@felicityr I would like to offer some observations in my own experience and things that are easily overlooked that relate to breathing issues. I am a Mayo surgical patient. I have allergies and asthma and a physical problem that affects breathing because of thoracic outlet syndrome. I had a whiplash injury about 20 years ago and worked a high stress desk job. I developed TOS, but it was unrecognized for a long time. I had neck stiffness for years and eventually C5/C6 ruptured and collapsed with an osteophyte complex pressing into my spinal cord. I came to Mayo for spine surgery. I had been working with a physical therapist for several years before the spine problem was evident and continued again after recovery from surgery. My PT does myofascial release and has shown me how to self treat and I do. My pattern of fascial tightness is from the left side of my jaw and neck, through my chest and rib cage and into the left side of my pelvis. All of that affects my ability to properly move my ribs enough on the left side and can pull my pelvis out of alignment, and I was getting a repeating pattern of chest infections because of trapped phlegm. Stress can cause more tightness in these affected muscles. I have made a lot of progress, and I think my chest movements are better now. Previously, I have had muscle spasms that start in the neck and go into the chest near the sternum that can cause my heart to jump a little bit, but stretching out these spasms manually resolves this. The first time I experienced this, I was evaluated at the emergency room and found to have a chest infection, but no heart problems.

Recently, I also realized how much dental issues were affecting my breathing and the amount of phlegm that was my baseline. I have been doing allergy shots compounded specifically for me by my doctor for several years. I had the old silver amalgam dental fillings removed and replaced which improved my thyroid function. I have hashimoto's and the blood tests could not measure the level of antibodies because it was off the charts. After the fillings were gone, the blood tests could accurately measure the antibodies which were low, but still present, and my dosages of desiccated pig thyroid could be reduced.

As a kid, i broke my teeth which had resulted in a lot of dental work over the years with 4 root canals, crowns and bridgework. One of the root canals failed after my spine surgery, and the infection ate a hole in my jaw bone. I thought it was sinus problems, and it progressed pretty far before I realized what was happening. An oral surgeon cleared out he infection to save the tooth which was about a year after my spine surgery. Recently, a second of the root canals began failing and I realized this was just going to continue with the affected teeth. I made the decision to have the 4 teeth extracted in preparation for dental implants that will be bio-compatible zirconia. As soon as all of the root canal teeth were removed, I could breathe much easier, and my baseline of excess phlegm wasn't there any more. I still am affected by allergies and asthma, but not as badly, and I can get it under control much faster and get my lungs clear. I breathe at a slower rate now as my norm.

I mention this because physical problems in breathing are easily missed and a lot of doctors are unfamiliar with the benefits of myofascial release therapy which gets tissues re-hydrated and moving properly. No one ever questioned how a dental infection could impact chest congestion. TOS is aggravated by poor posture which tightens the front of the chest and the scapula wing out instead of staying where they belong and tight muscles tend to pull up on the first ribs holding them there. My pattern of breathing wrong is to try to use my neck and upper chest muscle rather than the diaphragm, and doing that does increase anxiety. I am one of those patients who likes to figure out how things work and I read medical information. My parents wanted me to be a doctor and I earned a pre med biology degree, but I chose not to go into medicine and I worked in research instead for awhile. When I came to Mayo for spine surgery it was after I found medical literature with cases similar to mine. All of the local surgeons who evaluated me missed understanding the problem and would not help me.

Here are some links that may be of interest and my patient story.
https://www.painscience.com/articles/respiration-connection.php
https://connect.mayoclinic.org/discussion/myofascial-release-therapy-mfr-for-treating-compression-and-pain/
https://mskneurology.com/how-truly-treat-thoracic-outlet-syndrome/ (there are lots of good articles on this website)
https://sharing.mayoclinic.org/2019/01/09/using-the-art-of-medicine-to-overcome-fear-of-surgery/