OBJECTIVE: To investigate the relationship between heart rate variability and hypercapnia. PATIENTS AND METHODS: We measured the coefficient of variation of R-R interval (CVrr) and arterial blood gas pressures in 73 patients with Duchenne muscular dystrophy. RESULTS: CVrr was negatively correlated with arterial partial pressure of carbon dioxide (PaCO(2)). In patients whose CVrr was larger than 5%, 84% of them had no hypercapnia while the other 16% had hypercapnia (PaCO(2) >45 mmHg). In contrast, 27% of those with CVrr smaller than 3% had no hypercapnia, 73% had hypercapnia and 47% had severe hypercapnia (PaCO(2) >50 mmHg). CONCLUSION: We first showed that CVrr was negatively correlated with PaCO(2), and propose that abnormally low CVrr indicates respiratory insufficiency in patients with Duchenne muscular dystrophy.