what changes have been made to the calculated total co2 levels of patient 9566 second measured on the current hospital visit compared to the value first measured on the current hospital visit?
select ( select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 9566 and admissions.dischtime is null ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'calculated total co2' ) order by labevents.charttime asc limit 1 offset 1 ) - ( select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 9566 and admissions.dischtime is null ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'calculated total co2' ) order by labevents.charttime asc limit 1 )