If I remember correctly, infants can't breathe through their nose, so when they get congested, it must be really difficult to breath at all.
i wonder if there might be a homeopathic remedy that might work? I can't remember if we tried something like that.
You could also try squirting some breastmilk into her nose and then suctioning it out. Have you tried suctioning? I have had good luck with that with DD and she doesn't even seem to mind it - it's like she knows it is helping her.
If there is some way to safely prop her up at night so she is not lying flat, that can help too (helps keep the mucus from draining down her throat).
We went through all sorts of trips to the dr - everytime DS got a cold, he got this horrid cough. We ended up in the emergency room once too. Basically all they ever did there or at the dr. office was give him an inhaler treatment with Albuterol. Once they did give him steroids. The Albuterol does help relieve the coughing as it opens up the airways, but it also makes the baby kind of hyper. So be forewarned if that is what your cough syrup is.
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Oh, one more thing. Hopefully you won't, but if you ever do get to the point where they prescribe an inhaler - some food for thought: We had an awful time with the aerosol-type inhaler that comes with this little nose/mouth mask. It was seriously like child abuse trying to use it on DS. So I never did. Finally I got a Dr. who prescribed for us a motorized inhaler like the kind they always used at the doctors office. It is so much gentler, it makes a mist that you can just hold in front of the baby's face. This is probably way too much information - but just in case. If your insurance covers the motorized inhaler it is SO the better way to go.