Objective To identify electrocardiographic findings, especially deep Q and S waves in lead III, that differentiate athletes from patients with hypertrophic cardiomyopathy (HCM).

Methods Digital ECGs of athletes and patients with HCM followed at the Stanford Center for Inherited Cardiovascular Disease were studied retrospectively. All patients with HCM had an echocardiogram performed. A multivariable logistic regression model was used to calculate ORs for various demographic and ECG characteristics. Linear regression was used to correlate ECG characteristics with echocardiogram findings.

Results We studied 1124 athletes and 240 patients with HCM. The average Q+S wave amplitude in lead III (III_Q+S) was significantly higher in patients with HCM compared with athletes (0.71±0.69 mV vs 0.21±0.17 mV, p<0.001). In patients with HCM, III_Q+S directly correlated with interventricular septal (IVS) thickness on echocardiography (ρ=0.45, p<0.001). In a multivariable analysis adjusted for demographic and ECG characteristics, higher III_Q+S values remained independently associated with HCM compared with athletes (OR=4.2 per 0.5 mV, p<0.001). In subgroup analyses of young patients, African–American subjects and subjects without left axis deviation (LAD), III_Q+S remained associated with HCM. The addition of III_Q+S>1.0 mV as an abnormal finding to the International Criteria for athletic ECG interpretation improved sensitivity from 64.2% to 70.4%, with a minimal decrease in specificity.

Conclusion Large Q and S waves in lead III distinguished athletes from patients with HCM, independent of axis and well-known ECG markers associated with HCM. The correlation between IVS thickness in patients with HCM and III_Q+S suggests a partial explanation for this association.