The Sokolow-Lyon index is one of the oldest and most widely used ECG-based screening tools for left ventricular hypertrophy (LVH) — a simple sum of two voltage measurements that can be read directly off a standard 12-lead ECG. This guide explains how the index is calculated, what its cutoff means clinically, and why a negative result doesn't rule out LVH.

Where the formula comes from

In 1949, Maurice Sokolow and Thomas Lyon published an American Heart Journal paper correlating ECG voltages with autopsy-confirmed left ventricular mass. They found that summing the S-wave depth in lead V1 with the R-wave height in lead V5 or V6 (whichever was larger) tracked reasonably well with hypertrophy: SLI = SV1 + max(RV5, RV6). A sum greater than 35 mm (35 small boxes on standard ECG paper at 10 mm/mV calibration) became the conventional adult cutoff for a positive screen.

Because it requires no special equipment beyond a standard ECG and a ruler, the Sokolow-Lyon index remains in routine clinical and research use more than 70 years later, despite newer and more sensitive criteria having since been developed.

What the cutoff does and doesn't tell you

An SLI greater than 35 mm is generally considered a positive screen for LVH. But voltage criteria trade sensitivity for specificity: Sokolow-Lyon's sensitivity for detecting true LVH (confirmed by echocardiogram) is only around 22–42% in most studies, while its specificity is high, often 90–100%. In plain terms, a positive result is fairly trustworthy, but a negative result is not — many people with true LVH will still have an SLI under 35 mm, particularly if they are obese, elderly, or have a conduction abnormality like left bundle branch block that distorts precordial voltages.

Cornell voltage criteria (RaVL + SV3, with separate cutoffs for men and women) is a common alternative that generally performs somewhat better, and many clinicians and studies report both criteria side by side rather than relying on either alone.

How this fits into a real evaluation

ECG voltage criteria like the Sokolow-Lyon index are a low-cost first-pass screen, not a diagnosis. Echocardiography, which directly measures left ventricular wall thickness and mass, is the reference standard when LVH is suspected or needs to be confirmed. A positive Sokolow-Lyon result in a patient with a history of hypertension, for example, would typically prompt an echocardiogram rather than a treatment decision on its own.

This calculator performs the SV1 + max(RV5, RV6) arithmetic and applies the standard 35 mm cutoff — it does not read an ECG, diagnose LVH, or replace a clinician's interpretation of the full tracing and clinical picture.