Dogs can hyperventilate when they become very excited. Some dogs, the Chihuahua is certainly one of the breeds, become overexcited very easily. If this is the case, there are a couple of things that you can do.
When a person is hyperventilating, they breathe into a paper bag. You can help your Chihuahua by slightly covering his mouth, so that he breathes back in a bit of carbon dioxide to slow down or stop this.
You can also try to establish what is happening at the time when your Chi begins to do this. Does it happen when you come home and greet him? Does it happen when you are going to take him for a walk?
If you can pinpoint the situations that are causing your Chihuahua to become overexcited, you can then begin to train your Chihuahua to become used to those situations, at a slow and steady pace, so that he is better able to handle them.
For example, if your dog gets very excited when you enter the home, you will want to show your Chi that while this is a good thing, it is not a reason to lose control.
Train him by entering and exiting the home. Each time you enter, do not greet your dog (yes, it is tempting, but do not do it). Just read the mail for a minute, get a drink or do some activity that is quiet and takes a minute or so. Then, kneel down to your Chihuahua and in a very calm and matter-of-fact voice, greet him. Do not shower him with hugs and kisses. Just gently pat him and say hello. Once he is used to your presence, you can then begin to play with him.
You can use this type of training for just about any situation that is causing his excitement and hyperventilating. Always look at the situation, break it into smaller pieces and give your dog once piece at a time. Within a few weeks, your dog will learn that there is reason to be happy, but will be able to control himself and should no longer be overexcited.