12.5 year old Chihuahua

Hello, I have a senior chihuahua (12.5 years old) and he has a heart murmur, I believe grade 2. 3 days ago I let him outside to use the restroom before bed. He did his business while it’s 15 °F out. Walked up the flight of stairs to come back inside and once inside he started coughing, and fell to the floor. His body got all stiff and after about 20 seconds was able to regain himself. I just thought he got lightheaded and dizzy. Then he did it again tonight.   I will be bringing him to the vet first thing Monday.

Hello there and thank you for this interesting description of a new clinical sign in your older chihuahua.  Indeed, it could have been caused by a heart problem - a lack of Oxygen flow to vital muscles, for example.  However, this is not necessary the only explanation as fitting can sonetimes look surprisingly similar, as might hypoglycaemia (rare at this age) or very occasionally the effect of the pressure from coughing on certain nerves.  Your vet will take a history and examine your dog and take things from there.  If you are not sure how soon a consultation is merited, it is always a valid choice to ask for phone triage - this way the vet or nurse can listen to the case over the phone and help you to decide when to go in.  I hope that something there is of use.