What is the difference between a left and lateral electrocardiogram (EKG)?

Understanding "Left" vs "Lateral" in ECG Terminology

The terms "left" and "lateral" in ECG refer to different anatomical perspectives: "left" typically describes limb lead positioning (left arm electrode) while "lateral" describes the anatomical region of the left ventricle viewed by specific ECG leads, particularly the lateral precordial and limb leads (I, aVL, V5, V6). 1

Anatomical Lead Terminology

Left Limb Lead Components

• Lead aVL (augmented Vector Left) represents the potential difference between the left arm electrode and a modified central terminal, mathematically derived as (lead I + lead III)/2 2
• The left arm electrode is placed in the left infraclavicular fossa medial to the deltoid insertion for standard 12-lead ECG recording 2
• Lead I directly measures the potential difference between the left arm and right arm electrodes 2

Lateral Lead Territory

• The lateral leads (I, aVL, V5, V6) view the lateral wall of the left ventricle and are among the most sensitive for detecting lateral myocardial ischemia or infarction 2, 1
• The lateral precordial leads (V4-V6) can detect approximately 90% of all ST depression observed in multiple lead systems 2
• Lead V5 is positioned at the anterior axillary line in the horizontal plane of V4, while V6 is at the midaxillary line 2

Clinical Significance of the Distinction

Diagnostic Implications

• Lateral leads provide critical spatial information about left ventricular electrical activity that cannot be obtained from other lead groups 1
• The lateral precordial leads (V5, V6) are independent measurements that cannot be mathematically derived from other leads, unlike the augmented limb leads 2
• Lead aVL, despite being derived from limb electrodes, provides a frontal plane view of lateral cardiac activity 1

Common Clinical Pitfalls

• Electrode misplacement significantly affects lateral leads, particularly V5 and V6, where inferior displacement (sixth intercostal space or lower) can alter voltage criteria used for ventricular hypertrophy diagnosis 2
• The left arm electrode position is particularly sensitive to placement variation, with more marked ECG alterations compared to other limb electrodes 2
• When comparing serial ECGs, inconsistent electrode placement in lateral positions can lead to false interpretation of new pathology 3

Practical Electrode Positioning

Standard Lateral Lead Placement

• V5: Anterior axillary line at the horizontal level of V4 (fifth intercostal space), or midway between V4 and V6 if the anterior axillary line is ambiguous 2
• V6: Midaxillary line at the horizontal level of V4 and V5 2
• Left arm electrode: Left infraclavicular fossa medial to the deltoid muscle border 2

Modified Positioning Considerations

• Mason-Likar positioning (used during exercise testing) places the left arm electrode in the left infraclavicular fossa and the left leg electrode at the left iliac fossa, which can alter QRS morphology and should not be used interchangeably with standard ECGs for serial comparison 2
• Torso placement of limb electrodes reduces motion artifact but distorts the central terminal, thereby altering both augmented limb leads and precordial leads 2

Integration in Clinical Practice

Ischemia Detection

• Lateral lead ST-segment changes (particularly in V5, V6, I, aVL) indicate lateral wall ischemia or infarction and require immediate clinical correlation 4
• The lateral precordial leads are essential for comprehensive ST-segment monitoring in acute coronary syndromes 2

Axis Determination

• Lead I (left-right axis) and aVL are critical for determining left axis deviation, characterized by positive deflection in lead I and negative deflection in aVF 1
• Normal cardiac axis ranges from -30° to +90°, with lead I providing the primary left-right orientation 1