Which leads are septal on an electrocardiogram (ECG)?

Septal Leads on ECG

The septal leads on an ECG are V1 and V2, which are positioned over the interventricular septum and record electrical activity from this region of the heart. 1, 2

Anatomical Positioning and Significance

• V1 and V2 are positioned at the 4th intercostal space on the right and left side of the sternum, respectively, and reflect electrical activity from the interventricular septum 3
• Proper placement of these leads is critical, as superior misplacement is a common technical error that can lead to false diagnosis of septal infarction 1
• The H→N maneuver (placing the patient's hand against the base of their neck) can help identify the correct anatomical landmarks for proper V1-V2 placement 3

Septal Electrical Activity

• Initial electrical forces from interventricular septal depolarization are normally directed to the right and anteriorly, producing an initial positive deflection (R wave) in leads V1 and V2 4
• In normal septal depolarization, these same forces produce initial negative deflections (Q waves) in leads I, aVL, and V6 4
• QS complexes (absence of R waves) in V1 and V2 without other ECG abnormalities are often misinterpreted as septal infarction but may be due to lead misplacement or other non-pathological causes 2

Clinical Significance in Myocardial Ischemia/Infarction

• In proximal left anterior descending (LAD) coronary artery occlusion, ST segment elevation appears in V1-V4, I, aVL, and often aVR, indicating involvement of the basal interventricular septum 5, 6
• In mid-LAD occlusion (between first septal and first diagonal branches), the basal interventricular septum is spared, and ST elevation is absent in V1 but present in V2-V4 5, 6
• In distal LAD occlusion, ST elevation is more prominent in V3-V6 and less prominent in V2 5, 6

Important Considerations

• The American Heart Association recommends avoiding labeling specific leads as "septal," "anterior," "inferior," or "lateral," and instead using their original nomenclature (V1, V2, etc.) 5
• Anatomical location of ischemia/infarction should be based on the pattern of ST-segment alterations across multiple leads rather than assuming a one-to-one correspondence between lead location and myocardial region 5
• Threshold values for abnormal ST-segment elevation in V1-V3 vary by age and gender, with higher thresholds in younger patients and males 5

Pitfalls in Interpretation

• Depression of the ST segment in V1 and V2 can represent reciprocal changes from posterior or lateral wall infarction rather than primary septal involvement 5
• QS complexes in V1 and V2 without other ECG abnormalities have multiple possible causes and represent true septal infarction in only a minority of cases 2
• In asymmetric septal hypertrophy, distinctive findings may include an R/S ratio >0.2 in lead V1 and Q waves in lead V4 7