How do you determine if left axis deviation is new or old on an electrocardiogram (ECG)?

Determining if Left Axis Deviation is New or Old

The only reliable way to determine if left axis deviation (LAD) is new or old is by comparing the current ECG to previous ECGs—without prior tracings, you cannot definitively establish chronicity. 1

Practical Approach to Assessment

Step 1: Obtain and Compare Prior ECGs

• The gold standard is direct comparison with previous electrocardiograms. This is the only method that definitively establishes whether LAD is new or longstanding. 1
• Look specifically at the QRS axis measurements on prior tracings to determine if the leftward deviation was present previously. 2
• Note that LAD is defined as a QRS axis less than +30° (normal adult range is +30° to +90°). 1

Step 2: Evaluate Clinical Context When No Prior ECG Exists

If no prior ECG is available, assess the following to estimate likelihood of chronicity:

Age-Related Considerations:

• The QRS axis naturally shifts leftward with increasing age, making mild LAD more common in older adults and potentially a longstanding finding. 3, 1
• In children, LAD is distinctly abnormal (normal pediatric axis ranges from 10° to 110° in ages 1-5 years), suggesting a congenital or acquired structural abnormality if present. 3, 4

Associated ECG Findings Suggesting Chronicity:

• Left ventricular hypertrophy (LVH) criteria with LAD suggests a chronic process, as LVH develops gradually over time. 3
• Left anterior fascicular block (LAFB) criteria (qR pattern in aVL, R-peak time ≥45 ms in aVL, QRS duration <120 ms) indicates a conduction system abnormality that is typically chronic. 3, 1
• The degree of axis deviation matters: marked LAD (-45° to -90°) is more likely to represent established LAFB than mild deviation. 1

Clinical History Indicators:

• Absence of acute symptoms (chest pain, dyspnea, syncope) suggests the LAD is not related to an acute event. 4
• Known chronic conditions (hypertension, valvular disease) make chronic LAD more likely. 3, 5
• Previous cardiac imaging showing structural abnormalities indicates the LAD is likely longstanding. 6

Step 3: Identify Red Flags for Acute/New LAD

Consider LAD potentially new if:

• Acute clinical presentation with chest pain, heart failure symptoms, or hemodynamic instability. 5
• QRS axis ≤-42° in a pediatric patient without known heart disease warrants urgent evaluation. 4
• Additional acute ECG changes such as ST-segment elevation/depression, new T-wave inversions, or new bundle branch block patterns. 3
• ECG evidence of chamber enlargement or hypertrophy in a previously asymptomatic child. 4

Step 4: Determine Need for Further Workup

Echocardiography is indicated when:

• LAD is discovered in a child with abnormal cardiac physical examination findings (75% of pediatric patients with LAD and heart disease had abnormal exams). 4
• ECG shows chamber enlargement or hypertrophy criteria alongside LAD. 4
• QRS axis is ≤-42° in pediatric patients. 4
• Clinical suspicion exists for underlying structural heart disease. 5, 4

Echocardiography may not be necessary when:

• LAD is isolated (no other ECG abnormalities), the patient is asymptomatic, and physical examination is normal—particularly in adults where mild LAD can be a normal age-related variant. 1, 4

Critical Pitfalls to Avoid

• Overdiagnosing LAD as pathologic rather than recognizing it as a finding requiring clinical correlation, especially in older adults where mild leftward axis shift is common. 1
• Failing to account for age-related changes in QRS axis—what is abnormal in a child may be normal in an elderly patient. 1, 2
• Assuming LAD alone equals left anterior fascicular block without confirming additional diagnostic criteria (qR in aVL, prolonged R-peak time). 3, 7
• Missing biventricular hypertrophy when LAD is present with LVH criteria, as this combination can mask right ventricular involvement. 8
• Relying solely on automated ECG interpretations at axis borderlines, as these can be inaccurate. 2