What are the guidelines for performing and interpreting an electrocardiogram (ECG) in a patient with a history of heart conditions, considering their age, symptoms, and medical history?

ECG Guidelines for Patients with Heart Conditions

In patients with known cardiovascular disease, obtain a 12-lead ECG at baseline, whenever symptoms change, before and after therapeutic interventions, and periodically based on disease severity—with all interpretations requiring physician verification regardless of computer analysis. 1

When to Perform ECG in Cardiac Patients

Class I Indications (Must Perform)

Baseline and initial evaluation:

• All patients with known cardiovascular disease or dysfunction require a baseline ECG 1
• Patients with suspected cardiac disease based on symptoms (chest pain, syncope, near-syncope, palpitations, dyspnea), abnormal physical findings, or previous abnormal ECGs 1

Symptom-driven repeat ECGs:

• Syncope or near-syncope 1
• Unexplained change in usual angina pattern 1
• New or worsening dyspnea 1
• Extreme unexplained fatigue, weakness, or prostration 1
• Palpitations 1

Therapy monitoring:

• When prescribed therapy produces ECG changes correlating with therapeutic response or disease progression 1
• When therapy may produce adverse cardiac effects detectable by ECG (digitalis, dopamine, dobutamine, antiarrhythmics, doxorubicin, lithium, antidepressants, antipsychotics) 1, 2
• After initiation, changes, or addition of interacting drugs with cardiac effects 1

Procedural ECGs:

• Just before and immediately after cardioversion (electrical or pharmacologic) and before hospital discharge 1
• Soon after pacemaker insertion/revision, when malfunction suspected, after lead threshold maturation, and at periodic intervals throughout patient lifetime 1
• Serial ECGs after cardiac or extensive pulmonary surgery until stable and before discharge 1

Preoperative assessment:

• All patients with known cardiovascular disease undergoing cardiac or noncardiac surgery 1

Class II Indications (Reasonable to Perform)

• Patients with hemodynamically insignificant heart disease, minimal-to-mild hypertension, or infrequent premature complexes without organic heart disease undergoing surgery 1
• Patients with implanted pacemakers or antitachycardia devices for routine follow-up 1

Class III Indications (Not Indicated)

• Patients receiving therapy not known to produce ECG changes 1
• Adult patients with benign, stable cardiovascular conditions unlikely to progress, unless clinical status changes 1

Follow-Up Frequency

The interval for periodic ECGs depends on:

• Natural history of the specific disease 1
• Patient age 1
• Therapy effectiveness 1
• Disease severity at last evaluation 1

General recommendation: Yearly ECGs for progressive cardiovascular diseases, with more frequent monitoring based on clinical changes 1

Critical Interpretation Requirements

Physician Oversight is Mandatory

Computer interpretation alone is never acceptable—all ECGs require qualified physician verification. 1, 3, 4 Computer programs provide accurate heart rate, intervals, and axes, but rhythm disturbances, ischemia, and infarction interpretations require careful physician over-reading 1

Systematic Interpretation Approach

Rate and rhythm:

• Normal heart rate: 60-100 bpm 3, 4
• Calculate by counting QRS complexes in 6-second strip × 10, or 300 ÷ number of large boxes between R waves 4

Intervals:

• PR interval: 120-200 ms (3-5 small boxes) for AV conduction assessment 3, 4
• QRS duration: <120 ms (<3 small boxes) for ventricular conduction 3, 4
• QTc: <450 ms (men), <460 ms (women) using Bazett's formula 3, 4

Axis determination:

• Normal: +90° to -30° (positive in leads I and aVF) 4
• Left axis deviation: -30° to -90° (positive I, negative aVF) 4
• Right axis deviation: +90° to +180° (negative I, positive aVF) 4

Ischemia/infarction:

• ST elevation: >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads suggests acute injury 4
• Pathological Q waves: >0.04 seconds or >25% of R wave amplitude suggests MI 4
• Location determines coronary territory involved 4

Technical Standards

Minimum frequency response:

• Adults/adolescents: 150 Hz 3, 4
• Children: 250 Hz 3, 4

Inadequate high-frequency response causes systematic underestimation of signal amplitude and smoothing of Q waves and notched QRS components. 4

Common Pitfalls to Avoid

Electrode misplacement (especially precordial leads) significantly alters interpretation and causes false diagnoses 3, 4

Over-reliance on computer interpretation without physician verification leads to frequent diagnostic errors 3, 4

Interpreting ECG in isolation without clinical context may lead to inappropriate management decisions 1, 4

Failure to compare with previous ECGs when available misses important changes 4

Ignoring technical artifacts (baseline wander, electrical interference, poor electrode contact) compromises interpretation 4, 5

Clinical Context Integration

The same ECG pattern may represent different pathophysiologic states—always integrate clinical data including:

• Age and gender 6
• Cardiovascular diagnosis 6
• Current medications 6
• Electrolyte abnormalities 6
• Symptoms and their temporal relationship to ECG findings 1, 7

Physician judgment may transcend specific guideline recommendations when acute illness presents without typical cardiac symptoms or risk factors. 1

Special Populations

Athletes: Sinus bradycardia ≥30 bpm is a normal variant 4

Children >1 month: T wave inversion in V1-V3 is often normal 4

Neonates: Normal axis ranges 55-200° at birth, decreasing to ≤160° by 1 month 4

Maintaining Competency

Continuing medical education through seminars or self-assessment programs is essential for maintaining current ECG interpretation skills, especially for physicians who read ECGs infrequently. 3, 4 Interpretation varies greatly even among expert electrocardiographers, underscoring the need for ongoing education 3, 4