what is the last value of the calculated total co2 lab test of patient 3403 since 05/2102?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3403 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'calculated total co2' ) and strftime('%y-%m',labevents.charttime) >= '2102-05' order by labevents.charttime desc limit 1