It depends on how rapidly it progresses and how severe it is. But it is a treatable condition and most people do recover from it. It is the failure of the left ventricle to pump out blood sufficiently to meet the needs of the body. It leads to damming up of blood in the left atrium and the lungs so that fluid collects inside the tiny air passages of the lung due to back pressure. This can cause severe breathlessness. A very severe form of left ventricular failure which is rapidly progressing can be life threatening. Milder forms might even resolve with rest. The cause for the left ventricular failure has to be found out and treated to prevent a recurrence. It could be due to a massive heart attack, severely elevated blood pressure or due to a heart muscle disease.