Is My Dad High Risk For Lung Reduction Surgery Due To Severe Bullous Lung Disease Or Low PO2?

In order to be a candidate for lung volume reduction surgery or to have a bullectomy you have to have severe lung disease which my dad does but when testing for the reduction surgery they said he is to high risk due to severe bullous lung disease and low pO2 I m confused by that is it the bullous or his oxygen level that makes him high risk