When is an electrocardiogram (ECG) necessary and how is it interpreted?

Indications and Interpretation of Electrocardiogram (ECG)

An electrocardiogram (ECG) is essential for all patients with known cardiovascular disease, suspected cardiovascular disease, or those presenting with symptoms such as chest pain, dizziness, or syncope, as it provides critical information about cardiac electrical activity that can directly impact morbidity and mortality outcomes. 1

When is an ECG Necessary?

Patients with Known Cardiovascular Disease

• Required during initial evaluation of all patients with known cardiovascular disease, dysfunction, or arrhythmia 1
• Necessary when monitoring response to therapy, especially for conditions where ECG changes correlate with therapeutic responses or disease progression 1
• Indicated for follow-up of progressive cardiovascular diseases, with frequency determined by the natural history of the disease, patient age, and effectiveness of therapy 1

Patients with Suspected Cardiovascular Disease

• First-line diagnostic tool for patients presenting with chest pain, dizziness, or syncope - symptoms that may predict sudden death or myocardial infarction 1
• Indicated for patients at increased risk of developing cardiovascular disease 1
• Essential before starting medications known to produce cardiac effects, particularly psychotropic agents, anti-infective agents, and certain cardiovascular medications 1, 2

Specific Clinical Scenarios Requiring ECG

• Before and after electrical or pharmacologic cardioversion of arrhythmias 1
• After pacemaker insertion or revision, and whenever pacemaker malfunction is suspected 1
• Before surgery in patients with known cardiovascular disease 1
• When new symptoms develop, including syncope, changes in angina pattern, new/worsening dyspnea, extreme fatigue, or palpitations 1

Asymptomatic Patients

• Generally not recommended for asymptomatic patients without risk factors for cardiovascular disease 3
• May be considered as a baseline in selected high-risk asymptomatic patients, though evidence for this is less robust 3

How to Interpret an ECG

Systematic Approach to ECG Interpretation

• Follow a methodical analysis to ensure accurate interpretation 4:
  1. Assess heart rate and rhythm
  2. Evaluate PR interval, QRS duration, and QT interval
  3. Examine P wave morphology
  4. Analyze QRS complex morphology
  5. Evaluate ST segments and T waves
  6. Compare with previous ECGs when available

Key Diagnostic Information Provided by ECG

• Arrhythmias and conduction disturbances 1
• Chamber enlargement and myocardial hypertrophy 1
• Myocardial ischemia and infarction 1
• Pericardial inflammation 1
• Electrolyte disturbances 1
• Drug effects, particularly QT prolongation with certain medications 2

Special Considerations

• Additional leads may be necessary in children and selected adults 1
• Computer-interpreted ECGs should always be reviewed by a qualified physician, as automated algorithms can produce erroneous information 1, 3
• In specific conditions like hypertrophic cardiomyopathy, ECG abnormalities may be the only manifestation of disease at an early stage 5

Common Pitfalls in ECG Use and Interpretation

• Failing to obtain a baseline ECG before starting medications known to affect cardiac conduction 2
• Not recognizing patient-dependent errors, operator-dependent errors, or artifacts 4
• Neglecting to monitor electrolyte levels when interpreting ECG changes, particularly in patients on medications affecting the QT interval 2
• Overreliance on computer interpretations without physician verification 1

ECG in Modern Clinical Practice

• Despite technological advances, the ECG remains an irreplaceable first-line diagnostic tool due to its broad availability, non-invasiveness, and good sensitivity 6
• Particularly valuable in emergency settings where rapid diagnosis can significantly impact mortality outcomes 6
• Continues to be the most widely used non-invasive diagnostic and prognostic test for suspected or proven coronary artery disease 7

Remember that ECG interpretation requires understanding its sensitivity and specificity limitations, as diagnoses of structural or pathophysiologic changes are made by inference and therefore subject to error 1.