What would keep a pulse ox low on a hospital patient with emphysema who is on all the necessary meds.?

Exacerbation: Even when a patient is on "all the right medications"; the underlying lung disease can cause inflammation and lower the pulse oximetry readings (known as a COPD exacerbation). Most of the time the exacerbation will resolve over the course of 5-7 days but sometimes more powerful medications given intravenously (like steroids & antibiotics) are necessary.

Lots of reasons: Sometimes, despite all the right treatments, the lungs of a patient with emphysema just can't deliver the necessary oxygen and needs help with extra oxygen. Also, if a patient with emphysema gets pneumonia or heart failure or fluid in the lungs, this will also cause the oxygen to go low. Further, it is quite common for oxygen levels to go down when pts with emphysema sleep.