What are the key considerations and steps for interpreting an electrocardiogram (ECG) in patients with symptoms suggestive of cardiac disease or risk factors for cardiac disease, such as hypertension, diabetes, or high cholesterol?

ECG Interpretation in Patients with Cardiac Symptoms or Risk Factors

An ECG is a Class I indication (strongest recommendation) for all patients with suspected cardiac disease based on symptoms, abnormal physical findings, or cardiac risk factors such as hypertension, diabetes, or hyperlipidemia, and should be obtained as an integral part of the initial evaluation. 1

Systematic Approach to ECG Interpretation

Rate Assessment

• Calculate heart rate by counting the number of QRS complexes and applying standard methods (300 divided by number of large boxes between R waves, or count R waves in 6 seconds × 10) 2
• Normal range: 60-100 beats per minute 2

Rhythm Determination

• Assess P wave presence and morphology to determine if rhythm is sinus or non-sinus origin 2, 3
• Evaluate P-QRS relationship to identify AV conduction patterns and blocks 2
• Look for irregular rhythms suggesting atrial fibrillation or frequent ectopy 2

Axis Calculation

• Determine QRS axis using leads I and aVF to identify left axis deviation (suggesting left anterior fascicular block or LVH) or right axis deviation (suggesting RVH or left posterior fascicular block) 2, 3

Interval Measurements

• PR interval (normal 120-200 ms): Prolongation suggests first-degree AV block; shortening suggests pre-excitation 2
• QRS duration (normal <120 ms): Widening indicates bundle branch block or ventricular conduction delay 4, 2
• QT interval (correct for heart rate using QTc): Prolongation >450 ms (men) or >460 ms (women) increases risk of torsades de pointes, particularly relevant with medications like antipsychotics, macrolide antibiotics, or antiarrhythmics 4, 2

Morphologic Analysis

• Q waves: Pathologic Q waves (>40 ms duration or >25% of R wave amplitude) suggest prior myocardial infarction 5, 2
• ST segments: Elevation ≥1 mm suggests acute MI or pericarditis; depression ≥0.5 mm suggests ischemia 4, 2
• T waves: Inversion in contiguous leads suggests ischemia or ventricular strain; peaked T waves suggest hyperkalemia 4, 2, 3

Chamber Abnormalities

• Left ventricular hypertrophy: Voltage criteria (S in V1 + R in V5 or V6 >35 mm) with strain pattern in patients with hypertension 5, 2
• Right ventricular hypertrophy: Dominant R wave in V1 with right axis deviation 2
• Atrial enlargement: P wave duration >120 ms (left atrial) or amplitude >2.5 mm (right atrial) 2

Risk Factor-Specific Considerations

Hypertension

• Obtain baseline ECG in all hypertensive patients to assess for LVH, which indicates target organ damage and higher cardiovascular risk 1, 5
• LVH with strain pattern warrants more aggressive blood pressure control 5

Diabetes Mellitus

• ECG is reasonable (Class IIb) in diabetic patients with abnormal baseline ECG to assess cardiac structure and function 1
• Silent ischemia is more common; look for ST-T wave changes even without symptoms 1

Hyperlipidemia/High Cholesterol

• Baseline ECG recommended when combined with other risk factors (age >40, hypertension, diabetes, smoking) to establish pre-treatment cardiac status 1, 5
• Presence of Q waves or ischemic changes upgrades risk category and intensifies lipid management 1

Symptom-Driven Urgent Evaluation

The following symptoms mandate immediate ECG with urgent cardiology referral if abnormalities are present: 1, 4

• Syncope or near-syncope: Suggests arrhythmia or conduction disease 1, 4
• New or worsening dyspnea: May indicate heart failure, ischemia, or arrhythmia 1, 4
• Unexplained change in usual angina pattern: Represents unstable coronary disease 1, 4
• Extreme unexplained fatigue, weakness, or prostration: Often manifests cardiac ischemia 1, 4
• Palpitations: Especially when accompanied by any ECG abnormality 1, 4

Medication-Related ECG Monitoring

Serial ECGs are Class I indicated when patients receive medications known to produce cardiac effects: 1, 4

• Psychotropic agents: Phenothiazines, tricyclic antidepressants, lithium, paliperidone (obtain ECG before and during therapy) 6, 4
• Anti-infective agents: Erythromycin, pentamidine, fluoroquinolones (QT prolongation risk) 4
• Cardiac medications: Digitalis, antiarrhythmics, beta-blockers (therapeutic monitoring) 1, 4
• Diuretics: Can cause electrolyte disturbances affecting conduction 4

Comparison with Prior ECGs

Always compare with previous ECG when available—new changes are far more significant than chronic findings. 4, 7 Acute ischemic patterns (ST elevation or depression) require immediate action, while chronic bundle branch block in asymptomatic patients may only warrant cardiology follow-up rather than urgent referral. 4

Common Pitfalls to Avoid

• Do not rely on automated interpretation alone: Computer algorithms have significant error rates, particularly for ischemia detection 7
• Clinical information does not bias ECG interpretation: Studies show no significant effect of clinical history on diagnostic accuracy for acute MI, so interpret the ECG objectively first 8
• Uninterpretable ECGs (left bundle branch block, ventricular pacing, baseline artifact) require alternative testing such as stress echocardiography or nuclear imaging rather than standard exercise ECG 1
• Screening ECGs in truly asymptomatic low-risk adults (no symptoms, no risk factors) are not recommended and may lead to false positives 1

Follow-Up ECG Timing

Repeat ECGs are Class I indicated for: 1

• Change in symptoms, signs, or relevant laboratory findings 1
• After initiation or change in cardiac medications 1
• Periodic assessment (e.g., yearly) in progressive cardiovascular diseases 1

Routine reassessment <1 year is not recommended in patients with no change in clinical status and no contemplated change in therapy. 1