what is the yearly minimum value of patient 26720's pco2 until 2102?
select min(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 26720 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'pco2' ) and strftime('%y',labevents.charttime) <= '2102' group by strftime('%y',labevents.charttime)