when did patient 739 last have pco2 minimum since 12/2104?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 739 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'pco2' ) and strftime('%y-%m',labevents.charttime) >= '2104-12' order by labevents.valuenum asc, labevents.charttime desc limit 1