Researchers in the UK have discovered that moderate to severe obstructive sleep apnea (OSA) can cause changes in the heart's shape and function, similar to the effects of hypertension. These changes include increased mass, thickening of the heart wall and reduced pumping ability. British researchers evaluated 40 patients with moderate to severe obstructive sleep apnea and compared the results with those obtained from 40 people with high blood pressure and 40 healthy people.The OSA patients had abnormal cardiac structure and performance changes typically associated with chronic high blood pressure, even though their blood pressure was only moderately elevated.The National Sleep Foundation estimates that more than 18 million Americans have some form of OSA.