Why does congestive heart failure cause diuretics to be mal-absorbed?

Swelling: Swelling in the intestinal wall. Some diuretics are better absorbed in this setting by mouth. If this fails then IV diuretics. Hope this helps.

Blood flow: Congestive heart failure can cause decreased efficacy from oral medicines in a couple of ways. 1) if the right atrial pressure is high, then that pressure will be transmitted back into the systemic venous circulation and there will be increased water in the tissues of the body, including the walls of the small intestine, creating an absorption block. 2) also, there can be relative gut ischemia.