BACKGROUND: We investigated the presence of low QRS voltage (LQRSV) in a large sample population presenting for cardiovascular diseases. Further studies on LQRSV prevalence and clinical implications are warranted. METHODS: We conducted a cross-sectional study using ECG data from the National Center for Cardiovascular Diseases of China, collected from January 2015 to December 2023. LQRSV was defined as QRS amplitude <
0.5 mV in limb leads (L-LQRSV), <
1.0 mV in precordial leads (P-LQRSV), or both (C-LQRSV). The primary endpoint was a composite of in-hospital death or discharge against medical advice. RESULTS: We analyzed ECG data from 712,362 patients, with echocardiographic data from 454,717, cardiac magnetic resonance (CMR) results from 17,712, and hospitalization data from 187,125 patients. LQRSV prevalence was 2.0%, with 81.4% L-LQRSV, 12.3% P-LQRSV, and 6.4% C-LQRSV. The most common cardiomyopathies associated with LQRSV were cardiac amyloidosis (49.4%), arrhythmogenic cardiomyopathy (31.2%), and Takotsubo syndrome (25.7%). C-LQRSV patients exhibited worse echocardiographic parameters and higher prevalence of myocardial fibrosis on CMR compared to those with L-LQRSV or P-LQRSV or normal QRS voltage (NQRSV). LQRSV was an independent predictor of in-hospital death or discharge against medical advice (OR 3.27, 95% CI 2.94-3.64, P <
.002), and C-LQRSV conferred the highest risk of primary endpoint (12.41%, OR 8.63, 95% CI 6.72-11.07, P <
.002). CONCLUSIONS: LQRSV is an uncommon ECG pattern associated with severe cardiac dysfunction and the majority presents in limb leads, few in precordial leads, a tiny minority in both. C-LQRSV might be an ominous marker indicating the highest risk of adverse clinical outcomes.