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Introduction Advances in multidisciplinary care have significantly improved survival in Duchenne muscular dystrophy (DMD), leading to a growing adult population with complex cardiac and respiratory involvement. In clinical practice, respiratory disease severity—particularly the need for non-invasive ventilation (NIV)—is assumed to parallel cardiac disease progression. However, whether respiratory dependence reliably reflects the severity of left ventricular systolic dysfunction in adults with DMD remains unclear. This assumption may lead to delayed cardiac surveillance or initiation of medications for cardiomyopathy.The purpose of this study is to evaluate the relationship between respiratory dependence measured through NIV use and left ventricular systolic function in adults with DMD, ultimately to assess whether NIV use identifies patients with advanced cardiomyopathy.Methods We conducted a single-centre observational study of adults (≥18 years) with genetically confirmed DMD under specialist neuromuscular follow-up in Central London, UK as of January 2020. Left ventricular ejection fraction (LVEF) was obtained from the most recent clinically indicated transthoracic echocardiogram and categorised as normal (≥50%), mildly reduced (41–49%), or reduced (≤40%). Use of nocturnal non-invasive ventilation (NIV) and co-morbidities were also recorded. The distribution of respiratory dependence across LVEF categories was examined.Results A total of 123 adults with DMD were included (mean age 23.5 ± 4.2 years). NIV use was common across all cardiac phenotypes, affecting 31/50 (62.0%) patients with normal LVEF, 14/24 (58.3%) with mildly reduced LVEF, and 28/49 (57.1%) with reduced LVEF. There was no significant association between NIV use and LVEF category (χ 2 = 0.20, p = 0.88). This shows that the severity of cardiac systolic dysfunction cannot be reliably inferred from respiratory dependence alone in adults with DMD.Conclusions In adults with Duchenne muscular dystrophy, respiratory dependence does not reliably reflect the severity of left ventricular systolic dysfunction. Using respiratory status as a surrogate marker for cardiac disease may risk under-surveillance or delayed initiation of treatment for cardiomyopathy. These findings highlight the need for proactive and systematic cardiac surveillance in adult DMD patients, regardless of respiratory disease severity.