How do I interpret this ECG for a patient with possible acute coronary syndrome and a history of cardiovascular disease?

ECG Interpretation in Suspected Acute Coronary Syndrome

Obtain a 12-lead ECG within 10 minutes of patient presentation and interpret it immediately in conjunction with clinical symptoms, cardiac biomarkers, and risk factors—never rely on the ECG alone or on computer interpretation without physician verification. 1, 2, 3

Immediate Acquisition and Technical Considerations

• Acquire the 12-lead ECG as soon as possible after first patient contact, ideally within 10 minutes for patients with chest pain or symptoms suggestive of ACS 1, 2, 3
• Verify proper electrode placement and adequate filtering settings (minimum high-frequency response of 150 Hz for adults) before interpretation, as misplaced leads can cause false diagnoses 2
• Repeat the ECG on arrival if initially obtained in the prehospital setting, as serial ECGs combined with cardiac biomarkers improve diagnostic accuracy from 68% sensitivity to higher levels 1, 2
• The ECG has 76% sensitivity and 88% specificity for acute cardiac ischemia in chest pain patients, with 68% sensitivity and 97% specificity specifically for acute MI 1

Systematic Interpretation Framework

Rate and Rhythm Assessment

• Calculate heart rate by counting QRS complexes in a 6-second strip and multiplying by 10, or using 300 divided by the number of large boxes between R waves 2
• Confirm sinus rhythm by identifying a P wave before each QRS complex with consistent PR interval (normal: 120-200 ms) 2
• Normal sinus rhythm is 60-100 bpm; bradycardia <60 bpm; tachycardia >100 bpm 2

Intervals and Axis

• Measure QRS duration (normal: <120 ms) to identify bundle branch blocks that may complicate ST-segment interpretation 1, 2
• Calculate corrected QT interval (QTc) using Bazett's formula; normal <450 ms for men, <460 ms for women 2
• Determine electrical axis using leads I and aVF: normal axis when both positive (+90° to -30°) 2

Critical ST-Segment Analysis for Triage

STEMI Pattern (Requires Emergent Reperfusion):

• ST-segment elevation ≥0.1 mV (1 mm) in limb leads or ≥0.15-0.2 mV (1.5-2 mm) in precordial leads indicates complete coronary occlusion requiring immediate reperfusion therapy 1, 3, 4
• New or presumed new left bundle branch block with symptoms should be treated as STEMI equivalent 1
• Activate the cardiac catheterization laboratory immediately for primary PCI within 120 minutes of first medical contact 1, 5, 3
• If PCI cannot be achieved within 120 minutes, administer fibrinolytic therapy (alteplase, reteplase, or tenecteplase at full dose for patients <75 years; half dose for ≥75 years) 1, 3

NSTE-ACS Patterns (Require Urgent but Not Emergent Management):

• ST-segment depression (especially horizontal or downsloping) ≥0.5 mm in any lead except V1-V3 suggests subendocardial ischemia from partial coronary occlusion 1, 4, 6
• Marked symmetrical precordial T-wave inversion ≥2 mm suggests acute ischemia, particularly from critical LAD stenosis 1
• Transient ST changes ≥0.5 mm during symptoms strongly suggest severe CAD and require high-risk classification 1
• Widespread ST-segment depression with ST elevation in aVR indicates diffuse subendocardial ischemia from severe multivessel disease requiring urgent angiography 6

De Winter's Pattern (STEMI Equivalent):

• Upsloping ST-segment depression with tall, positive T waves in precordial leads represents critical proximal LAD occlusion 5, 6
• Classify as high-risk NSTE-ACS requiring immediate catheterization laboratory activation within 120 minutes 5
• These patients may respond poorly to fibrinolytics; primary PCI is strongly preferred 5

Additional ECG Findings

• Pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggest prior myocardial infarction 2, 7
• Consider additional leads V7-V9 for suspected posterior wall involvement when initial ECG is non-diagnostic but clinical suspicion remains high 1, 5
• Nonspecific ST-T changes (ST deviation <0.5 mm or T-wave inversion <2 mm) are less diagnostically helpful and require integration with biomarkers and clinical context 1

Integration with Clinical Context

Critical Principle: Never interpret the ECG in isolation. 1, 2

• Clinical signs and symptoms alone have only 35-38% sensitivity and 28-91% specificity for ACS—insufficient to rule in or rule out disease without ECG and biomarkers 1
• Compare with previous ECGs when available, as dynamic changes significantly improve diagnostic accuracy 1, 2
• Obtain cardiac troponin T or I (preferred biomarker) immediately and repeat at 6-12 hours if initial measurement is negative 1, 3, 7
• Consider echocardiography to assess regional wall motion abnormalities (occur within seconds of coronary occlusion) and exclude alternative diagnoses like aortic dissection or pulmonary embolism 1

Computer-Assisted Interpretation

• Computer interpretations must always be verified by a qualified physician—they are helpful adjuncts but not substitutes for clinical judgment 1, 2, 8
• Computer programs provide accurate measurements of heart rate, intervals, and axes, but interpretations of ischemia or infarction require careful physician over-reading 2
• Computer interpretation may increase specificity of STEMI diagnosis, especially for less experienced clinicians 1

Nonphysician Interpretation in Emergency Settings

• Paramedics and nurses can independently identify STEMI on 12-lead ECG only if there is mandatory initial training and ongoing concurrent medical oversight of all interpretations 1, 2
• Field-transmitted ECG for expert interpretation is reasonable if on-site interpretation is unavailable 1

Common Pitfalls and How to Avoid Them

Do not confuse ST elevation with non-ischemic causes:

• Left ventricular hypertrophy, early repolarization, pericarditis, and ventricular aneurysm can all cause ST elevation 4, 6
• Clinical context and comparison with previous ECGs are essential 2, 4

Do not miss STEMI equivalents:

• De Winter's pattern, Wellens' syndrome (deeply inverted T waves in V2-V4 with QT prolongation), and posterior MI (ST depression in V1-V2) all require emergent management despite lacking classic ST elevation 1, 5, 6

Do not rely on response to nitroglycerin for diagnosis:

• Reduction in chest pain after nitroglycerin is unrelated to presence or absence of ACS and should not be used as a diagnostic test 1

Do not fail to obtain serial ECGs:

• Repeat ECG with any change in symptom severity or quality, as initial ECG may be non-diagnostic in up to 41% of NSTE-ACS cases 1

Do not over-rely on computer interpretation without clinical context:

• Noncardiologists are more influenced by patient history when interpreting ECGs, making systematic approach critical 2
• Errors in computer interpretation remain common and can lead to inappropriate management decisions 2, 8

Risk Stratification and Disposition

High-Risk Features (Require Urgent Invasive Strategy within 24-48 hours):

• ST-segment depression ≥0.5 mm 1
• Elevated cardiac troponin 1, 3
• TIMI risk score ≥3 or GRACE risk score indicating high mortality risk 1
• Hemodynamic instability or ongoing ischemic symptoms 1, 6

Intermediate-Risk Patients:

• Undergo structured evaluation in chest pain unit with serial ECGs and biomarkers 7

Low-Risk Patients:

• May be discharged with appropriate follow-up if two troponin measurements are negative and ECG remains non-ischemic 1, 7