Im scared that my dog is going to die ! ?

My dog has heart murmur and some disease in his teeth that goes to his bloodstream . Idk how old he is cause we got him from someone else . Hes a **** tzu . And this morning he was on my bed and he ran upstairs and started coughing and then he just fell down and started shaking like crazy and started crying so much and i think he had a seizure ! It stopped after a few seconds and i took him for a walk after a few hours . Idk what to do ! Im so scared and i love him so much !