what is the monthly minimum amount of po2 patient 6921 has until 2104?
select min(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 6921 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'po2' ) and strftime('%y',labevents.charttime) <= '2104' group by strftime('%y-%m',labevents.charttime)