Apnoea puts strain on the heart, as it races to pump blood more quickly to compensate for the lack of oxygen. Fluctuating oxygen levels also cause plaque to build up in the arteries, increasing the risk of cardiovascular disease, hypertension and stroke. In the mid-1990s, the US National Commission on Sleep Disorders Research estimated that 38,000 Americans were dying every year of heart disease worsened by apnoea.