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Causes
Temporary issues include poor air quality or an odd sleeping position. This is especially true for snub-nosed (brachycephalic) breeds.
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If this is a new thing for your dog, an airway exam should be performed by a vet. They will listen closely to your dog’s heart and lungs and may order some tests such as a chest x-ray.
Breathing that sounds more congested when a dog is asleep may be associated with the position in which your dog is sleeping, especially if they are older and/or over-weight.