Obstructive sleep apnea (OSA) increases the risk of cardiovascular disease and CPAP has been shown to improve heart structure in the short-term. In the journal Chest, there is an article about how CPAP improves heart structure when used long-term. Researchers studied 47 patients with severe OSA who did not have heart failure. The patients' heart structure was evaluated with echocardiography and cardiac MRI at the start of the study and then serially for one year.
After as little as three months of CPAP use, echocardiography showed improvement in heart structure and lung blood pressures - and these changes continued to improve over the one year follow up. Cardiac MRI showed improvement in heart structure starting at 6 months of CPAP use, and improvements continued over the one year follow up.
The researchers point out that the results are limited by the fact that none of the patients had heart failure at the start of the study, so we really don't know if the same improvements would be seen in those with heart failure and long-term CPAP use.