Understanding a High LEWIS Index in EKG: Causes & Implications

The Lewis Index (or Lewis Lead configuration) is a specialized ECG technique used to enhance atrial activity recording, primarily for atrial pacing studies and atrial flutter/fibrillation analysis.

Definition

The Lewis Lead places the right arm electrode at the 2nd intercostal space, right sternal border; left arm at the 4th intercostal space, right sternal border; left leg on the right lower abdomen. This orientation maximizes atrial vector projection.

Lewis Index Calculation

Lewis Index = (Atrial Pacing Rate in bpm) / (Sinus Rate in bpm) during incremental atrial pacing.

Interpretation

• Index ≈ 1.0: Normal. Atrial conduction time ≈ ventriculoatrial (VA) interval. Intact AV node and His-Purkinje system.
• Index < 1.0 (Abnormally Low): Atrial pacing rate < sinus rate at same pacing cycle length. Suggests prolonged atrial conduction or AV nodal delay disproportionate to ventricular response. May indicate atrial myopathy, fibrosis, or drug effect (e.g., beta-blockers, calcium channel blockers).
• Index > 1.0: Atrial pacing captures at rate higher than sinus rate. Less common; may reflect enhanced automaticity or measurement artifact.

Clinical Utility

• Assess AV nodal conduction properties during EP study.
• Differentiate atrial tachycardia mechanisms.
• Evaluate sinus node function (SNRT, corrected SNRT).
• Guide ablation strategy for atrial flutter/AF.

Limitations

Not a standard diagnostic index in routine clinical ECG. Used primarily in electrophysiology labs. Interpretation requires expertise in cardiac conduction physiology.

Note: This is distinct from the "Lewis Lead" used to amplify P-waves in surface ECG for atrial rhythm diagnosis.