What is the systematic approach to interpreting an electrocardiogram (ECG) as a Resident Medical Officer (RMO) for a patient with potential cardiac conditions, such as hypertension, diabetes, or a history of cardiovascular disease, presenting with symptoms like chest pain?

Systematic ECG Interpretation for RMOs

As an RMO evaluating a patient with chest pain or suspected cardiac disease, interpret the ECG using a structured approach that prioritizes life-threatening conditions first, always correlates findings with clinical presentation, and never relies solely on computer interpretation. 1

Initial Assessment: Rate and Rhythm

• 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 consecutive R waves 2
• Identify the underlying rhythm by confirming a P wave precedes each QRS complex with consistent PR interval for sinus rhythm 2
• Recognize normal variants: sinus rhythm is 60-100 bpm in adults; bradycardia (<60 bpm) may be normal in athletes, tachycardia (>100 bpm) during physiological stress 2
• Note any irregularities including premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 2

Critical Step: Identify Life-Threatening Findings First

In patients presenting with chest pain, immediately assess for ST-segment elevation (>0.1 mV in limb leads or >0.15-0.2 mV in precordial leads) indicating acute injury requiring emergent intervention. 2

• Look for pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggesting myocardial infarction 2
• Assess for low voltage ECG (<5 mm in limb leads, <10 mm in precordial leads), which demands immediate evaluation for cardiac tamponade, infiltrative cardiomyopathy, or advanced heart failure 3
• Evaluate hemodynamic stability if low voltage is present, looking specifically for signs of tamponade, acute decompensation, or syncope 3

Intervals and Conduction Analysis

• Measure PR interval (normal: 120-200 ms) to assess AV conduction and identify heart blocks 2
• Evaluate QRS duration (normal: <120 ms) to detect ventricular conduction delays or bundle branch blocks 2
• Calculate corrected QT interval (QTc) using Bazett's formula; normal is <450 ms for men, <460 ms for women 2
• Identify conduction abnormalities including AV blocks, bundle branch blocks, or pre-excitation patterns 2

Axis Determination

• Determine electrical axis by examining leads I and aVF: normal axis (+90° to -30°) shows positive deflections in both leads 2
• Identify left axis deviation (-30° to -90°): positive in lead I, negative in aVF 2
• Identify right axis deviation (+90° to +180°): negative in lead I, positive in aVF 2
• Recognize extreme axis deviation (+180° to -90°): negative in both leads I and aVF 2

Chamber Enlargement and Hypertrophy

• Apply voltage criteria for left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV (Sokolow-Lyon criterion) 2
• Assess for atrial enlargement patterns including P wave morphology changes 2
• Note that QRS voltage criteria decline with age and vary by population, affecting sensitivity 2

Ischemia, Injury, and Infarction Evaluation

• Examine ST-segment deviations systematically across all leads to determine affected coronary territory 2
• Assess T-wave abnormalities including inversion, hyperacute changes, or flattening 2
• Identify the location of abnormalities to determine which coronary artery is involved 2
• Always correlate ECG findings with clinical presentation: the same ECG pattern has different implications depending on symptoms 1, 2

Critical Pitfalls to Avoid

Never accept computer interpretation without physician verification—automated systems produce frequent errors, particularly with arrhythmias (0-94% accuracy). 1, 2

• Verify electrode placement before interpretation, as misplaced precordial leads significantly alter findings and cause false diagnoses 2
• Check for technical artifacts including inadequate filtering, baseline wander, and electrical interference that distort waveforms 2
• Always compare with previous ECGs when available to identify new changes 2
• Recognize that noncardiologists are more influenced by patient history than cardiologists when interpreting ECGs, making clinical context particularly important 2

Special Considerations for High-Risk Patients

In patients with hypertension, diabetes, or cardiovascular disease presenting with chest pain, obtain an ECG immediately as it is Class I indication. 1

• Perform serial ECGs if the condition is evolving rapidly (acute MI) or symptoms are intermittent (angina) 1
• Consider normal variants in athletes such as sinus bradycardia ≥30 bpm 2
• Account for age-specific findings: T waves often inverted in V1-V3 in children >1 month 2

When Low Voltage ECG is Present

Order transthoracic echocardiography immediately to evaluate for pericardial effusion, left ventricular dysfunction, and infiltrative cardiomyopathy 3

• Assess hemodynamic stability first and perform emergent pericardiocentesis if tamponade is suspected 3
• Order cardiac troponins, BNP/NT-proBNP, complete metabolic panel, CBC, and TSH as initial laboratory evaluation 3
• Consider cardiac MRI when echocardiography is non-diagnostic, patient age >70 years, or infiltrative disease suspected 3

Quality Assurance

• Verify signal quality before interpretation by checking electrode contact and baseline stability 2
• Ensure proper bandwidth settings: minimum 150 Hz for adults, 250 Hz for children 2
• Recognize that ECG diagnoses of structural changes (MI, hypertrophy) or pathophysiologic changes (electrolytes, drugs) are made by inference and subject to error 1