tell me the first value of bicarbonate in a lab test of patient 25997 until 64 months ago?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 25997 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'bicarbonate' ) and datetime(labevents.charttime) <= datetime(current_time,'-64 month') order by labevents.charttime asc limit 1