My first thought is sleep apnea. Have you been tested? This just requires having your sleep monitored in a sleep lab to determine if you stop breathing while sleeping. Your metabolism will be thrown out of whack if you are experiencing sleep apnea. It will cause weight gain, and can cause sudden death. Please rule this out before you do anything else.
If you can't do any other activity, I suggest lifting light weights with your arms while on high flow oxygen. Exercise With Oxygen Therapy can be very beneficial. Come by Heart Failure Solutions for more helpful ideas. The simple things you can do for yourself deserve some attention before you risk surgery again.
Many blessings.
A low arterial blood oxygen level due to paralysis of a hemi-diaphragm is unusual, but not rare, in the absence of underlying lung disease. However, the ill-effects of such paralysis is accentuated by your being overweight. Nevertheless, that may not be enough to explain your need for relatively high flow oxygen of 3.5 liters per minute. Were that the cause, however, the surgical procedure you mention, plication of the diaphragm, might result in improved blood oxygen levels, less need for the use of oxygen and a reduction in shortness of breath.
The mitral valve replacement, itself should not have resulted in your need for oxygen unless it resulted in an alteration of blood flow within the upper chambers of your heart of a type that has also been associated with paralysis of the right hemi-diaphragm and that is called a patent foramen ovale (PFO). The following, from the medical literature, is an abstract that describes that circumstance. I know this may be hard to follow and understand but your doctors will understand it.
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Authors
Full Name Ghamande, S. Ramsey, R. Rhodes, J F. Stoller, J K.
Institution Department of Pulmonary and Critical Care Medicine, the Cleveland
Clinic Foundation, Cleveland, OH 44195, USA.
Title Right hemidiaphragmatic elevation with a right-to-left interatrial shunt
through a patent foramen ovale: a case report and literature review.
Source Chest. 120(6):2094-6, 2001 Dec.
Abstract A right-to-left shunt (RLS) is an uncommon complication of a patent foramen ovale (PFO) that may cause hypoxemia from venous admixture and ischemic complications from paradoxic embolization. This report presents the third described patient whose RLS through a PFO and profound hypoxemia developed in association with right hemidiaphragm dysfunction (but without a pressure gradient driving the right-to-left flow). In addition to extending the available experience with this unusual clinical event, we report on the successful closure of the PFO by a catheter-deployed double-umbrella device, after the positioning of which the patient's oxygenation normalized.
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The primary action to be taken at this time is to accurately identify the cause of your low oxygen levels and to seek consultation with a thoracic, commonly known as a chest, surgeon, familiar with and experienced with the surgical procedure of diaphragmatic placation. Unfortunately, your being overweight might preclude this surgery and even without the surgery, you should make an intensive effort to lose weight as that does have the potential to change your health and the quality of your life.
Good luck.