Your mum obviously has a failing heart due to her valve problem though I note she's elected not to go the surgical way. Your cardiologist can advise how severe this problem is and recurrent episodes of the lungs flooding with more fluid than normal may account for the cough. This can be quite subtle and may not be easily measurable (in terms of both timing and tools). Of course there could be completely unrelated causes for her cough. Sometimes blood pressure lowering medication (and depending on the heart problem she has she might be on one of those) could also causes a cough. Other common causes include post-nasal drip from chronic hay fever and acid coming up the food pipe (GORD). There are also other lung causes. You should discuss these with your consultant and they can investigate further if appropriate.
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