How should an electrocardiogram (ECG) be interpreted and what are the subsequent management steps for a patient with a potential acute myocardial infarction (AMI) or other cardiovascular conditions?

ECG Interpretation in the Emergency Setting

Immediate Acquisition and Initial Assessment

Obtain a 12-lead ECG within minutes of first medical contact for any patient with suspected acute coronary syndrome, and interpret it immediately to guide triage and treatment decisions. 1, 2

Timing and Technical Requirements

• Acquire the ECG as soon as possible after patient contact, particularly for chest pain, syncope, or symptoms suggesting myocardial infarction 2
• Verify proper electrode placement and adequate filtering settings (minimum 150 Hz high-frequency response for adults) before interpretation, as misplaced leads can cause false diagnoses 2
• Repeat the ECG on arrival if obtained prehospitally, as serial recordings improve diagnostic accuracy 2
• If the initial ECG is equivocal or non-diagnostic, repeat recordings every 15-30 minutes or continuously monitor for dynamic ST-segment changes 1

Systematic Interpretation Framework

Step 1: Rate and Rhythm Analysis

• Calculate heart rate by counting QRS complexes in a 6-second strip and multiplying by 10, or use 300 divided by the number of large boxes between R waves 2, 3
• Normal sinus rhythm: 60-100 bpm with P wave before each QRS and consistent PR interval 2
• Bradycardia <60 bpm; tachycardia >100 bpm 2
• Assess rhythm regularity by examining R-R intervals for consistency 3

Step 2: Interval Measurements

• PR interval: Normal 120-200 ms (3-5 small squares); prolongation suggests AV conduction delay 2, 3
• QRS duration: Normal <120 ms (<3 small squares); widening indicates ventricular conduction abnormality or bundle branch block 2, 3
• QT interval: Calculate corrected QT (QTc) using Bazett's formula; normal <450 ms for men, <460 ms for women 2, 3

Step 3: Axis Determination

• Use leads I and aVF for rapid quadrant assessment 2, 3
• Normal axis: both leads positive (+90° to -30°) 2
• Left axis deviation: lead I positive, aVF negative (-30° to -90°) 2, 3
• Right axis deviation: lead I negative, aVF positive (+90° to +180°) 2

Step 4: Critical Pattern Recognition for Acute MI

ST-segment elevation meeting STEMI criteria mandates immediate reperfusion therapy without waiting for biomarker results. 1

STEMI Diagnostic Criteria

• ST-elevation at J-point in at least two contiguous leads: 1
  • ≥2.5 mm in men <40 years in V2-V3
  • ≥2.0 mm in men ≥40 years in V2-V3
  • ≥1.5 mm in women in V2-V3
  • ≥1.0 mm in all other leads
• These criteria apply only in the absence of left ventricular hypertrophy or left bundle branch block 1

Territory-Specific ECG Findings

• Inferior MI: Record right precordial leads (V3R, V4R) to identify concomitant right ventricular infarction 1
• Posterior MI: ST-depression in V1-V3 with positive terminal T-wave (ST-elevation equivalent); confirm with ST-elevation ≥0.5 mm in V7-V9 1
• Circumflex occlusion: May present without ST-elevation; extend to V7-V9 leads if suspicion remains high despite non-diagnostic standard ECG 1

Additional Acute Ischemia Patterns

• Look for hyperacute T-waves, which may precede ST-elevation in very early presentation 1
• Pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggest prior or evolving infarction 2
• Dynamic T-wave changes including inversion, hyperacute morphology, or flattening 2

Step 5: Special Circumstances

Left Bundle Branch Block (LBBB)

• LBBB obscures standard ST-segment analysis 1
• Apply modified Sgarbossa criteria to identify acute MI in the presence of LBBB 4
• In hemodynamically stable patients with LBBB and chest pain, integrate high-sensitivity troponin results before deciding on immediate angiography 1
• Critical pitfall: More than 50% of patients with chest pain and LBBB will have a non-MI diagnosis 1

Ventricular Pacing

• Pacemaker rhythm prevents interpretation of ST-segment changes and may require urgent angiography if clinical suspicion is high 1
• Consider reprogramming the pacemaker to allow evaluation during intrinsic rhythm in non-pacemaker-dependent patients, but do not delay invasive investigation 1

Right Bundle Branch Block (RBBB)

• ST-elevation remains indicative of STEMI 1
• ST-depression in leads I, aVL, and V5-V6 indicates NSTE-ACS 1

Integration with Clinical Context

Never interpret the ECG in isolation—clinical presentation, ECG findings, and cardiac biomarkers must be integrated for accurate diagnosis and triage. 2

• Clinical signs and symptoms alone have insufficient sensitivity (35-38%) and specificity (28-91%) to rule in or rule out ACS 2
• The ECG has 76% sensitivity and 88% specificity for acute cardiac ischemia, with 68% sensitivity and 97% specificity for acute MI 2
• A skilled physician must provide differential diagnosis and compare with previous tracings, as the same ECG pattern may occur in different pathophysiologic states 2, 3

When Clinical Suspicion Remains High Despite Non-Diagnostic ECG

• Suspicion of ongoing myocardial ischemia is an indication for primary PCI strategy even without diagnostic ST-elevation 1
• Serial ECG monitoring and repeat troponin measurements are mandatory 1
• Consider extending to V7-V9 leads for posterior involvement 1
• Echocardiography can identify regional wall motion abnormalities occurring within seconds of coronary occlusion 1

Computer-Assisted Interpretation

• Computer interpretations must always be verified by a qualified physician—they are adjuncts, not substitutes 2, 3
• Computer programs accurately calculate heart rate, intervals, and axes, but interpretations of rhythm disturbances, ischemia, or infarction require physician over-reading 2
• Critical pitfall: Erroneous computer interpretations affect clinical decision-making even when physicians correctly interpret the ECG, leading to inappropriate management 5
• Field-transmitted ECG for expert interpretation is reasonable if on-site interpretation is unavailable 1, 2

Nonphysician Interpretation in Emergency Settings

• Paramedics and nurses can independently identify STEMI with mandatory initial training and ongoing concurrent medical oversight of all interpretations 1, 2
• This allows earlier catheterization lab activation and reduced door-to-balloon times 2

Subsequent Management Based on ECG Findings

STEMI (ST-Elevation Myocardial Infarction)

Initiate reperfusion therapy immediately—do not wait for biomarker results. 1

• Activate the cardiac catheterization laboratory immediately upon STEMI diagnosis 1
• Begin ECG monitoring to detect life-threatening arrhythmias and allow prompt defibrillation 1
• Administer aspirin and P2Y12 inhibitor unless contraindicated 1
• Provide pain relief with titrated intravenous opioids (morphine 4-8 mg with additional 2 mg doses every 5 minutes until pain relieved) 1
• Administer oxygen only if dyspnea, hypoxemia (monitor with pulse oximetry), or signs of heart failure or shock are present 1
• Avoid empirical high-flow oxygen in uncomplicated AMI without hypoxemia 1

NSTE-ACS (Non-ST-Elevation Acute Coronary Syndrome)

• Measurement of high-sensitivity cardiac troponin is mandatory in all patients with suspected NSTE-ACS 1
• Dynamic elevation of cardiac troponin above the 99th percentile with compatible clinical presentation indicates MI 1
• For patients presenting within 6 hours of symptom onset with initially negative troponin, remeasure between 6-12 hours after symptom onset 1
• Risk stratify for timing of invasive approach based on troponin levels, ECG changes, and clinical features 1
• Serial ECGs are warranted until the condition stabilizes 1

Non-Diagnostic or Equivocal ECG

• Repeat ECG recordings and compare with previous records when available 1
• Continue serial cardiac biomarker testing 1
• Consider noninvasive imaging (CT angiography, cardiac MRI, myocardial perfusion imaging, or echocardiography) in hemodynamically stable patients with negative biomarkers 1
• Proceed to stress testing if history, ECG, and serum markers are not diagnostic of acute infarction 1

Critical Pitfalls to Avoid

• Do not rely solely on computer interpretation without physician verification—errors remain common and affect clinical decisions 2, 5
• Do not fail to compare with previous ECGs when available—this can miss important dynamic changes 2
• Do not interpret findings without clinical context—this leads to inappropriate management and adverse outcomes 2
• Do not wait for biomarker results to initiate reperfusion therapy in STEMI—time is myocardium 1
• Do not assume LBBB or ventricular pacing rules out acute MI—use modified criteria and maintain high clinical suspicion 1, 4
• Do not dismiss patients with high clinical suspicion despite non-diagnostic ECG—occluded circumflex, vein graft occlusion, or left main disease may not show ST-elevation 1
• Do not administer empirical high-flow oxygen to all MI patients—reserve for those with hypoxemia, dyspnea, heart failure, or shock 1