Shortness of breath is caused by fluid accumulation and congestion in the lungs. Wet lungs cannot exchange oxygen easily. In the early stages of heart failure you will probably only experience breathlessness after exercise, but if your heart failure progresses you may feel breathless when resting as well.
You may feel more breathless lying down because the fluid in your lungs moves with gravity, making more of your lungs wet (think of liquid in a bottle upright and then lying on its side).
If you do feel breathless at night or when lying down, try supporting yourself with pillows so you are lying in a more upright position. If you consistently feel breathless at night contact your doctor or nurse.
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