, 4

Initiate high-intensity statin therapy immediately. 4

Start beta-blockers if heart failure or LVEF <40% is present. 4

Begin ACE inhibitors within 24 hours if heart failure, LV dysfunction, diabetes, or anterior MI is present. 4

Continue aspirin 75-100 mg daily indefinitely plus a second antiplatelet agent for 12 months. 1, 4