Why would someone with severe copd/emphysema have 100% o2 sats and still severe sob at time even at rest. He's 71 has had increasingly difficulty breathing. Been in hosp for 4 days and had every test. Only thing came back positive was d-dimer. Was sen

On oxygen . That is possible. Off oxygen should decline saturation. Ct may help define embolism! or destroyed lung areas.
COPD . COPD or chronic obstructive pulmonary disease is subdivided into chronic bronchitis and emphysema. Most patients with emphysema do have ch bronchitis to some extent as well. First he needs a full pulmonary function tests to asses severity of his disease and to determine prognosis/life expectancy. 100 saturation on room air or supplemental o2 is only one measure of lung function. Dyspnea or shortness of breath is a symptom. It could be caused by underlying lung disease/ heart disease/vascular disorders like blood clots or anemia ( lack of enough hemoglobin) and last but not the least is deconditioning. You need to discuss with his pcp and if appropriate will need to see pulmonologist. There are certainly more treatment options at this time and age to help patients with emphysema. Pulmonary rehab should also be considered if all treatment options have been optimized and cardiac causes have been ruled out as cause of dyspnea.