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Older labs can be prone to a few things that can affect their repiratory system or cause panting include laryngeal paralysis (loud pant noise from the throat because the cartilage in the larynx does not open properly, pain (does will pant if they are dealing with arthritis pain etc), Allergies, pleural effusion (fluid around the lungs which can be from a tumor, heart disease, etc), pulmonary edema (fluid in the cells of the lungs which typically occurs with heart disease), stress, tracheal issue, infections, etc. While this may require a vet visit to evaluate his respiratory system, if you are noticing a blue/purple to his inner lip, gums, or toungue then this is a much more urgent situation and he should be seen immediately. I willl sometimes offer advice to give Benedryl in case this is allergies, but I would prefer you didn't treat with anything at this stage so we do not limit your vets treatment options or alter the symptoms. I hope this helps and gives you direction.