Dilated cardiomyopathy (DCM) is a condition where the heart chambers become enlarged, which reduces its ability to pump properly. The left ventricle of the heart becomes enlarged (dilated) and the muscle wall becomes thinner. This gives the heart a more rounded (rather than the normal cone) shape. This causes left ventricle to become weaker is not able to pump as effectively as normal. The following image (courtesy of Wikipedia) illustrates the difference between a normal heart and a heart with dilated cardiomyopathy.