15 year old dog, coughing and falling over. What's wrong?

My Shelty dog, whose 15 years old, is coughing periodically and at random times passes out. I've considered heart murmur or lung cancer. Does anyone know anything about what's wrong with her?

    15 year old dog, coughing and falling over. What's wrong?

    My Shelty dog, whose 15 years old, is coughing periodically and at random times passes out. I've considered heart murmur or lung cancer. Does anyone know anything about what's wrong with her?...
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