I understand what the OP is saying. I get your thinking that if the pt was ambulatory, then why use heparin. The answer is not necessarily about the pts ability to move around, but it depends more on how diseases affect their hematological status. There are a number of factors that can cause DVT like surgery or immobility, pts can clot because of hormone production, stress, infection, sepsis, and newly prescribed medications. Cytokines and other anti-inflammatory pathways can begin a cascade that can lead to clots. Endothelial injuries (which can be caused by hypertension or a decrease in blood flow like anemia) can lead to bad clotting events. Endothelial injury is basically what surgery does...the cutting and manipulation of vessels and tissue. Dead tissue, like an area of necrosis, or even a case of vasculitis from a medication can release factors that initiate the clotting cascade. Mobility is a huge factor with clot prevention, but you can't d/c heparin just because they are walking around. Mobility just helps with prevention, it's not the end-all and be-all of prevention.