tell me the difference of patient 594's amount of po2 measured at 2105-05-20 04:34:00 compared to the value measured at 2105-05-15 19:03:00?
select ( select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 594 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'po2' ) and labevents.charttime = '2105-05-20 04:34:00' ) - ( select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 594 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'po2' ) and labevents.charttime = '2105-05-15 19:03:00' )